INTRODUCTION……………………………………………………………4

&#&#Receiving Basin…………………………………………………….............15

Deltaic Process-Form Variability: A Brief Summary…………………............….29

DELTA DESCRIPTIONS……………………………………………….........37

Amu Darya River System……………………………………………….......…45

Baram River System……………………………………………………...…...49

Burdekin River System…………………………………………………….....53

Chao Phraya River System……………………………………….………..…57

Colville River System…………………………………………………...……62

Danube River System…………………………………………………..….…66

Dneiper River System…………………………………………………...……74

Ebro River System……………………………………………………....……77

Fly River System………………………………………………………....…..79

Ganges-Brahmaputra River System……………………………………...……83

Girjalva River System…………………………………………………..….…91

Krishna-Godavari River System………………………………………..…… 94

Huang He River System………………………………………………....……99

Indus River System…………………………………………………………105

Irrawaddy River System……………………………………………………113

Klang River System……………………………………………………...…117

Lena River System……………………………………………………….…121

MacKenzie River System………………………………………………..…126

Magdelena River System……………………………………………..….…130

Mahakam River System………………………………………………….…134

Mahanadi River System………………………………………………….…137

Mangoky River System…………………………………………………..…140

Mekong River System…………………………...………………………… 146

Mississippi River System………………………………………………...…151

Niger River System…………………………………………………………160

Nile River System…………………………………………………………..165

Ord River System…………………………………………………………...171

Orinoco River System………………………………………………………176

Parana River System………………………………………………………..180

Pechora River System………………………………………………………183

Po River System……………………………………………………………186

Pungoe River System…………………………………………………….…189

Red River System………………………………………………………...…192

Salenga River System…………………………………………………….…195

Sao Francisco River System…………………………………..…………….198

Senegal River System…………………………………………………….…202

Shatt el Arab River System……………………………………………….…207

Volga River System…………………………………………………………213

Yangtze-Kiang River System…………………………………………….…221

Yukon River System…………………………………………….…………..225

Zambezi River System………………………………………………………230

CONCLUSIONS…………………………………………………………..235

REFERENCES CITED…………………………………………………..…238

Deltas vary because of variations in associated process environment. Understanding delta variability involves (1) understanding the causal links between process environment and morphologic response (deterministic studies) and (2) knowledge of the actual process environments and responses and their global associations. In order to understand the reasons for the existence of deltaic forms, the details of deterministic associations between process and form and the conditions which exist in different parts of the world must be examined simultaneously. The study of deterministic associations can lead to establishment of universal cause-effect relationships and development of theoretical models. Investigation of global variability of process environments and delta forms permits evaluation of variations in intensity of forcing agents and morphologic responses and determination of the realistic spectrum of probable process-form models.

The purpose of this paper is to present the results of a comparison of delta forms and their associated process environment. The various morphometric data presented in the report was gathered from a variety of sources and was heavily dependent on delta data acquired in the early to mid 70’s. These data were acquired under an Office of Naval Research contract with the Coastal Studies Institute, Louisiana State University. The principle investigators were J. M. Coleman and L. D. Wright. The data was published in two documents, both technical reports of the Coastal Studies Institute (Coleman and Wright, 1971 and Wright, Coleman, and Erickson, 1974).

MAJOR RIVER SYSTEMS AND THEIR SUBSYSTEM COMPONENTS

This report is concerned primarily with river deltas and their variability. However, a delta is an integral part of a larger total river system and, to be adequately understood, must be considered in that context. Each component subsystem of a river system contributes in varying degrees to the characteristics of the delta. Because of the numerous interactions which take place between subsystems, the river system as a whole is more than the sum of its parts.

Figure 1 diagrammatically illustrates the spatial relationships between the basic river subsystem. A river system consists of at least four subsystems: (1) the drainage basin; (2) the alluvial valley; (3) the receiving basin; and (4) the delta plain. Within each of these subsystems, climatic, geologic, geomorphic, hydrologic, and biologic events mutually interact. Some of these interactions are illustrated in Figure 2. The drainage basin supplies water and sediment to the remainder of the river system and is characterized by net erosion; the alluvial valley is a graded conduit which, over the long term, experiences neither significant erosion nor deposition and through which water and sediment are transported en route from the drainage basin to the sea. The receiving basin serves as a sink for the water and sediment discharged by the river and supplies energy which opposes the seaward-directed riverine energy. The delta itself is, for the purpose of this study, regarded as the response to these various subsystem contributions. The delta is characterized by sediment dispersal and accumulation and results from the interactions between riverine and marine forces.

Drainage Basin. The drainage basin, or catchment, is the source of the water and sediment ultimately supplied to the delta. The amount and temporal distribution of river discharge are of paramount importance to deltaic sedimentation and are functions of basin climate, area, and shape. The sediment yield of a basin is affected by these same factors, as well as by basin geology, relief, and hypsometry.

A total of seven different variables were measured and recorded for drainage basins and are presented for each river system described. The seven parameters are: 1) drainage basin area, 2) stream length, 3) average basin elevation, 4) average basin relief, 5) tributary drainage density, 6) number of days with below freezing temperatures, and 7) average annual rainfall. The drainage basin area was obtained by digitizing the perimeter of the basin on geo-referenced topographic maps at a scale of 1:1,000,000 and imported into a software program that was used to calculate area. Figure 3 illustrates a graph of the drainage basin areas used in this report.

Stream length was digitized on the 1:1,000,000 scale maps and was traced from the headwaters to the river mouth outlet. Figure 4 illustrates a graph of the river lengths for the river systems analyzed. The average basin elevation was based on a random selection of elevations within the drainage basin. The number of points selected varied and no set number of points can be used to obtain an average elevation. In basins having low relief, few points are needed to determine the average, but in basins having extreme relief, a greater number of points are required. The major procedure was as follows: a) ten random points are generated and mean elevation and standard deviation are calculated, b) ten more points are randomly selected and the mean of the total (20 points) and standard deviation are calculated, and c) this process is continued until the standard deviation ceases to change significantly as additional points are measured. The number of points needed ranged from 60 to 900. Figure 5 illustrates the average elevation of the drainage basins in the river systems described. The average drainage basin relief was calculated in a similar manner to that described for calculating the average basin elevation. The relief in the vicinity of the random point is used for the calculation. Figure 6 illustrates the average basin relief for the river systems described. The drainage density is the length of the channels per unit area: D_{d =} ∑L/A_; where ∑L is the summed length of channels per given area A in square kilometers. Drainage densities were measured at the same random points used in the relief analysis. At each point, a given area, usually 500 sq km, is blocked off and the lengths of all streams within this area are digitized. From the densities at each individual point, the mean and standard deviation are calculated. Figure 7 illustrates the drainage density for the river systems described. The number of days that the temperature is below freezing was gathered from a variety of climatic sources. Figure 8 illustrates the number of days the temperature in the drainage basin was below freezing for the river systems described. The average annual rainfall was derived from various climatic maps. Figure 9 illustrates the average annual rainfall, in mm, for each drainage basin in the river systems described.

Alluvial Valley. Unlike the drainage basin, which functions as a supplier of sediment and water and experiences net erosion, the alluvial valley is approximately a balanced system in which the river flows through its own deposits. Typically, there is neither net erosion nor deposition. It is through this conduit that riverine energy, fluid, and sediment mass, derived from the drainage basin, are transmitted to the delta. Sediment sorting and trading processes, which take place within the alluvial valley, affect the texture of delta sediments. In addition, the discharge regime, which ultimately affects deltaic processes, assumes its salient characteristics and, in a sense, matures within the alluvial valley. From a practical of view, more detailed discharge data are normally available for stations within the alluvial valley. Two discharge parameters were tabulated for each river system; a) average annual discharge, in cubic meters/second, and b) discharge range, in cubic meters per second. Figure 10 illustrates the average annual discharge for each river system and figure 11 illustrates the discharge range. Although the data for river discharge came from various sources, the most comprehensive data set was found in Vorosmarty, Fekete, and Tucker (1998) and this data was used extensively throughout the document.

Receiving Basin. The receiving basin serves as a sink for sediments and energy discharged by the river; however, it is far from passive. In a large number of cases, marine forces play the major roles in molding the delta and determining its landscape and geometry. Of fundamental importance are the wind, wave, and tide regimes of the seas and the subaqueous morphology of the continental shelves fronting the deltas. Four parameters were computed for the receiving basins for each river system described. These parameters are: a) tidal range, b) average offshore slope fronting the delta, c) root mean square wave height, and d) wave power computed at the 10 meter contour. The average tidal range along the front of the delta was derived from a variety of sources, usually from published articles on specific deltas. Figure 12 illustrates the tidal range for each delta described. The average offshore slope (in degrees) was derived by measuring offshore profiles at selected intervals along the delta front on hydrographic maps. The average of these measurements is illustrated in Figure 13. The root mean square wave height (H_rms) was derived from a variety of sources, mostly from marine atlases. Using these data, other parameters can be calculated. Significant wave height (H_s) can be calculated using the formula: H_s = 1.416 H_rms and maximum wave height (H_max) with the formula: H_{max =} 2.366 H_rms. Figure 14 illustrates the root mean square wave height for the deltas described. Wave power calculated at the 10 meter contour offshore of the delta shoreline was based on wave height, period, and direction of incidence in deep water were determined from wind data by hindcasting procedures (Sverdrup-Munk-Bretschneider technique). These wave characteristics, together with digitized bathymetric data, were input into a computer program that calculates the effects of refraction, shoaling and frictional attenuation for any given set of wave conditions and bottom configuration. Details concerning this program can be found in Coleman and Wright (1971). Figure 15 illustrates the wave power, in 10⁷ ergs/sec/m coastline, at the 10 meter contour for each delta in which wave data were available.

The slope and shape of the subaqueous profile fronting a delta naturally affect the rate of horizontal sediment accumulation: deltas can prograde faster across flat, shallow shelves than across steep ones. In addition, previously compiled comparisons of deltas and their associated wave-power climates (Wright and Coleman, 1971b, 1972, 1973) have demonstrated that, owing the effects of refraction and frictional attenuation, wave power which reaches the delta shoreline is as much or more a function of offshore slope as a deepwater wave characteristics.

