Wetland Loss in World Deltas
James M. Coleman
Oscar K. Huh
DeWitt Braud, Jr.
Coastal Studies Institute
Louisiana State University
Baton Rouge, LA 70803
Geologic and geomorphic data on forty-two world deltas were compiled for a NASA-sponsored research project. Satellite images from fourteen of these deltas (Danube, Ganges-Brahmaputra, Indus, Mahanadi, Mangoky, McKenzie, Mississippi, Niger, Nile, Shatt el Arab, Volga, Yellow, Yukon, and Zambezi) were analyzed for delta plain wetland loss caused by natural causes and conversion of wetlands for agricultural and industrial use. These analyses indicated that a total of 15,845 sq km of wetlands have been irreversibly lost during the past fourteen years and the average rate of loss is 95 sq km/yr. If a similar trend is present in the other deltas, a total wetland loss in the delta plains of the forty-two deltas would be on the order of 364,000 sq km over the past 15 to 20 years.
The world’s deltaic plains contain some of the most productive and sensitive ecosystems found on the planet, yet because of natural causes and man-induced changes, these ecosystems are being modified and lost at an astonishing rate. An extensive amount of information was collected for some forty-two deltas worldwide through a NASA research contract (Coleman, et al, 2003). One aspect of this project was to detect changes in delta plain wetlands by using change detection techniques on satellite images of fourteen deltas. Images from two different dates (generally the early ‘80s and 2002) were analyzed by geo-registering the high-resolution satellite images. Photoshop software was then utilized to delineate the two major causes of wetland loss, expansion of open water in the delta plain and agricultural and industrial expansion in the delta plain. The two images of each delta were then imported into ArcView software for calculation of areas. This data could then be used to quantify the net total wetland loss for each delta in the two major categories. Figure 1 illustrates the results of the analysis of open water comparisons in the Yellow River for the period 1989-2000. Because of the size of some of the deltas and cost and time constraints, only selected images from Ganges-Brahmaputra, Indus, and Mississippi deltas delta were analyzed, while the entire delta plain of the other eleven deltas were analyzed. On those where only one image was analyzed, qualitative review of adjacent images in the entire delta plain tended to indicate that the images analyzed were indeed representative of the entire delta.
The fourteen deltas analyzed and the dates of comparison included the Danube (1987-2001), Ganges-Brahmaputra (1989-2001), Indus (1992-2000), Mahanadi (1989-2002), Mangoky (1985-2000), McKenzie ( 1995-2002), Mississippi (1985-1997), Niger (1987-2002), Nile (1984-2001), Shatt el Arab (1984-2000), Volga (1984-2001), Yellow (1989-2000), Yukon (1985-1992), and the Zambezi (1986-2000). These deltas span various climatic regions, display varying rates of subsidence, vary considerably in size, and have varying population densities (Figure 2). The Ganges-Brahmaputra and Shatt el Arab deltas have relatively high population densities, while the Yukon and Mangoky have little or no industrial/agricultural use in the delta plain. The total area of the delta plains examines was 30,225 square kilometers. Table 1 summarizes the results of these analyses, showing for each delta the net loss due to expansion of open water, net loss as a result of conversion of wetlands by agricultural and industrial expansion, the average rate of wetland loss in each category, and the total net wetland loss and average annual rate of loss.
DELTA DESCRIPTIONS AND LAND LOSS ANALYSIS
Danube River. 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. The geology of the drainage basin is complex; the western part of the basin is dominated by Pre-Cambrian 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. The drainage basin has an extremely dense tributary pattern . 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. 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. 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 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 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. 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. 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.
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.
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. 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 (Table 1) and included a
significant portion of the entire delta plain. Thus before man influenced the delta plain, there was approximately 4,345 sq km of wetlands within this portion of the delta plain. By 1987, there was a total of 1,630 sq km of open water within the delta plain, or a reduction of some 62% 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 1,570 sq km, mostly because of filling of open water areas by man. Agricultural and industrial use of the delta plain comprised some 493 sq km on the 1987 image. By 2001, the total wetlands loss due to agricultural and industrial expansion was 410 sq km or a loss of 83 sq km in the 14 year period. Thus, since the delta was first occupied by man, a total of 3 percent of the wetlands in the delta plain has been destroyed and the average rate of wetland loss is 6 sq km/year.
Ganges-Brahmaputra River. 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 and the combined length of the two major rivers exceeds 3,900 km. The drainage network is exceedingly complicated and highly dense, 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. 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 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 eastern side of the country of Bangladesh, where large bell-shaped distributaries are present. 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.