Delta Plain. The three subsystems just described all contribute to the production of the delta plain. The geometry, landforms, and environments of the delta plain and delta shoreline result from accumulation of sediments dispersed by the river and the reworking of these sediments by marine forces. Delta morphology reflects the totality of hydrologic regime, sediment load, geologic structure and tectonic stability, climate and vegetation, tides, winds, waves, density contrasts, coastal currents, and numerous other spatiotemporal interactions of all these factors. As a consequence of spatial variations in magnitudes and intensities of these factors, deltaic responses vary in terms of absolute dimensions, relative proportions of various delta components (see Coleman and Wright, 1971), distributary patterns and forms of river mouths, landform suites of interdistributary areas, and dimensionless geometry of the delta. The following variables were measured or calculated for each of the deltas described: a) delta area in sq km, b) ratio of subaerial/subaqueous delta area, c) ratio of abandoned/active delta area, d) ratio of actual shoreline length to the width of the delta plain, e) number of active river mouths, and f) distributary density. The boundary of the entire delta plain, interpreted from maps and satellite images, was digitized and the area, in sq km, was computed. Figure 16 illustrates the area of the delta plain, in sq km for each delta where data was available. The ratio of the subaerial delta plain to the subaqueous delta plain was calculated from the area of the delta plain as measured on maps and images and data derived from bathymetric maps for the area of the subaqueous delta. Figure 17 illustrates the ratio of the subaerial/subaqueous delta area. In most deltas, only a small portion of the delta plain is actively receiving and delivering sediment; the remainder of the delta plain is undergoing deterioration. These two components of the delta plain were interpreted from maps and satellite images and the boundaries of each were delineated and the area calculated. Figure 18 illustrates the ratio of the abandoned/active delta area. The ratio of the length of the shoreline to the average width of the delta is an indication of the smoothness of the delta shoreline. Those deltas with very large ratios are usually the deltas in which riverine processes are dominant, while those with very low ratios are deltas in which wave energy and currents smooth the delta shoreline. Figure 19 illustrates the ratio of the shoreline length/average delta width. The number of active river mouths was determined from examination of maps and satellite images and these data are shown in Figure 20. The total length of all distributaries in the subaerial delta plain is calculated from digitized data and this total length is divided by the total area of the subaerial delta plain to obtain the distributary density. The larger the ratio, the greater the total length of distributaries is in relation to the delta area. Figure 21 illustrates this distributary density.

DELTAIC PROCESS-FORM VARIABILITY: A BRIEF SUMMARY

The morphologic contrasts, similarities, and groupings of deltas can be attributed to corresponding differences and similarities in the process environments. However, it is typical of morpho-dynamic systems that the cause-effect relationships are bidirectional, so that the behavior of the dynamic forces in the immediate vicinity of the various deltaic forms can differ substantially from the incident, unmodified conditions. Furthermore, with a few possible exceptions, no single process subsystem (such as alluvial-valley discharge regime or receiving-basin energy regimes) can be invoked to explain deltaic patterns because all factors interact to produce the observed morphology. Attempts to demonstrate process-form relationships by inferential statistics were precluded by the small sample size, large number of important variables, and consequent insufficient degrees of freedom. An exhaustive discussion of these relationships would require a very lengthy report and in most cases, data is insufficient to make these comparisons. For more information, see Wright, Coleman and Erickson (1974). The purpose of this section is simply to summarize some of the more prominent relationships and describe the deltas as observed from satellite imagery.

The Drainage Basin and The Discharge Regime. The data indicate a direct correspondence between drainage-basin climate and size and the discharge regime of the alluvial valley; the largest and wettest basins yield the greatest discharge volume, and arid basins or basins with pronounced seasonal variations in precipitation are associated with erratic or seasonally variable discharge regimes. The actual roles of basin geomorphic factors are less obvious. Although basin morphology must certainly influence sediment discharge, the general lack of sediment data did not allow demonstration of the relationship.

Figure 22 suggests a rough tendency for the largest deltas to be associated with largest drainage basins. However, because factors such as the geologic framework of the delta and energy conditions in the receiving basin also affect the area of the delta, there is by no means direct correspondence between basin area, or even total discharge, and delta area. More important are the roles that the total discharge and discharge concentration per unit width of river mouth play in determining the degrees to which delta morphologies are river dominated or the products of marine forces. The discharge regime alone is insufficient to explain deltaic morphologic patterns; it must be considered in terms of its ability to prevail over marine forces (Wright and Coleman, 1971b, 1972, 1973). It is not

simply the strength of the river that matters, but the strength of the river relative to the strength of the opposing waves or tides of the receiving basin. Figure 23 indicates that there is a relatively good correlation between the volume of water discharged and the delta area. Those deltas that have erratic discharges are generally those that do not fit this relationship.

Near shore Marine Energy Climate And Discharge Effectiveness. Overall, the morphologic likenesses and contrasts of deltas exhibit only weak correspondence to the deepwater wave regimes of the receiving basins. This apparent lack of cause-effect association arises largely from the fact that the wave climate in the near shore region often differs substantially from the deepwater wave climate owing to wave modification by the subaqueous topography of the delta front and offshore regions. Figure 24. illustrates the relationship between waver power and average offshore slope in the delta. As can be seen in the diagram, a large amount of scatter occurs. These modifications result largely from wave refraction and attenuation by bottom friction; the significance of these effects on wave climates and resulting morphologies of deltas has been discussed in Wright and Coleman (1971b, 1972, 1973).

The morphologic similarities and contrasts among deltas show some gross tendencies to parallel the nearshore wave climate spectrum. Figure 25 and 26 illustrate the near shore wave power for each delta in which data were available. The wave power is show at the 10 meter contour and the shoreline. In addition, the alongshore wave power gradients are illustrated. In general, coastal barriers and interdistributary beach ridges become more abundant, delta shorelines become straighter, and deltaic protrusions become more subdued as near shore wave power increases and as offshore slopes steepen. However, delta morphology cannot be attributed solely to near shore wave climate inasmuch as deltas result from fluvial as well as marine forces. The delta form depends on the degree to which the river is able to play the dominant role as a morphologic control. In order to index the relative delta-molding ability of river discharge, a ratio referred to as the "discharge effectiveness index" was devised (Coleman and Wright, 1971; Wright, Coleman and Ericson, 1974). This quantity is the ratio of the discharge per unit width of river mouth to the near shore wave power per unit width of wave crest. Although the absolute value of this ratio probably has little or no physical meaning, the relative values and ordering provide a highly significant means of comparing the degree of riverine dominance between deltas.

Table 1 lists 16 deltas in order of decreasing discharge effectiveness with index values normalized relative to the maximum. The Mississippi Delta has the largest discharge effectiveness (normalized value of 1.0) and is consequently the one most dominated by the river, whereas the Senegal Delta exhibits the lowest value (normalized value of 5.99 x 10^-5) and is thus the one most dominated by waves. Delta morphologies define a broad range of patterns between the two extremes. At the river-dominated (high discharge effectiveness) end of the spectrum, deltas are highly indented and have extended distributaries and extensive marshes, bays, or tidal flats in interdistributary regions. With increasing nearshore wave power and decreasing discharge effectiveness, delta shorelines tend to become more regular, and gentle, arcuate protrusions and beach ridges become more common. The highest nearshore wave-power values and lowest discharge-effectiveness indices are associate with deltas which exhibit wave-straightened shorelines and abundant beach ridges.

Table 1

Discharge Effectiveness of Sixteen Rivers

River Discharge Effectiveness River Discharge Effectiveness

Index #9; Index

Mississippi…………..1.00 Indus………………..1.10 x 10^-2

Shatt el Arab…………6.98 x 10^-1 Ord…………………3.66 x 10^-3

Danube………………2.14 x 10^-1 Burdekin……………2.08 x 10^-3

Amazon………………1.17 x 10^-1 Chao Phraya………..1.77 x 10^-3

Ebro………………….4.87 x 10^-2 Niger………………..8.03 x 10^-4

Hwang Ho……………4.16 x 10^-2 Nile…………………5.86 x 10^-4

Irrawaddy…………….1.70 x 10^-2 Sao Francisco……….2.37 x 10^-4

Ganges-Brahmaputra…1.12 x 10^-2 Senegal……………...5.99 x 10^-5

Despite the generalities just discussed, wave and discharge regimes alone do not explain the entire range of deltaic landscapes, particularly under extreme conditions of climate, tide range, or subsidence rate. Deltas in macrotidal environments are characterized by abundant tidal flats and tidal creek networks in their interdistributary areas. Climatic effects are equally significant: those deltas with abundant swamps or other interdistributary vegetation tend to occur in humid environments, whereas arid delta climates are associated with barren evaporite flats. Numerous thaw lakes are common in arctic deltas.

River-Mouth Process-Form Variability. The forms of river mouths and lower river courses also exhibit a broad range of variability. Comparisons tend to suggest that river-mouth morphology reflects differences in the relative importance of the mechanisms of effluent buoyancy, inertia, bottom friction, and bi-directional bottom shear. The intensities of these effects are, in turn, dependent on the sharpness of water-density contrasts at the river mouth, the depth and slope of the bottom fronting the river mouth, and the tidal range and strength of tidal currents. Investigations at the mouths of the Mississippi River have suggested that flow tendencies associated with the expansion of buoyant effluents may be responsible for maintaining straight, parallel distributary levees and high depth-width ratios. These buoyant effects are dominant when the tidal prism is minor relative to river discharge, allowing salt wedges to intrude into channels without appreciable tidal mixing. This type of river-mouth system characterizes the Mississippi, Danube, Po, and Magdalena Rivers. The effects of bottom friction play a major role in controlling effluent expansion and deceleration when outflow velocities are high relative to water depths in and immediately beyond the outlet. Under these conditions, the river-mouth effluents expand and decelerate more rapidly, producing distributary levees which diverge downstream and middle-ground-type bars. River mouths in macrotidal environments tend to experience strong bi-directional flow and appreciable tidal mixing, which obliterates vertical density gradients. These river mouths characteristically exhibit downstream bank divergence at exponential rates and linear tidal ridges in and seaward of the mouths. The mouths of the Ord, Shatt-al-Arab, Chao Phraya, and Irrawaddy Rivers fall into this general category.

A large number of satellite images were obtained for some 41 river deltas and the information gained from various sources are used to describe each river delta. A large part of the factual data concerning the various river systems were obtained from the USGS Global GIS Database seven CD set (Hearn, Hare, Schruben, Sherriff, LaMar, and Tsushima, 2000). This Georegistered map data set proved to be invaluable in the compilations of the data concerning the river systems described. The map locations of the deltas to be described are shown in Figures 27A, 27B, 27C, 27D, 27E1, 27E2, and 27F and Table 2 lists the deltas, country, receiving basin (Delta numbers refer to the maps).

The Amu Darya River, known in ancient times as the Oxus, is the largest river in central Asia and is formed by the junction of the Pandj and Vakhsh rivers. It extends some 2,550 km from its headwaters in the high mountains of Afghanistan and Soviet Tadzhikistan to the end of the delta (Samajlov, 1956) where it enters the Aral Sea. The drainage basin is some 466,200 sq km in area and is dominated by a relatively dry climate. The average annual rainfall is 464 mm with a maximum of 2,009 mm (and a minimum of 69 mm. Rainy months occur from November through May. Vegetation in the central basin is mostly xeric shrublands and desert vegetation. The headwaters of the river originate in the Paleozoic Tian Shan Foldbelt and approximately 70% of the basin drains Tertiary sedimentary sediments. Over the centuries the river has shifted its course several times. In the 3^rd and 4^th millennia BC, the river flowed westward into Lake Sarykamysh, and from there to the Caspian Sea. Since the 17^th century, the river has emptied into the Aral Sea. The receiving basin, the Aral Sea, is some 64,000 sq km in area and experiences less than 1,100 mm of annual precipitation. Evaporation is dominant, with approximately 1 m of water stripped from the surface annually. This water is replaced by inflow from the two major rivers that drain into the Aral, the Amu Darya and Syr Darya.

In its alluvial valley, the river flows through the extremely arid Turan lowlands, where evaporation removes a high percentage of the total flow, resulting in extremely high sediment concentrations. The delta, illustrated in the Landsat image (Plate 1), is bordered by the Kara Kum desert on the west and the Kyzyl Kum desert on the east. The Kara Kum consists of a denuded desert plain with abundant salt flats, soft sediment bluffs, and karst topography. The initial flow of the river was into a low depression of this desert referred to as the Sarykamysh Depression. Some of the former river courses that fed into this depression can still be discerned on aerial photographs. To the east lie the eolian sand fields of the Kyzyl Kum desert. Quaternary linear dune fields, some of them presently active, are shown in the image. Note that many of these linear dune fields have been inundated by the Aral Sea, giving a striking example of the linearity of the dune fields.

The lower reaches of the river once contained a large delta that supported extensive vegetation, but most of the delta has dried up due to reduced water flow. The eastern part of the delta was active until the 10th century, when the major courses shifted to the west. The modern course began to form in the late 17th and early 18th centuries. The modern channel displays active meandering within the upper delta plain; in the lower delta plain, the channel bifurcates, and many distributaries feed water and sediment to the Aral Sea. The presently active part of the delta is characterized by relatively small bifurcated channels that deliver a high volume of sediment to the Aral Sea annually. The turbid river plumes are dramatically shown in the image. The river has an average discharge of 1,397 m³/sec, with a peak flow of 3,145 m³/sec during flood (May through September) and a minimum flow of 464 m³/sec during low river stages (October – April). The image was obtained near the end of May, a period of spring flooding just before the major floods that occur in June and July as a result of glacial melt. Sediment load is extremely high in the active channel; in excess of 100 million metric tons of suspended sediment and 5 million metric tons of bedload is delivered to the delta annually. The various abandoned channel courses are well defined and consist of abandoned meander scars. In many instances, land reclamation in this arid delta follows these courses, and irrigation schemes are abundant along their courses.

The relatively inactive abandoned delta plain dominates much of the scene in the image. The various abandoned channel courses are well defined and consist of abandoned meander scars. In many instances, land reclamation in this arid delta follows these courses, and irrigation schemes are abundant along their courses. Modifications by man are not as prominent in the image shown on Plate 1 scene as on low-level photographs. However, the general trend of the former courses can be easily discerned and mapped from the satellite imagery. The eastern part of the delta was active until the 10th century, when the major courses shifted to the west. The modern course began to form in the late 17th and early 18th centuries.

A large percentage of the delta plain is composed of evaporation flats and salt playas. Some of the lower lying depressions, formed during the prior delta progradation, have not been totally infilled and now exist as high- salinity algal flats and saline lakes. Other partially filled depressions are barren of higher vegetation and consist of algal flats and salt marshes. Along the coast, waves and currents have reworked parts of the older delta plains into a series of beach-barrier complexes, often covered with small coastal dunes and backed by salt and algal flats. Along the fringes of the older abandoned delta are coastal sand and mud flats

Since the 1950’s the Amu Darya has been heavily tapped for irrigation, which has greatly reduced its water level and the amount of water reaching the Aral Sea. During the 1980’s, several years passed in which little or no water reached the Aral Sea from the river. Inflows from the Syr Darya, which empties into the Small Aral sea from the east, have also drastically diminished in recent decades. As a result, the volume of the Aral Sea has dropped by about 80 percent between 1960 and 1996. In the 1960’s, the Aral Sea was fourth among the world’s largest lakes with an area of 66,458 sq km (25,660 sq mi). The lake is generally shallow, with an average depth of 16 m (53 ft) and a maximum depth of 69 m (226 ft). Its salinity averaged about 10 ppt or slightly less than a third of normal sea water. By the end of 1996, the Aral’s total area had decreased by 57 percent to 31,220 sq km (12,050 sq mi) and its water volume had decreased by 80 percent (P. P. Micklin, "Aral Sea", Microsoft Encarta Encyclopedia, 2001). As a result of water loss by evaporation and lack of fresh water input by the Amu Darya, the salinity rose to 35 ppt or normal sea water. With continued evaporation, the salinity will continue to increase.

The Baram River is located in the state of Sarawak, Malaysia. The drainage basin covers some 17,760 sq km and is located in a tropical rainforest region. The average elevation is the basin is 602 m, with a maximum elevation of 4,833 m. Rainfall in the basin is quite high, averaging 3,225 mm annually and rainfall is spread throughout the year. The river is quite sinuous in nature (Plate 2 and Figure. 28), particularly in its lower reaches. The river empties into the South China Sea. The west coast of Sarawak and Brunei to the north consists of well-developed sandy beaches, resulting from a strong southwest longshore drift and a relatively high offshore wave action. All major rivers contribute to this sand / sediment deposition while minor rivers are diverted south to flow for long distances behind sandbars before reaching the sea.

The majority of rivers and coastal waters have high concentrations of total suspended solids. Most coasts are soft sediment-depositing shores while generally there is accretion rather than erosion. However, the coast in the vicinity of the Baram river is being eroded at a rate of 4-8 m per year, and at the mouth of the Baram River, nearly one kilometer of shore has been lost since 1985.

  There is but a single distributary in the Baram delta and at the head of the delta, the river ceases to be meandering and the lower course of the river is relatively straight. The river mouth protrudes into the South China Sea some fifteen to twenty kilometers. The delta plain is generally free from cultivation and the entire region is covered by dense vegetation. Stranded beach ridges are the major geomorphic feature in the delta plain. The delta area is approximately 208 sq km in size. Evidence of relatively high wave energy is indicated by the extensive beach ridges that flank the river mouth.

The Burdekin River is located on the east coast of Australia and in the state of Queensland. The drainage basin is located in the arid region of Australia and has an area of 226,700 sq km. The average elevation in the drainage basin is 732 meters and average relief is 339 meters. The density of tributaries is 0.22 km stream length per 500 sq km, quite high for such a small basin. Average rainfall is very low, averaging 662 meters per year. A major dam has been constructed along the river and forms a large fresh water reservoir (Plate 3). From its headwaters to the river mouths, the river channel is 613 kilometers in length. The river discharges into the Coral Sea behind the Great Barrier Reef.

Average annual discharge is 7,752 m³/sec and the discharge range is extremely large, some 24,359 m³/sec. River discharge is quite peaked, with little or no flow in the months of March through December. Peak discharge occurs in the months of January through April. Figure 29 illustrates the annual discharge in cu m/sec. Note the extreme variation in discharge throughout the year. The alluvial valley is relatively narrow along most of its length and rocky gorges occur in several regions. Most of the river is highly braided and little or no meandering is present.

The delta of the Burdekin River is roughly triangular in shape (Plate 3) and covers some 2,112 sq km. The active delta is quite small relatively to the entire delta plain, the ratio of the abandoned delta to the active delta area is 3.96. The ratio of the shoreline length to delta width is 1.66, thus the delta front is relatively smooth. There are four active distributary mouths and distributary density is 0.25 km distributary length per 500 sq km. Wave energy along the delta front is quite high, the wave power calculated as 6.4 x 10⁷ ergs/sec/m coast. Figure 26-J illustrates the variation in wave power along the front of the delta coast. The root mean square wave height is 0.59 meters. Tidal range is relatively high, averaging 2.23 meters. The subaerial delta is much larger than the subaqueous delta, the ratio of the subaerial/subaqueous delta is 2.12. Offshore slope in front of the delta is extremely low, averaging some 0.0451 degrees.

Geomorphology of the delta plain is governed by the relatively dry climate, erratic discharge and moderate wave energy. The distributary channels are highly braided in nature and consist of gravels and coarse sands. Several older channels are apparent and are now the site of marshy deposits. Stranded throughout the delta plain are numerous stranded beach plains that have resulted from the relatively high wave energy that erodes the river mouth deposits and form relatively coarse beach deposits. It is apparent that wave reworking is constantly taking place along the delta front. The beach ridges are stranded when a major discharge event occurs; the river mouths prograde rapidly and strand the former active beach deposit. The distributary mouth bars consist of elongated sand ridges that front the active distributaries. The delta plain consists primarily of saline salt and algal flats that contain little woody vegetation. Along the active tidal channels, mangroves line the banks. Fronting the delta front are extensive low tide flats that extend several kilometers offshore at low tide. The tidal flats are composed of extremely well-sorted medium-grained sand.

The Chao Phraya River is located in the country of Thailand and empties into the Bay of Bangkok. The Chao Phraya basin is the most important river basin in Thailand (www.unesco.org/water/wwap/case_studies/chao_phraya/index.shtml). The Basin covers 30% of Thailand's land area, is home to 40% of the country's population, employs 78% of its work force, and generates 66% of its Gross Domestic Product (GDP). The total population of the Chao Phraya basin was 23.0 million inhabitants in 1996. The headwaters of the Chao Phraya river originate in mountainous terrain in the northern part of the country and consist of four large tributaries, the Ping, Wang, Yom and Nan rivers. The drainage basin comprises some 992,000 sq km in area and has a relatively low relief (ave. 176 m). The elevation of the basin, however, is quite high, averaging 1,033 m in elevation. Drainage density (0.19 km stream length per 500 sq km) is quite low for such a large basin. Over 90% of the area of the basin is either used for agriculture or covered with forest, with the proportions of these land uses being roughly equal.

The flows in the Chao Phraya and its tributaries are dependent on the monsoon rains during May to October and are highly seasonal. Average annual precipitation in the Chao Phraya basin varies from a minimum of 1,000 mm in the western part to about 1,400 mm in the headwaters and up to 2,000 mm in the eastern Chao Phraya delta. Variations from year to year, which are responsible for floods and droughts, are key factors in determining the availability of the basin's water resources. About 85% of the total runoff occurs in the months of July to December, and natural flows are small in the January to June period. Average annual runoff recorded in the upper Chao Phraya basin varies from about 250 mm in the sub-basin of the Ping above Bhumibol reservoir to some 450 mm in the sub-basin of the Nan above Sirikit reservoir. Average annual runoff for the Chao Phraya river at Nakhon Sawan is 226 mm. Annual average river discharge is 883 cu m/sec and the discharge range is 2,838 cu m/sec. The hydrological cycle starts in April when the discharge is typically at its minimum. From May to August the discharge gradually increases, while from August to October the increase is more rapid, peaking in October. The discharge then decreases fairly rapidly during November and December, with the rate of decrease then slowing until minimum flow conditions are again experienced in April. During the low flow periods from January to April the discharge typically ranges from 50 to 200 m³ sec^-1. Tidal intrusion extends to Angthong (175 km) during low stream flow conditions and to about 75 km upstream during high stream flow conditions. The river, from its headwaters to the delta mouth is 866 km long. The depth of the river ranges from 5 to 20 m and the width ranges from 200 to 1,200 m. The river traverses several large cities and the major agricultural region of the country, hence this river receives large amounts of wastes along its path.