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 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. 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 three satellite images were analyzed and covered some 5,930 sq km of the delta. In the 12 year period, some 783 sq km of delta wetlands had been converted to new open water (Table 1). The Ganges-Brahmaputra delta is one of the highest populated delta plains in the world. Wetland loss because of conversion to agricultural and industrial use totaled 3,507 sq km during this same 12 year period. 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.
Total conversion of wetlands to open water and agricultural lands during the 12 year period is 4,290 sq km. The average annual rate of wetland loss by natural causes and man’s modifications is 358 sq km/year. Although the analysis did not cover the entire delta plain, 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 is 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.
Indus River. 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. 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. 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. 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, 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.
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. 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. 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 1992, there was 1,030 sq km 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 satellite images. 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. However, the most significant change is taking place in the tidally dominated lower delta plain. In 2000, nearly 1,990 sq km of open water existed, an increase of 960 sq km of new open water (Table 1). Analysis of the images indicated that shoreline erosion was occurring all 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. Man’s intervention in destruction of wetlands has also been spectacular. In the eight year period, some 635 sq km of wetlands had been converted into agricultural and industrial use. This represents an average annual rate of wetland loss of 79 sq km/year. In the eight year period, a total of 1,595 sq km of the wetlands in the Indus River delta has been converted from wetlands to open water or agricultural use, an average rate of 199 sq km/year.
Mahanadi River. The Mahanadi river rises in the hills of central India in the Satpura Brahmani fold belt of the Indian Pre-Cambrian Shield. Drainage density is extremely dense and the Hirakud Dam on the river has formed a man-made lake 55-km long. The area of the drainage basin is 141,464 sq km. The interior coastal plain has a relatively low elevation and relief is extremely low. The average elevation of the drainage basin is 426 m, with a maximum of 877 m and a minimum of 193 m. The main soil types found in the basin are red and yellow soils, mixed red and black soils (laterite soils). The main channel of the river is 900 km long. Average annual rainfall in the basin is 1,463 mm with a maximum of 1,663 mm and a minimum of 1,331 mm. The rainy months are June through September, corresponding to the monsoon season. The remainder of the year, rainfall is extremely low, rarely exceeding 30 mm per month. The basin is heavily populated, with a population density of 3.6 people per sq km, with some areas exceeding 36 people per sq km.
The alluvial valley is poorly defined and the channel is predominantly meandering in nature. The average annual discharge is 1,895 m3/sec, with a maximum of 6,352 m3/sec during the summer monsoon. Minimum discharge is 759 m3/sec and occurs during the months October through June. The river is one of the most active silt-depositing streams in the Indian subcontinent. The area of the delta is 10,589 sq km. The delta is extremely complex with numerous abandoned delta lobes. The presently active delta lobe lies to the south and at least two other abandoned delta lobes are located to the north. The older delta lobe to the north is now dominated by tidal influence and numerous tidal channels are apparent on the image. Mangrove is the most common type of vegetation along the seaward edges of the delta plain. The delta plain is a major rice-growing region in India and population density is extremely high. Numerous lakes and bays are present on the delta plain, many of them the remnants of former river courses. Wave energy is quite high along the delta front and well-developed beaches and barrier islands are present along the coast.
Analysis of satellite images from 1999 and 2002 (only 3 years) indicated that the major change in wetlands was the result of new open water rather than conversion of wetlands to agricultural and industrial use. In the three year period, 116 sq km of new open water was computed or the annual rate of conversion being 39 sq km/year (Table 1). Agricultural and industrial use is a relatively new use of this delta plain and in the three year period only 22 sq km of wetlands had been converted. This is a relatively small delta and the rate of agricultural and industrial is 7 sq km/year and if this rate continues at the same rate, a high percentage of the delta plain will be destroyed by man’s activities. Total net loss during the three year period by both natural causes and man’s intervention has been 94 sq km or a rate of 31 sq km/year.
Mangoky River. The Mangoky River rises in the central Highlands of Madagascar and enters the Indian Ocean in the Mozambique Channel. The eastern one-half of the basin drains PreCambrian sediments while the central and western basin drains a prominent series of north-south oriented Mesozoic fold belts. The basin area is 58,155 sq km in area and drainage density is relatively low. From its headwaters to the delta mouth, the main channel has a length of 570 km. Average elevation in the drainage basin is 778 m, with a maximum of 1,440 m and a minimum of 240 m. Most of the higher relief areas are located in the eastern-most portion of the basin. Average annual rainfall is 831 mm, with a maximum of 1,882 mm and a minimum of 240 mm. The rainy months occur from November through March and the average annual monthly rainfall during this period is 130 mm, while the average monthly rainfall during the dry season is only 19 mm.