Plate 4 illustrates a satellite image of the Chao Phraya river delta obtained on January 8, 2002. As can be seen on the image, the river has but a single distributary and the river mouth is quite embayed. Tidal range is quite high, some 2.38 m in range. Offshore slope is extremely low, averaging only 0.0041 degrees along the delta front. As a result, wave energy is quite low. Calculated wave power along the shoreline is 0.736 x 10⁷ ergs/sec/m coastline and the root mean square wave height is 0.23 m. The delta is some 11,329 sq km in area and the subaerial delta is some seven times larger than the subaqueous delta and ratio of subaerial/subaqueous delta area is 7.38. The ratio of the abandoned to active delta is 8.95; only a small part of the delta is active today. The front of the delta is rather smooth with the ratio of the shoreline length to the width being 1.15. This smoothness is not the result of extensive wave action, but occurs of the extremely large fine-grained sediment load that is delivered to the coast each year. Figure 30, a satellite image (Photo # STS084-708-088), taken in May of 1997 shows the extremely muddy plume of the river during flood stage. The delta plain has been radically modified for industrial use and is heavily cultivated, primarily in rice and as can be seen in Figure 30. The only natural feature is the thin strip of mangrove covered muddy tidal flats that front the delta.

The Colville River, the largest river in northern Alaska, enters the Arctic Ocean about 240 km east of Barrow, Alaska. The river drains a basin that is 49,500 sq km in area (Figure 35). The entire basin lies in the zone of continuous permafrost and is snow covered for 8-9 months of the year. Snow melt and river flow begin in May with river breakup within the delta occurring in late May or early June. During breakup, as much as two-thirds of the delta is covered by flood waters. The basin displays moderate relief (470 m average relief) and has an average elevation of 410 m, with a maximum of 1,350 m and a minimum of 20 m. The main channel of the Colville River is some 567 km in length from its headwaters to the river mouth at the Arctic Ocean. The river channel is sinuous throughout its length and meandering is common in most of the alluvial valley. Discharge is quite peaked and the average annual discharge is 491 cu m/sec, with a range of 1,075 cu m/sec. Generally three major discharge peaks occur at the end of May and early June.

Plate 5 is a satellite image of the alluvial valley of the Colville River and as can be seen, the river leaves the foothills of the drainage basin and enters the North Slope Coastal Plain where a large number of oriented lakes and permafrost polygons (Figure 36) exist. The delta that has resulted from these deposits is roughly triangular in nature and covers an area of 1,687 sq km. It is composed primarily of sand and finer-grained silts and clays. Thick peat deposits are also common in the delta plain. The subaerial delta is much smaller than the subaqueous delta; the ratio of the subaerial/subaqueous delta is 0.51. The eastern part of the delta is the most active part of the delta and the ratio of the abandoned/active delta is quite high. The Colville delta is rather smooth and the shoreline length to delta width is 1.73. The most conspicuous geomorphic forms present in the delta are: 1) active distributaries, 2) lakes, great in number and varied in size and shape, 3) sand dunes, and 4) polygons (Walker, 1961). Most of the lakes are small and drain to the distributaries through seepage over the permafrost. The sand dunes vary greatly in size and age. The youngest dunes are found almost exclusively along the western banks of the present distributaries, whereas the older dunes are associated with older abandoned distributaries along the western margin of the delta plain. Within the delta plain there are nearly 60 bifurcations and 29 rejoinings. At normal river stage, there are approximately 34 active river mouths. The beds of the distributary channels are highly braided at low river stages, and often gravel lags occur.

The tides in the Arctic Ocean are low, averaging only 0.21 m. The offshore slope is extremely low (0.038 degrees) and thus wave action is extremely low. The root mean square wave height is 0.07 m. In addition, the bottom-fast ice that occurs the vast majority of the year precludes any significant wave action.

The Danube River was known to the Greeks as the Ister River, and Herodotus called it "the greatest of rivers". Napoleon referred to it as the "king of rivers." The Danube is the second largest river in Europe; it is approximately 2,900 km long and drains an area slightly larger than 779,500 sq km. The river rises in the Black Forest Mountains of Germany and empties into the Black Sea. At the Romanian border, the river once cut a channel through the mountain ridge that joins the Carpathian arc with the Balkan Mountains, and a large interior sea was formed. The geology of the drainage basin is complex; the western part of the basin is dominated by PreCambrain and Paleozoic sediments, the southern basin is Mesozoic in age, and the central and eastern part of the basin is dominated by Neogene sediments. The western part of the drainage basin lies within the West Molasse and Southwest German basin, while the central basin lies within the Central Pannonian and Caspian-Balkanean basins. The northern basin is bordered by a major zone of faulting, while the southern border of the basin displays a large number of earthquake epicenters. Not until Recent geologic times has the Danube lowered its channel through the gap to drain this interior sea. The drainage basin has an extremely dense tributary pattern (Figure 37). The density of the tributary pattern is 0.22 km stream length per 500 sq km and the average rainfall is 808 mm, with a maximum of 1,678 mm (July) and a minimum of 457 mm (January). The river, from its headwaters to the mouths of the river in the Black Sea is 2,536 km in length. The basin geology is complex. Relief in the basin is generally low, averaging only 292 m. The average elevation of the drainage basin is 462 m, with a maximum of 2,600 m and a minimum of 60 m. The average annual rainfall is 808 mm, with a maximum of 1,678 mm and a minimum of 457 mm. The wet months are March through August, while the dry months are December through February. Most of the western basin lies within temperate broadleaf and coniferous forests and the central basin consists primarily of temperate grasslands and savannas.

The alluvial valley of the river system is well-defined (Plate 6) and meandering of the channel is quite common. Numerous channels exist and from the satellite images, it is apparent that changes in the river course is quite a common occurrence. The average annual discharge is 6,499 m³/sec, with a maximum of 8,938 m³/sec and a minimum of 4,447 m³/sec (Vorosmarty, et al, 1998). Floods generally begin in late March and continue into the latter part of July. Lowest discharges occur in September and October. Settlements and population density is quite high within the alluvial valley and much of the area is under cultivation.

The delta area (4,345 km²) of the Danube was created in Recent times. Sediment discharge averages 122 million tons/year, of which 54 million tons consist of bed load (Samajlov, 1956). To the north and west of the river delta, primarily Pliocene and Miocene sedimentary rocks form north-south-trending low hills capped by Pleistocene loess up to 200 m high. East of the river and south of the delta, Paleozoic and Mesozoic sediments form high rolling hills that attain elevations of 450 m. The present density of farmlands, however, effectively masks the relief in the image (Plate 6).

The main channel of the Danube is highly migratory within the lower part of the alluvial valley, so that numerous meander scars are present within the valley. Small stretches of river braiding are found along the valley course. As the Danube turns abruptly east to form its delta plain, sedimentation has blocked numerous valleys of the north- south-trending topography, forming elongated freshwater lakes (Plate 6). The major distributaries of the Danube consist of three major channels, the St. George to the south, the Sulina in the middle, and the Kilia to the north (Almazov et al., 1963). The St. George arm is 120 km long and has widths ranging from 200 to 500 m, while the Sulina arm, prior to 1860, had a length of 100 km and a width of 250 m. The Sulina was artificially diked in the period 1860-1895 for navigation purposes. Kilia, the youngest of the distributaries, having formed within the past 600 years, now receives the major part of the flow. It is slightly longer than 100 km and ranges in width from 300 to 700 m. The distributary channels are bordered by well-developed natural levees that are quite narrow, generally less than 250 m wide. Downstream, the natural levees decrease in width and height. Sinuous distributary channels result from migration as the channel cuts through former beach-ridge trends. Like distributaries in many deltas, migration of the channel is controlled by the presence of coarser material. The Kilia distributary to the north is the youngest part of the delta. Numerous bifurcated channels and overbank splays are present, most of which are too small to show up in the satellite image. Offshore of these young prograding distributaries, slope is extremely low (0.152 degrees), the coastline is relatively muddy, but sandy beaches are located along the entire delta coast. A large sandy barrier spit is present south of the St. George distributary (Plate 6).

Several abandoned distributary channels are present within the delta plain. They indicate that the channels shift with time in response to subtle changes in slope, supplying sediment to all parts of the delta. In general, the delta consists of an older upstream part and a younger downstream part. Separating the two units is a large beach-ridge complex seen in the satellite image. These beach ridges display both progradational and transgressive characteristics. They are obviously associated with the prograding river mouths of the St. George, Sulina, and older distributaries. Once the Kilia distributary became the dominant channel, wave reworking of the older distributaries resulted in the formation of transgressive beaches at these river mouths. The Danube beach ridges have heights of 5 to 10 m, and many contain coastal eolian dunes.

Predominant in the delta plain are roseau cane (marsh cane, Phragmites) marshes and freshwater lakes. The marshes are an important resource of the delta, providing a major nesting place for waterfowl. The marshy areas of the delta plain are the largest continuous marshland in Europe which includes the greatest stretch of reedbeds probably in the world. The roseau cane forms thick root mats and results in organic content being exceptionally high in the delta deposits. Much of the delta area is occupied by freshwater lakes up to 3 to 4 m deep. These lakes are initially filled by overbank flow of organic-rich clays. As filling with organic-rich clays proceeds, the lakes become isolated from the overbank splays, and thick floating organic mats form the final fill.

Alongshore drift north and south of the delta has built linear barrier islands and spits, which enclose broad brackish and marine estuaries and lagoons. The barrier islands have relatively steep shore faces, and many are characterized by coastal eolian dunes. The estuaries are important biological environments in that they form the spawning ground for many economically valuable marine species.

Tides in the Black Sea are virtually absent and the only water variations result from wind driven surges. Wave power is moderate, with average wave power of 0.033 x 10⁷ ergs/sec/m coast. Figure 38 shows the distribution of wave power along the delta front for each month of the year. Wave energy is highest in the months of March – May and September through November. The subaerial delta is much larger than the subaqueous delta (ratio of subaerial/subaqueous delta is 8.6). The abandoned delta is approximately three times the size of the active delta.

In order to detect changes in open water and agricultural and industrial use within the Danube River delta, comparisons were made between a satellite obtained in 1987 and one in 2001. Changes in these parameters were completed for the 14 year period. The delta area covered by the georegistered images was 3,066 sq km (757,625 acres) and included a significant portion of the entire delta plain. Thus before man influenced the delta plain, there was approximately 757,000 acres of wetlands within this portion of the delta plain. By 1987, there was a total of 402,083 acres of open water within the delta plain, or a reduction of some 53% of the delta plain wetlands by formation of open water due to subsidence, changes in channel geometry and obviously some influence by man. Some 14 years after the 1987 satellite image was obtained, the open water was reduced to 389,105 acres, mostly because of filling of open water areas by man (Plate 6A). Agricultural and industrial use of the delta plain comprised some 121,925 acres on the 1987 image. By 2001, the total wetlands loss due to agricultural and industrial expansion was 155,379 acres or a loss of 33,454 acres in the 14 year period (Plate 6B). Assuming that most of the delta plain was intact before major modifications by man, a total of 524,008 acres of wetlands were loss because of increasing open water and modification by man until 1987. From 1987 to 2001, an additional 20,476 acres of pristine wetlands were lost primarily the result of modification by man. Thus, since the delta was first occupied by man, a total of 72 percent of the wetlands in the delta plain has been destroyed and the average rate of wetland loss is 1,462 acres/year.

The Dneipr River is one of the longest rivers in Europe, some 2,300 km long. It rives in the Valdai Hills, west of Moscow, Russia and flows into the Black sea. The drainage basin, some 495,500 sq km in size, is located in the rugged mountains of central Russia. The northern part of the basin is dominated by Devonian and Cretaceous sediments, the western basin lies with Cretaceous and Paleogene sediments, while the southern part of the basin lies within Paleogene sediments with scattered PreCambrian granites. The river’s headwaters are located in the Pripzat and Dneipr basins, while the northern boundary of the basin lies in the Belorussian-Voronezh High and the southern basin is defined by the Ukrainian Shield. The average relief in the basin is relatively low, being only 45 m and the average elevation of the basin is 150. Maximum elevation is 2,600 m, while the minimum elevation is 60 m. Tributary density in the basin is quite high (Figure 39). About half of the year, the temperature of the drainage basin is below freezing, thus reducing the river flow for a considerable amount of the year. The average annual rainfall is 489 mm, while the maximum and minimum are 631 and 461 mm, respectively. Rainfall is highest in the months of July through September and thus discharge highest in these months. Average annual discharge is 1,482 cu m/sec with a maximum of 2,893 cu m/sec (month of March) and a minimum of 842 cu m/sec in the month of July. Discharge is quite erratic year from year and yearly variation is quite high. Since the construction (1932) of the Dniprohes dam, the Dnieper is navigable for virtually its entire course and discharge is more controlled.

.

The alluvial valley is well defined and the river displays meandering tendencies along most of its length, with evidence of numerous meander belts (Plate 7). The river flows down a topographic low rift-like basin for most of its length. This is especially apparent in the lower river course where the delta enters the Black Sea well inland from the main coastline (Plate 7). Being sheltered from wave energy in the Black Sea, few beaches are present in the delta proper and root mean square wave height is 0.23 m. Beyond the delta, along the main northern coast of the Black Sea, long, narrow barrier spits and islands are present (Plate 7). The distributary pattern displays some bifurcations and several rejoinings. Several lakes and interdistributary bays are present in the delta. Marsh vegetation is extensive in the delta plain. This delta is a classic example of a river system confined by structural features and does not allow normal delta switching processes to occur.

The Ebro River system rises in the steep foothills of central Spain and flows for some 624 km before entering the Mediterranean Sea in a highly protruding delta plain. The western portion of the basin originates in the Western Spanish Trough and the Tajo-Dueres Basin composed Cretaceous and Carboniferous sedimentary rocks. The central and eastern basin lie in the Iberic Cordillera composed of Paleogene and Neogene sedimentary rocks. Drainage density is relatively low (Figure 40) and the main channel meanders through relatively narrow gorges for most of its length. Much of its course is determined by geologic structure. The drainage basin is some 85,100 sq km in size. The average relief in the basin 402 m and the average elevation is 854 m. Maximum relief 1,893 m and minimum relief is 260 m. Because of the relatively high altitude and mountainous nature of the drainage basin, temperatures are below freezing for a few months of the year. The average annual rainfall is 745 mm with a maximum of 1,337 mm and a minimum of 453 mm. Rainfall is spread evenly throughout the year, with a maximum of 75 mm in December and a minimum of 35 mm in July. The northern rim of the drainage basin is dominated by temperate broadleaf and mixed forests and the remainder of the basin consists of Mediterranean scrub vegetation.

The alluvial valley is not particularly defined and the main channel cuts through several narrow structurally controlled gorges. The average annual discharge is relatively low, being only 420 cu m/sec. Flow is relatively uniform all year long. The delta protrudes significantly into the Mediterranean Sea and has an area of 624 sq km. The offshore slope is quite steep, averaging some 0.31 degrees and is one of the higher offshore slopes seen in deltas. Thus wave energy is quite high, the wave power being 0.155 x 10⁷ ergs/sec/m/coast and the root mean square wave height is 1.53 m. Tides are virtually absent in the Mediterranean Sea and no evidence of tidal channels is present in the delta. Salt water intrusion, however, is quite evident from the numerous saline and evaporite flats that are present within the delta. Man has modified much of the original delta plain and much of the delta is used for agricultural purposes. Small dikes and levees surround these agricultural regions. Protruding from the river mouths are very large barrier island spits (Plate 8). The presence of these recurved spits indicate that strong littoral currents are present along the shoreline. No evidence is apparent on the satellite image of former river channels and presently, man has controlled the river and no migration of the channel occurs.

The Fly River and its tributaries head in the Mesozoic New Guinea Mobil fold and thrust belt. The average elevation of the drainage basin is 1,015 m with a maximum elevation of 2,317 m and a minimum elevation of 30 m. The river drains southward, descending through a narrow canyon thinly mantled with gravel, and emerges as a sand dominated river that crosses a low-relief but highly dissected plain (Papuan Basin) of uplifted Quaternary sediments to the Gulf of Papua. The basin has an area of 64,621 sq km and only the upper one-third is located in the rapidly eroding uplands. Tributary density is quite high (Figure 41). The basin receives, on the average, 2,848 mm of rainfall annually, with maximum and minimum of 3,342 mm and 2,439 mm respectively. Rainfall is relatively constant throughout the year, with monthly averages generally exceeding 200 mm. Most of the drainage basin is located in tropical and subtropical broadleaf moist forests.

The "natural" estimated load entering the Fly delta is estimated to be 85 million metric tons per year; 90 % of this material is fine-grained (Ok Tedi Mining Ltd, 1988). Mass-wasting processes may cause significant fluctuations in this load. The majority of the gravel entering the headwater streams breaks down (Parker, 1991) to fine sediment which is then carried away in suspension. There is limited gravel floodplain areas once the rivers leave the headwater gorge (at about an elevation of 500 m).

The alluvial valley displays abundant evidence of meandering bordered by numerous backswamp areas. Meander loop cutoffs are common as are large blocked valley lakes. Pickup (1984) estimated Holocene floodplain sedimentation rate along the Middle Fly to be about 1 mm/yr. The Middle Fly floodplain shifts progressively from rainforest dominated vegetative cover to swampland. Channel lateral migration rates are low throughout the system, averaging only 1 to 2 m/yr for the 250 m to 350 m wide channel and in the past 50 years it has been essentially zero in the swamp grass reach (www.vims.edu/margins/). Floodplain deposition rates were about 1 to 2 mm/year in the forested reach but much less in the swamp grass reach (Day and Dietrich, 1997). Local rates were locally much higher (Heffler et al., 1997).

The Fly River delta displays a classic funnel-shaped geometry of a tide-dominated system (Plate 9). The delta is mesotidal, with a local tide range from about 3.5 m at the mouth to 5 m at the apex (Wolanski and Eagle, 1991). The area of the delta is 6,230 sq km. Mid-channel islands, covered by mangrove vegetation, are common in the distributary system. Tidal mudflats abound at low tide. Fly River delta mangrove deposits are comprised of three facies: 1) a massive, bioturbated, clay-rich mud, 2) a thinly laminated sandy mud, and 3) a coarsely bedded sand (Walsh and Nittrouer, 1998).

The Ganges and Brahmaputra rivers are one of the largest river systems on the earth. The river channels drain some of the highest mountains present on the planet, the Himalayans. The Ganges River originates near the Tibet/India border, and then flows southeast across India to combine with the Brahmaputra in the country of Bangladesh. The Brahmaputra River has its source in Tibet along the northern slope of the Himalayas, and flows across Assam into Bangladesh. The drainage basin covers an area of 1,664,700 sq km (Figure 42) and the combined length of the two major rivers exceeds 3,900 km. As can be seen from Figure 42, the drainage network is exceedingly complicated and highly dense (tributary density = 0.24 km stream length per 500 sq km), one of the highest densities for a large drainage basin. The average elevation in the basin is 1,923 m, with maximum and minimum being 6,033 m and 180 m, respectively. The combined rivers drain both slopes of the Himalayan mountain chain. Paleozoic and Mesozoic, with scattered outcrops of Pre-Cambrian rocks make up the vast majority of the drainage basin. The headwaters are in the West Indian Shield and the Central Himalayan Foreland basins. In the northern part of the basin, mountain grasslands and alpine meadows are the dominant vegetation, while remainder of the basin is covered by deciduous forests and tropical and subtropical broadleaf forests. Average annual rainfall in the basin is relatively high (1,474 mm) with a maximum of 2,265 mm and a minimum of 341 mm. The rainy months range from June through September when monthly rainfall exceeds 100 mm. The dry months have average rainfall between 20 and 60 mm.

The Ganges is primarily a meandering river, while the Brahmaputra is primarily a braided channel. Figure 43 is a satellite image of the intersection of the two major rivers, the Ganges on the west and the Brahmaputra on the east. Their average annual combined discharge into the Bay of Bengal is approximately 29,692 m³/sec, with a

maximum during flood of 80,984 m³/sec and 6,041 m³/sec during low water periods. The major floods occur during the months from June through September. The image shown in Figure 43 was taken during low water period; during flood season, most of the channel islands are inundated with flood waters. The channels of both rivers are extremely unstable and banklines can migrate as much as 400 m in a single season (Coleman, 1969). Sediment load is extremely high, with suspended sediment load during flood stage reaching as high as 13 million tons per day (Coleman, 1969). Bedload has never been measured, but is obviously extremely high and consists of fine and medium grained sand. Most of the land in the alluvial valley is cultivated in rice and jute and population density is quite high, with 100 to 500 people per one-half degree area.

The delta, one of the largest in the world, covers some 105,640 sq km and has one of the highest population densities of all deltas. Throughout Pleistocene times, the site of active deltaic sedimentation has switched. Today, the Ganges merges with the Brahmaputra, and the site of active sedimentation lies to the east (Plate 10), where large bell-shaped distributaries can be discerned. The major area of abandoned deltaic plain lies to the west and is the site of one of the largest mangrove regions in the world, the Sunderbans. The abandoned delta is approximately 1.6 times the size of the active delta plain. Numerous abandoned channel scars dominate the surface morphology of the abandoned delta plain. These scars are apparently remnants of former courses of the Ganges River and many of its distributaries. Most of the scars indicate that a meandering channel was dominant, now extensively modified by man. Channel scars are of similar size to channels presently active along the Ganges and its distributaries. Many of these former riverine channels are now tidally dominated (Plate 10).

The inland part of the tidal plain has been diked, and the former saline lands have been converted to various agricultural and marine farming practices. This reclaimed land has retained some of the general morphology of the original deltaic channel scars, but it has been modified by tidal drainage networks. Originally, the entire surface of the abandoned delta formed an extreme expanse of mangrove forests. The only remaining large expanse of tidally dominated mangrove forests is shown on the image by the dark-green color and here dense stands of many species of mangrove exist. The mangrove swamp is dissected by an intricate network of tidal drainage channels. The larger tidal channels form bell-shaped estuaries that are quite deep, and many of them serve as major transport arteries. Inland, the estuarine channels display highly sinuous channel patterns, but appear to be stable rather than migratory. Comparison of old maps and aerial photographs with present- day imagery indicates that some major channel patterns have not changed in tens of years.