The channel in the well-defined alluvial valley displays a braided pattern, with numerous mid-channel islands. The average annual river discharge is 526 m3/sec with a maximum of 1,621 m3/sec, which occurs in January and a minimum of 93 m3/sec which occurs in October. Thus the discharge pattern is displays extremely erratic discharge characteristics. Examination of satellite images of differing dates indicates that changes in channel pattern is quite common in the alluvial valley as well as in the delta plain. The climate in the alluvial valley is quite dry and exposed sandy islands dot the main river course.
The delta displays a fan-shaped pattern and has two main distributaries. The area of the delta is 1,547 sq km. Braiding is prevalent in the distributary channels and it is obvious that the distributaries change course frequently as numerous abandoned channel courses can be seen on the satellite images. The abandoned delta is some four times larger that the active delta plain. Most of the delta plain is devoid of vegetation because of the harsh arid climate; salt pans and barren algal flats are common throughout the delta plain. In the area of the abandoned delta plain to the north, tidal channels are prominent and mangrove vegetation dominate the lower delta region). Wave energy is quite high and beach ridges front the entire delta plain, while stranded beaches can be seen in the delta plain, especially in the southern part of the delta. The barrier islands are relatively unstable and are constantly changing their geometry. The barriers average 3 – 10 km in length (Stutz and Pilkey, 2002) and occur as new islands that form at active river mouths that grow laterally through the accretion of recurved spits. Along the inactive delta, the barriers are exceedingly narrow and show evidence of active alongshore migration.
Geo-registering the 1985 and 2000 satellite images and importing them into ArcView allowed interpretation of various changes between the two dates and to calculate the changes. The delta plain analyzed was 1,449 sq km in extent and thus covered most of the delta plain. In the fifteen year period, a net loss of some 43 sq km of wetlands had been converted to open water, primarily by natural causes (Table 1). This represents a rate of 3 sq km/year. Most of the new open water resulted from shoreline erosion and changes in the channel pattern of the river course. A total of 130 sq km of the original delta plain had been reclaimed by 1985. Between 1985 and 2000, an additional 90 sq km of wetlands had been converted into agricultural and industrial use or a rate of 6 sq km/year. Thus, between natural change and man-induced change, a net total change of 133 sq km of former delta plain had been converted to either open water or man-induced landuse. This represents an average annual rate of 9 sq km/year.
McKenzie River. The Mackenzie River is the longest river in Canada, covering a distance of 1,470 km. The river originates at the Great Slave Lake in the Northwest territories and flows north in the Arctic Ocean. The drainage basin covers an area of 1,448,400 sq km and originates in the Canadian Shield of Canada. PreCambrian basement rocks are dominant in the eastern part of the basin, while Devonian and Cretaceous sedimentary rocks are found with the central part of the basin. Drainage density of the tributaries is quite high and several large rivers such as the Peace, Athabasca, Liard, and Slave are part of the drainage pattern. Average elevation in the basin is 620 m, with maximum elevations attaining a height of 2,167 m and minimum elevations of 80 m. Relief in the upper basin is quite high, average relief being 730 m. Annual average rainfall is 335 mm with a maximum of 893 mm and a minimum of 119 mm. The rainy months are July through September, when precipitation rarely falls below 30 mm. There are about 300 days during the year when the temperature is below freezing. The drainage basin is covered by Boreal forests and taiga and results in an extremely large volume of woody debris flowing down the river.
The channel in the well-defined alluvial valley is predominantly braided in nature, but meandering is present in the lower part of the valley. Average annual discharge is 8,561 m3/sec with a maximum of 18,188 m3/sec and a minimum of 2,873 m3/sec. Discharge is rather peaked because of the rapid thaw in the drainage basin and the flood season lasts from May through September during which monthly discharge generally exceeds 10,000 m3/sec. The month of March has the lowest discharge (2,873 m3/sec).
The delta has an area of 8,506 sq km and is formed in a narrow embayment of the general coast. The climate of the area is very cold, with mean temperatures of -29.6°C in January and 13.6°C in July at Inuvik (Pannatier, 1997). Much of the Mackenzie Delta is underlain by permafrost. The permafrost is approximately 100 m thick in land areas in the delta and well away from river channels or lakes. The delta is dominated by approximately 25,000 lakes. These lakes are not static features, but are constantly changing. Only northern deltas have such a large number of lakes, as most temperate and tropical deltas have large areas of marshes and swamps, and few lakes. The main distributary of the delta is quite complex and displays a sinuous pattern with a few meandering stretches. The delta is home to one of the world's largest concentration of pingos, with about 1,450. Pingos are large, volcano-shaped mounds of solid ice, which are thrust up through the permafrost terrain by the growth of their ice cores from below. Most of the delta plain is characterized by patterned ground or ice polygons. Tides are very low in the Arctic Ocean and spring tides are less than 0.3 m. Wave action is also relatively low, with the root mean square wave height being only 0.15 m. Drill holes reveal that there is about 70 to 80 m of deltaic sediment overlying bedrock (http://www.nwtresearch.com/simply/scirep6a.htm). Wood found at a depth of 38 m in one of these holes was dated using radiocarbon techniques at around 6,900 years of age.