Typical of many high tidal estuaries is the bell- or funnel-shaped river mouths in the active delta plain (Plate 10). The tidal range varies considerably along this coast, mean tidal range is 3.6 m. Wave energy is relatively low, wave power being 0.585 x 10⁷ ergs/sec/m coastline and the root mean square wave height is 1.4 m. As a result of the low wave energy, few beaches are present along the shoreline and muddy tidal flats are common. The coastline is extremely irregular as a result of the large number of tidal channels that dissect the coast. Broad mud and silt flats border the coast. At low tide, many of these flats are exposed as fluid mudbanks. Most of the banks display elongated patterns, aligned in an onshore- offshore direction. This type of subaqueous morphology is common along many high tidal estuaries, and these shoals have been called "tidal ridges." The few beach deposits that exist are most commonly composed of reworked shell debris and fine sand. These types of beach ridges have been referred to as cheniers on other delta coasts.

The offshore slope fronting the delta is extremely low, averaging 0.011 degrees. The ratio of the subaerial to subaqueous delta is 2.42, the subaerial delta being nearly 2.4 times the size of the subaqueous delta. Offshore of the delta is a large submarine canyon, the Swatch of No Ground. This submarine feature is a broad canyon that was formed during Pleistocene low sea levels, feeding fluvial sediment to one of the largest submarine fans in the world’s oceans.

Georegistered satellite images from 1989 and 2001 were compared to detect changes to the delta plain during this 12 year period. Because of the size of this delta, only a single satellite image was analyzed and is shown in Plate 10A. Note that this image is inland from the river mouth and does not include much of the coastal delta plain of the delta. The wetland area included in this image is 33,405 sq km (8,254,555 acres). Analysis of the 1989 image indicated that a total of 1,080,991 acres of open water existed in this image. By 2001, a total of 1,201,952 acres of open water was present and thus some 120,961 acres of new water was present in only this small portion of the delta plain. The Ganges-Brahmaputra River is a highly actively migratory river channel (Coleman, 1969) and most of the new open water was the result of natural migration of the river channel and formation of new channels. Plate 10A illustrates the changes detected. Note the major changes in the position of the main river channel shown in Plate 10A Note also, that some of the changes in open water is associated with enlargement of small lakes that are found throughout the delta. Thus in a 12 year period, an increase in open water was nearly 2 percent. The Ganges-Brahmaputra delta is one of the highest populated delta plains in the world. Plate 10B illustrates the primarily agricultural land use (colored green in Plate 10A) as determined on the 1989 image. A total of 1,975,207 acres (or 24%) of the original wetlands had been converted to agricultural use. Some 12 years later, a total of 2,523,100 of agriculture was analyzed on the 2001 image. This is an increase of 547,893 acres of wetlands that had been converted to agricultural land in the 12 year period or an increase of 22 percent during this time. These changes are shown in Plate 10B. Enlargement of the original high resolution georegistered image indicates that a high percentage of the agricultural land is divided into small family parcels, generally on the scale of a few acres at most.

By 1989, some 3,056,198 acres of the original wetlands in this image scene along had been converted from a delta plain wetland to agricultural use. This represents approximately 37 percent loss. By 2001, some 3,725,052 acres of the original wetlands had been converted to open water or agricultural use, approximately a 45 percent loss. The average annual rate of wetland loss by natural causes and man’s modifications is 55,738 acres/year. Although this is but a single scene, browse images of the entire delta show a similar use of the delta plain. The only area in which agricultural expansion has not taken place in those areas where saline tidal waters intrude into the delta plain. The largest such area is the main mangrove covered tidal plain referred to as the Sunderbands. Examination of browse images show, however, that even in a short period of time, agricultural land is expanding into this region by construction of levees to prevent salt water intrusion.

The Grijalva River is located in Central America and empties into the Gulf of Mexico. The drainage basin is 134,400 sq km in size and the river runs through the basin for a length of only 82 km. The drainage density of the tributaries within the basin is quite low (Figure 44). Numerous large lakes exists within the headwaters of the river system and an extremely large lake, providing most of the fresh water for the country’s agriculture, is located in the northeast part of the basin (Figure 44). Most of the drainage basin lies within Paleozoic granites, but the central basin is located within relatively soft and easily eroded Jurassic and Cretaceous sediments. The western boundary of the basin is defined by the Chiapas Massif, while the southern basin is defined by the complex Sierra Madre de Chiapas-Peten foldbelt. The eastern basin lies within the carbonates of the Yucatan Platform. The western and southern boundary display a large number of earthquake epicenters. Tropical and subtropical moist broadleaf and coniferous forests and the average annual rainfall is exceptionally high, 1,591 mm, with a maximum of 2,801 mm and a minimum of 916 mm. The rainy months are May through October.

The river channel is highly meandering and flows within a well-defined alluvial valley. The average annual discharge is 3,079 cu m/sec with a maximum of 6,286 cu m/sec and a minimum of only 87 cu m/sec during the dry months of November through March. Although sediment load is not available, the unconsolidated sedimentary rocks of the drainage basin yields extremely large amounts of sediment as defined by the sediment plumes seen in many of the satellite images. Plate 11 is a satellite image of the delta and lower part of the alluvial valley. The highly meandering river channel is well defined in the image and enlargement of the images indicate numerous abandoned river channels.

The delta is quite large for such a small river system and covers an area of 3,341 sq km (Plate 11). A former active distributary is shown in Plate 11 and is located east of the presently active channel. Although the wave energy of the delta front has not been calculated, it must be quite high as most of the delta plain displays a series of seaward prograding beach ridges (Plate 11). Note the erosion displayed along the eastern edge of the delta plain in the vicinity of the former active river mouth. A considerable amount of erosion as taken place as the truncated beach ridges are quite well-displayed. Coastal lagoons are present all along the coast and lie behind prominent barrier islands or prograded beach-ridge complexes. Because of the large variation in river discharge, floods must frequently top the river bands as crevasse splays and well-defined natural levee ridges are prominent in the satellite images. Tidal range is relatively low and although a few tidal channels are present, most of the vegetation within the delta plain consists of fresh-water reed marshes. Most of the small lakes seen in Plate 11 are the result of abandoned distributaries and meander cut-off lakes.

The Godavari and Krishna rivers drain the Deccan Trap and Indian Shield area interior of southern India and flow adjacent to each other until they enter the Bay of Bengal. These two rivers are the largest rivers in South India, rising some 100 kilometers North East of Bombay and only 80 kilometers from the Arabian sea, it runs in a generally south east direction across the peninsula till, after a course of nearly 656 kilometers it discharges into the Bay Of Bengal about 400 kilometers north of Madras. The Krishna River has a length of 968 km and is located just south of the Godavari; it’s drainage basin shares a common border with the Godavari River. The combined drainage basin area of both rivers is 432,100 sq km. The Godavari River drains 126,800 sq km while the Krishna River drains 305,300 sq km (Figure 45). Drainage density is quite high both basins, being 0.22 km stream length per 500 sq km in the Krishna and 0.18 km stream length per 500 sq km in the Godavari. The average basin elevation in the two drainage basins is 470 m with maximum and minimum being 860 m and 177 m respectively. Average annual rainfall in the basins is 1,090 mm, with a maximum of 2,851 mm and a minimum of 466 mm. The rainy months are July through September when average monthly rainfall exceeds 100 mm. The months of November through March are relatively dry, with monthly rainfall rarely exceeding 30 mm. Most of the drainage basin is covered with dry deciduous forests. Population density is generally low with 50 – 100 people in a one-half degree area. However, localized population density in some areas ranges from 2,500 to 5,000 people per one-half degree area.

Figure 46 is a satellite image illustrating the Godavari River delta to the north and the Krishna River delta to the south. The alluvial valleys of both rivers are well-defined and display braided patterns (Plate 12). Average annual discharge in the Godavari River is 3,079 m³/sec while the discharge of the Krishna River is 1,656 m³/sec. Maximum discharge of the Godavari River is 11,567 m³/sec in the month of August and the minimum is 122 m³/sec in the month of April. The Krishna River has a maximum discharge of 6,266 m³/sec in the month of August and a minimum of only 34 m³/sec in the month of April. Thus there is high variation in discharge volumes in both rivers and the flood stage is quite peaked during the months of July through October.

Both deltas display a lobate form (Figure 46) and protrude significantly into the Bay of Bengal. The Godavari River delta is some 17,028 sq km in size, while the Krishna River delta is 6,322 sq km in size. Both deltas display a larger abandoned delta than an active delta, the ratio in the Godavari being 1.37 and 1.17 in the Krishna. Offshore slope is relatively high in both deltas, averaging 0.194 degrees and thus the ratio of the subaerial delta to the subaqueous delta is 6.7 (Godavari) and 4.55 (Krishna). Average tidal range is 0.79 m in the Godavari and 1.19 m in the Krishna. Wave energy is quite high offshore of the delta and root mean square wave height is 0.64 m for the Godavari and 1.42 m for the Krishna. As a result, prominent beach ridges are found along the coast (Figure 46 and Plate 12). Inland, numerous stranded beach ridges can be discerned on the satellite image shown in Plate 12. Both deltas display bifurcated distributary channel patterns and interdistributary bays are prominent between the distributaries. Sediment yield is quite high, the sediment discharge plumes being prominently displayed in Figure 46. Numerous abandoned distributary channels are present in the delta plain of both deltas, suggesting that channel switching is a common process. Mudflats, covered with mangrove vegetation are present in the swales between the stranded beach ridges and are common along the coastal fringe. Population density in the delta region is quite high, averaging 18 people per sq km, with localized areas of 36.5 people per sq km.

The Huang He or Yellow River is the second largest river in China after the Yangtze and has a total length of 5,464 km. The Huang He rises in northern China in the Kunlun Mountains in Qinghai Province, south of the Gobi Desert. The upper drainage basin originates in Ordos Basin and flows through numerous fold belts before reaching the Bohai Gulf. From its source, the river first flows east through deep gorges onto the Ordos Desert and finally through a relatively young valley in deposits of loamy soil known as loess. In this portion of its course, the river picks up and carries in suspension yellow silt, which colors the water. The drainage basin has an area of 865,100 sq km and the tributary density is extremely dense, 0.23 km stream length per 500 sq km (Figure 47). The average elevation in the basin is 1,547 m with maximum elevations reaching 4,240 m. Rainfall is relatively low, the average annual rainfall being only 300 mm a year, with a maximum and minimum of 754 mm and 6 mm respectively. The rainy months are May through September and the dry months are December through March when less than 25 mm of rainfall occurs. Most of the drainage basin is semi-desert or steppe grasslands.

The alluvial valley is well-defined in the lower part of the drainage basin and is heavily populated. The average annual river discharge is 2,571 m³/sec, with a maximum of 2,858 m³/sec and a minimum of 543 m³/sec. The major high water period is from July through October. More than 100 million people live along the banks of the Huang He. In some places the water level of the river is higher than the land and dikes have built to try and stop the river from flooding. The river is often called "China's sorrow" because millions of people have been killed by flooding. The worst flood disaster in world history occurred in August, 1931 along the Huang He River in China and killed an estimated 3.7 million people. Between July and November, some 88,000 sq km of land were completely flooded, and about 21,000 sq km more were partially flooded.

The Huang He has changed course in the eastern portion a number of times. For several centuries before 1852, it emptied into the Yellow Sea, south of the highlands of Shandong Province. The course shifted north that year, and, from that time until 1938, the river emptied into the gulf of Bohai. In 1938, during the Second Sino-Japanese War, the Chinese forces, seeking to impede the invading Japanese, destroyed the dikes and diverted the Huang He into the former course. The Chinese rebuilt the dikes in 1946-1947, rediverting the river to the Bohai. The present delta is 36,272 sq km in size (Plate 13). Although there are several distributary channels, the southern branch is the presently most active (Plate 13). This distributary has changed considerably in the past twenty years. The delta grew nearly 400 km2 between 1989 and 1995, then began eroding back Evans, 2002). Between 1995 and 1997, the delta area eroded back about 255 km2. In 1997 a new channel was cut near the tip of the delta. From 1997 to February 2000, the delta tip again grew nearly 100 km2. Two factors contribute to the changes: 1) the river carries a heavy sediment load, leading to clogged channels and frequent river course changes; 2) the river is heavily engineered and water is oversubscribed, resulting in little flow to the coast in recent years (Evans, 2002).

The river carries one of the largest sediment load of all major river systems. One hundred miles from the river mouth, the sediment discharge has been calculated to be 1.1 x 10⁹ tons/year, exceeded only by the Ganges-Brahmaputra and Amazon rivers (Qian and Dai, 1980). The sediment plume that emanates from the river mouth literally blankets the Bohai Gulf with suspended sediment (Figure 48). The high sediment loads and concentrations of suspended sediment (often greater than 50 g/l) result from the rapid erosion of the loess plateau in the drainage basin and partly from historically poor agricultural practices (Yang, et al, 1998). This sediment discharge figure is based on data collected between 1950 and the late 1970’s. Recently, during the past ten years, the sediment load was only 0.018 x 10⁹ tons/year, less than 1% of the annual sediment loads in the early 1950’s (Yang, et al, 1998). The most likely reason for this decrease is related to the decrease in rainfall and the corresponding increased use of the rivers water.

The channel patterns in the delta are extremely complicated (Figure 49 and Plate 13) and are constantly changing. Intensive modification of the original wetlands have taken place in the delta plain and few areas are in a pristine condition (Figure 49). Tides at the river mouth are 1.13 m in range and wave power is extremely low because of the extremely low offshore slope (0.026 degrees) and limited fetch in the Bohai Gulf. Wave power is calculated at 0.218 x 10⁷ ergs/sec/m coast and the root mean square wave height is 0.58 m.

The 1,487 km long Indus River rises in the Himalaya Mountains of western Tibet at an elevation of about 5,700 m. It follows a precipitous course west through Tibet and then northwest across Kashmir. In western Kashmir, it flows down a narrow passage nearly 396 m deep in places through the mountains until it enters Pakistan and proceeds almost due south to the point where it is joined by the Panjnab River. Shifting to the southwest, the Indus follows a contorted path before emptying into the Arabian Sea and creating a complicated protuberance of terrigenous clastic sediments known as the Indus delta. The drainage basin represents an extremely complex basin, the northern basin dominated by an east-west trending Himalayan fold belt, while the central and southern basin is dominated by relatively low relief Quaternary sediments. Drainage density of the tributaries is relatively high, averaging 0.37 km stream length per 500 sq km (Figure 50). One extremely interesting aspect of the southeastern basin is the presence of the Sulaiman-Kirthar thrust and fold belt, which has controlled the pattern of the river channel (Figures 50 and 51). Figure 51 is a view southeast from the upper alluvial valley toward the delta and illustrates the classical geologic control of a river course by geologic structure. The drainage basin occupies some 1,086,000 sq km and has an average elevation of 1,721 m with a maximum of 5,700 m and a minimum of 30 m. The average relief in the basin is 606 m. The average annual rainfall is relatively low, only some 396 mm with a maximum of 1,580 mm and a minimum of 39 mm. The rainy months are late June through September and the driest months are November through March, when the average monthly rainfall rarely exceeds 30 mm. Most of the vegetation in the drainage basin and the alluvial valley consists of thorn-scrub forests and desert with the exception of the northern part of the basin, which is dominated by alpine steppe vegetation.

Almost 90 % of the water in the Upper Indus River Basin comes from remote glaciers tucked in the majestic Himalayan and Karakorum mountain ranges, which border China and India, and the Hindu Kush, which borders Afghanistan. The rest comes from rains, especially during the monsoon season from July to September. The average annual discharge is 2,644 m³/sec with a maximum of 10,128 m³/sec and a minimum of 189 m³/sec, quite a large range illustrating the erratic nature of the discharge regime. River floods occupy the months of June through September, coinciding with monsoon rains and glacial melt. The river is lowest in December through February when rainfall is lowest. In its upper valley, the Indus flows primarily as a braided stream because of a high gradient associated with the river course and an erratic pattern of discharge. As the river approaches the Arabian Sea, it becomes a meandering system in its lower reaches (Plate 14). Oxbow lakes, meander loops, and abandoned channels, plus ridge- swale scrollwork associated with the deposition of coarse point-bar sediments, are formed during the lateral migration of the river. These morphologic features are illustrated in the satellite image shown in Plate 14. In historic times, the Indus River has switched its location, thus contributing to the construction of a broad deltaic plain some 29,524 sq km in area, the largest part of which does not receive active sedimentation from the modern river (Wells and Coleman, 1984). The abandoned delta is some 6.4 times the size of the active delta region and the subaerial delta is 8.2 times larger than the subaqueous delta. Within the abandoned deltaic plain (northeast portion of the delta in Plate 14), many remnants of once-active distributaries and their associated alluvial features are still apparent. Numerous small lakes, representing former interdistributary bays dot the abandoned delta plain. Tidal processes are now the most active process in the seaward-most region of the delta plain.

The delta has formed in an arid climate under conditions of high river sediment discharge (~400 million metric tons of sediment per year), a moderate tide range (2.62 m), extremely high wave energy (14 x 10⁷ ergs/sec/m coast and a root mean square wave height of 1.84 m), and strong monsoonal winds from the southwest in the summer and from the northeast in the winter. The resultant rather coarse- grained delta, which has acquired a lobate shape, is lacking in luxuriant vegetation and is dissected by numerous mangrove- lined tidal channels in the lower deltaic plain. Estimates of delta building over the last 5000 years indicate an average progradation rate of approximately 30 m/year. Morphology of the Indus River delta lie midway between that of a fluvially dominated delta, with distributaries that protrude into the basin of deposition, and a wave-dominated system, with little distributary expression along the coast, except where characterized by beach and dune deposits.

Plate 14 clearly shows the distinction between the Indus delta's abandoned and active deltaic plains, as well as the desert uplands fold belt (left side of the image) that form the delta's western boundary. In recent years, a high proportion of water from the Indus has been diverted for irrigation, thus considerably reducing the effective discharge. Water storage areas and manmade canals for diverting Indus River water are apparent along the west margin of the delta. The lower or active deltaic plain is roughly delineated by the landward boundary of salt-water intrusion, the position of which is easily seen on Plate 14 by the abrupt color change associated with vegetative cover. This lower deltaic plain is crossed by a complicated network of meandering tidal channels that daily inundate the region with salt water and fine- grained suspended sediment. The margins of these tidal channels are commonly lined by salt- tolerant mangrove vegetation on a sand to silt substrate, while barren flats are common in the inter-channel areas. Along the creek margins, small crevasses/splays build sediment wedges into inter-channel regions. These features are generally too small to be clearly delineated on Plate 14. Even though the tide range of the Indus is not extreme (~2 to 3 m), when combined with the effects of the storm tides of the southwest monsoon in summer, vast areas of both the active and lower abandoned deltaic plain are inundated with salt water. As a result of this yearly cycle, combined with an arid climate, low-relief areas trap salt water that evaporates to create rather extensive salt flats. The bell-shaped channels associated with river mouths and tidal creeks are other indicators of tidal influence on this delta's morphology.

Waves are the single most important process variable in shaping the Indus delta. Intense monsoonal winds arriving from the southwest (May-September) are responsible for an abnormally high level of wave energy at the coast. The effect of this wave energy has been to concentrate the coarse sediments at the shoreline, produce strong longshore currents, and generally straighten the configuration of the coastline. The result has been the development of beach, barrier, and dune complexes at the leading edge of the subaerial delta. Sandy sediments that were originally concentrated at the shoreline by wave activity have been transported into dunes by eolian processes. These dunes reach heights of several meters and are in a state of active migration. They occur along the seaward and western margin of the Indus delta.

Because of man's intervention in the natural delta- building processes of the Indus, this delta's future is uncertain. Extensive use of fresh water for irrigation during the 20th century has decreased the Indus River discharge approximately fourfold. If this trend continues, we can expect the delta to evolve into a more wave-dominated form characterized by extensive beach, beach ridges, and dune formation, probably accompanied by substantial coastal retreat.

In order to detect changes in open water and land use, 1992 and 2000 satellite images were analyzed using ArcView to determine changes during this eight year period. In 1992, there was 255,155 acres of open water in the Indus River delta plain. The interior of the delta plain consists mostly of vegetated soils and little open water is apparent on the image. Most of the open water is located in the tidally dominated lower delta plain. By the year 2000, or some eight years later, significant changes have taken place. First, small areas of new open water begin appearing within the delta plain (Plate 14A). However, the most significant change is taking place in the tidally dominated lower delta plain. In 2000, nearly 500,000 acres of open water existed, an increase of 237,111 acres of new open water, or an increase of nearly 93 percent (Plate 14A). Also note that there has been considerable shoreline erosion along the entire delta front. This erosion is probably due to a lack of sediment that presently reaches the coast because of dams on the middle and upper channels of the river. Some new land has also been formed, mostly the result of shifting of the river course, which has been significant in this eight year period (Plate 14A). Thus in an eight year period, there has been a net loss of wetlands of 162,583 acres. Plate 14B illustrates the changes in wetlands that have occurred because of man’s intervention and reclaiming of the wetlands primarily for agriculture use. In 1992, a total of 63,495 acres of land had been reclaimed for agricultural purposes. Some eight years later, the total wetlands under agricultural use had increased to 220,513 acres, a loss of 157,018 acres of wetlands. Note that this loss has occurred throughout the delta plain, with the exception of the large coastal tidal plain in the lower delta. Thus in an eight year period, a total of 319,601 acres of wetlands loss due to natural causes and man’s intervention. The average annual rate of wetland loss is 39,950 acres/year. The delta plain area shown in Plate 14A is some 13,497 sq km (3,335,181 acres) in extent. In the eight year period, nearly 10 percent of the wetlands in the Indus River delta has been converted from wetlands to open water or agricultural use.

The Irrawaddy River is located in the country of Myanmar (formerly known as Burma) and is formed in the north by the confluence of the Mali and Nmai rivers. The drainage basin is located in the northern part of Myanmar and has an area of 404,100 sq. km. The river originates in the north-south trending Tebasserun-Shon fold belt. From its headwaters, the river flows generally south for 1,295 km.. Near the town of Henzada it branches into several mouths, forming an extensive delta, and empties into the Andaman Sea. The average elevation of the drainage basin is 898 m and relief in the basin is quite high, with an average relief of 353 meters. Maximum elevation is 4,433 m and minimum elevation is 30 m. Tributaries are quite dense in the drainage basin and stream density is 0.21 km stream length per 500 sq km (Figure 52). The basin is located in a tropical climate and average annual rainfall is quite high, some 1,684 mm per year, with a maximum of 3,529 and a minimum of 665 mm. The rainy months are June through August and coincide with the seasonal monsoons.

The alluvial valley is well defined and the upper channel of the river is braided in nature. Further south, the river displays meandering tendencies and has numerous small tributaries entering the main river channel. The most active region of the delta is located on the western edge of the delta plain (Figure 53 and Plate 15). The delta comprises an area of 20,571 sq. km. and average discharge is approximately 12,564 cu m/sec, with a peak discharge of 36,000 cu m/sec. The delta is located in a tropical rainforest and sediment delivery is extremely high, the river delivering 265 x 10⁶ tons per year to the coast. Tidal range is extremely high, some 2.71 meters in amplitude resulting in an extremely dense network of tidal channels (Figure 53). Mangrove forest cover is dense and dominates the vegetation in the lower delta. Average offshore slope is quite low ((0.02 degrees) and thus wave action along the front of the delta is moderate, and only small beaches are present along the fringes of the distributaries. The root mean squared wave height is 1.42 m and wave power was calculated to be 0.193 x 10⁷ ergs/sec/meter of coast. A well-developed series of beach ridges flank the southeastern edge of the delta and represent reworking of deltaic deposits by the dominant southeastern swell. Much of the delta plain is cultivated in rice fields and the delta is one of major rice-growing regions of the world. The light pink areas on the image represent agricultural land and make up a large portion of the delta surface. The active delta region is approximately three times the size of the inactive delta region in the central and eastern part of the delta plain. There are approximately 10 active river mouths and distributary density is 0.17 km stream length per 500 sq km, quite low for a delta of this size. The shoreline of the delta is quite long compared to the width of the delta, shoreline length being about 1.7 times the delta width. Based on analysis of bathymetric charts, the subaqueous delta is quite large, probably resulting from the heavy sediment load delivered to the coast. The ration of the area of the subaerial/subaqueous delta is 0.09.

The Klang River system originates in the central highlands of Malaysia and drains into the Straits of Malacca. This is one of the smaller rivers described, but is typical of the large number of small river systems that drain adjacent coastal highlands and debouch into the adjacent ocean or strait. The drainage basin is only 900 sq km in size and the river length is 56 km. The drainage basin has few tributaries as the main river drains a few incised valleys and then debouches onto an alluvial plain. The coastal highlands that is drained by the Kang River is predominantly Paleozoic in age, with a few scattered outcrops of Triassic sediments. The basin vegetation consists of tropical and subtropical moist broadleaf forests. Population in the basin is extremely heavy and rubber plantations dominate the agricultural landscape. The average elevation in the drainage basin is 165 m and relief averages 446 m. Drainage density is extremely low, averaging only 0.01 km stream length per 500 sq km (Figure 54). Rainfall is extremely high, the average annual rainfall is 758 mm with a maximum of 1,010 mm and a minimum of 575 mm. The rainfall is controlled by the monsoons that are typical of the region of the world.

The river discharges throughout the year and has an average annual discharge of only 17.2 m³/sec. and discharge is generally uniform all year long. The channel in the alluvial valley displays braided characteristics and is chocked with clean medium-grained sands, even though the sediment load is heavily charged with fine-grained clays and silts. The river debouches onto a coastal plain bordering an elongate receiving basin open at both ends (the Malacca Straits). The delta has an area of 1,817 sq km and the ratio of the abandoned delta to the active delta is 1.37. The subaerial and subaqueous delta plains are equal in size. The most important factors affecting the Klang River are marine processes, particularly the tides and tidal currents. Tides in the Malacca Straits have a mean spring range of 4.3 m. Tidal currents are bi-directional, but set to the northwest, and average tidal current velocities range from 1.5 m/sec to the northwest to less than 1.0 m/sec to the southeast. This constant tidal reworking of bottom sediments in the shallow (120-180 m) straits results in concentration of a large sand body characterized by large asymmetrical sand waves along the bottom of the Malacca Straits. Often, many of these large bedforms coalesce to form large, linear tidal-ridge sands, which are essentially parallel to the axis of the seaway.

The mouths of the Klang River display a broad funnel shape and are tide dominated (Plate 16). The river mouth bars are skewed alongshore in the direction of the net drift. Within the distributaries, the strongest currents during low river stage are directed upstream, attaining velocities in excess of 1.5 m/sec. The distributaries are choked with well-sorted fine to medium sands in areas of maximum velocity and poorly sorted clayey sands along the edges of the channels. Adjacent to the channel the interdistributary areas consist of broad, saline mangrove and nipa palm swamps characterized by high organic deposition (Plate 16). Downdrift from the mouths of the active delta are broad muddy tidal flats and banks (Plate 16) that display definite alongshore biological zonations. These tidal flats extend alongshore some 20 km and are up to 3-4 km wide. Near the river mouth, broad tidal flats exist and are nearly devoid of fauna because of the high sedimentation rates. These flats are composed primarily of fine-grained, organic rich clays. Farther downcurrent, the tidal flats display hummocky surfaces and consist of large biomasses of the brachiopod, Lingula.

The interior parts of the delta, in the areas lying between the major channels are broad freshwater swamps. In such a tropical climate plant growth is rapid and the rate of accumulation of peat is high (0.1 m per century). Peat deposits attain higher elevations than the bordering clastic meandering belts, and thus the interdistributary areas are raised rather than depressed.

The Lena River, with a length of 4,319 km, is the longest river in the U.S.S.R. A great delta has formed where it empties into the Laptev Sea, a part of the Arctic Ocean. The river rises a few kilometers west of Lake Baikal, on the western slopes of the Baikal Mountains, at an elevation of 1800 m. The average elevation in the drainage basin is 570 m, with a maximum of 1,697 m and a minimum of 90 m. The drainage basin comprises some 2,421,400 sq km, one of the larger drainage basins of arctic rivers. The tributary drainage network is quite dense (Figure 55), average drainage density is 0.19 km stream length per 500 sq km, and is controlled primarily by geologic structure. The southern rim of the drainage basin is comprised of PreCambrian and Cambrain sedimentary rocks, while the central basin is composed of Jurassic and Cretaceous sedimentary rocks. The northwest part of the drainage basin displays complex geology, but is comprised mostly of Cambrain, Ordovician, and Triassic sedimentary rocks. The Aldon Shield and the Baikal Patom fold belt lie in the southern part of the basin, whereas the Nepa-Botuoba Arch and thee Anabar-Olenek high occupies the western part of the basin. Rainfall is relatively low, the average annual rainfall being 406 mm. Maximum rainfall is 700 mm and minimum rainfall is 226 mm. Heaviest rainfall occurs in the months of July through September. For some 285 days of the year, basin temperatures are below freezing and no runoff occurs. Throughout much of its course, the river widens into broad braided stretches 3 to 4 km wide and then narrows as it cuts through more resistant strata (Figure 56). The alluvial valley is well-defined, relatively narrow, and braiding is the most common channel pattern. Numerous small lakes dot the alluvial valley. Figure 56 illustrates the control that geologic structure has on the river’s course. Most of the basin is covered by Boreal forests and taiga, while the delta plain consists of ice bound tundra. Near its delta, the river narrows considerably as it cuts through the Kara Ulak Mountains, which are composed of Permian and Cretaceous sediments. Even in this stretch, narrow Pleistocene river terraces are present along the Lena's course.

Discharge is extremely high, the average annual discharge being 16,630 m³/sec, with a maximum of 73,996 m³/sec and a minimum of 1,203 m³/sec. Discharge abruptly increases from 5,578 m³/sec in May to a maximum of 73,997 m³/sec in June, when spring melt begins. Nearly every year in the late spring, ice blocks the flow of water at the mouth of the Lena River in northeastern Russia and gives rise to floods across the Siberian plains. In the spring, however, water upstream thaws earlier than water at the mouth of the river. As the southern end of the river begins to melt, blocks of ice travel downstream to the still frozen delta, pile up, and often obstruct the flow of water (Figure 56). Emerging from the highlands, this long north-flowing river crosses the taiga of Siberia, one of the most extensive forests on the Earth. After passing through the narrow gorge near its delta, the Lena cuts through broad coastal Pleistocene terraces (left side of image, Plate 17), many remnants of which are present as isolated highs within the delta. The fan-shaped delta covers some 43,563 sq km and is a maze of small distributaries (Plate 17). The river splits off into several distributaries, the only one of significance being the Trotimorskaya, which flows into the eastern part of the delta. The delta can be divided generally into two major parts, the western delta and the eastern delta (Antonov, 1959). The western delta is relatively high, is moderately crossed by distributary channels, and has had many of its lakes tapped and drained, resulting in dry land or former lake surface over approximately 85 percent of the delta. The presently more-active eastern part of the delta is characterized by relatively low elevations and a large number of anastomosing channels and sand bars. In both parts of the delta, the ground is permanently frozen, often to depths in excess of 15 m.

At the apex of the delta, the Lena River is complexly braided, with many large braid bars and small channels. Within the delta plain, the major channels split and rejoin, forming a maze of anastomosing channels (Suslov, 1961). Near the distal ends of the distributaries, where the channels often broaden out, the river mouth bars are composed of barren sand and mudflats. Tidal range is low, generally less than 0.21 m, and does not significantly influence the sediment distribution pattern. Wave action at the coast can be considerable, with root mean square wave heights up to 0.24. As a result, small beaches and complex offshore barriers (Plate 17) are present at most of the river mouths of arctic deltas. The channels in the lower delta are often quite deep, on the order of 15 to 18 m, being much deeper than the upstream segments. Sediment load is quite high during the short flood season, with the suspended load reaching 11.7 million tons per year. Offshore currents are relatively low and generally flow to the northwest, carrying large quantities of fine-grained sediment westward along the coast.

The delta plain consists of a maze of anastomosing channels, between which are broad areas of tundra surfaces containing patterned ground or polygons, freshwater lakes, and overbank splays. The tundra surface is composed of relatively large and complex patterned ground, which results from the annual freezing and thawing process. Ice wedges and other similar arctic landforms are present. As a result of thawing, many small freshwater lakes now dot the tundra surface. The freshwater lakes cover some 16 percent or 6,970 sq km of the delta plain. Because ice jams in the distributary channels cause abnormally high water in the delta, overbank splays are quite common, often filling many of the small lakes. After the major flood has subsided, water levels drop considerably, exposing a large number of sandy midchannel bars. Wind erosion removes the finer sands and forms small oriented dunes along the channel margins, often extending onto the adjacent tundra surface and into the lakes. The river traverses large taiga forests and thus carries large quantities of woody debris. This organic debris is often left stranded on the midchannel bars and within the channels.

On the Pleistocene surface to the west (Plate 17), oriented lakes are present, similar to those found along most of the Arctic tundra. Pingos or "ice-core hills" are also found on this surface. Broken pack ice, which is nearly always present, can be seen offshore of the delta (Figure 56).

The Mackenzie River is the longest river in Canada, covering a distance of 1,470 km. The river originates at the Great S