Net sedimentation rates on distributary channel levees vary between 1.3 and 2.3 cm/yr in the middle delta, while they range from 0.5 to 1.4 cm/yr in the outer delta (Pannatier, 1997). Lateral channel migration is limited by the development of fine grained levees covered by vegetation and stabilized by the presence of perennially frozen ground. Net sedimentation rates in lakes connected to the channel system vary between 0.36 and 1.16 g/cm/yr in the middle delta and between 0.15 and 0.64 g/cm/yr in the outer delta.
Georegistered satellite images from 2000 and 2002 (only a 3 year period) were analyzed to detect changes in open water as little or no parts of the delta plain are used for agricultural and industrial use. In the three year period, some 24 sq km of tundra wetlands were converted to new open water (Table 1). This represents an annual average rate of 12 sq km/year.
Mississippi River. The Mississippi River, the largest river system in North America, drains an area of 3,226,300 km2; this broad drainage area lies between the Appalachian Mountains (east), the Rocky Mountains (west), and the pre-Cambrian Shield of Canada (north). The river rises in the foothills of the Rocky Mountains and flows southward for a distance of 6,211 km and enters the Gulf of Mexico. The density of the tributary network in the basin is relatively dense, the average drainage density being 0.19 km stream length per 500 sq km. Average elevation in the drainage basin is 659 m, with a maximum of 2,980 m and a minimum of 30 m. Average relief in the drainage basin is quite high, averaging some 915 m. Average annual rainfall in the basin is 688 mm with a maximum of 1,532 mm and a minimum of 169 mm. The rainy season lasts from May through August when the rainfall rarely falls below 80 mm. The driest month is January, with an average rainfall of only 34 mm.
The alluvial valley is extremely well defined all along its course and has a length of 870 km. Meandering is the most common type of channel process, but some braided stretches occur in the upper valley. The lower alluvial valley is characterized by an abundance of abandoned meander belts, each belt marking a former course of the river. Within each meander belt are well-developed abandoned meander loops and oxbow lakes. Separating the meander belts are broad wetlands composed of water tolerant trees and are referred to as backswamps. Only fine-grained suspended sediments are deposited in these regions and organic accumulations are common. The major geologic work on the alluvial valley was conducted by the H. N. Fisk of the U.S. Army Corps of Engineers (Fisk, 1944). This paper is a classic study of processes and sedimentation in alluvial valleys.
The average discharge of the river at the delta apex is approximately 17,704 m3/sec, with a maximum and minimum of 28,161 and 9,579 m3/sec, respectively. Sediment discharge has been estimated to be about 2.4 billion kg annually. The sediment load brought down by the river consists primarily of clay, silt, and fine sand (approximately 70 percent of the load).
During the past 7000 years, the sites of maximum deltaic sedimentation (delta lobes) have shifted and occupied various positions (Kolb and Van Lopik, 1966). The currently active delta lobe is the Birdfoot or Balize delta. An older abandoned lobe, the St. Bernard delta lobe is located north of the active lobe, while a younger abandoned lobe, the Lafourche delta lobe flanks the modern delta to the west. In Recent times, the seaward progradation and lateral switching of the deltas has led to the construction of a broad coastal or deltaic plain that has an area of 28,568 km2, of which 23,900 km2 is subaerial.
The modern Birdfoot or Balize delta of the Mississippi River is the youngest of the Recent delta lobes; it commenced its seaward progradation some 600 to 800 years ago (Fisk and McFarlan, 1955). This newest delta has prograded over a relatively thick sequence of prodelta clays and, as a result of differential sediment loading, has built a relatively thick but laterally restricted deltaic sequence. In contrast, the older Recent deltas, mostly built over shallow bay and shelf deposits, are laterally widespread and relatively thin. The main channel of the river is almost 2 km wide, is 30 to 40 m deep, and displays relatively well-developed natural levees. At image top, the natural levees are up to 1 km wide and have heights of 3 to 4 m. Along the active distributaries in the lower delta, the natural levees narrow considerably, to widths less than 100 m, and display heights generally less than 0.5 m.
The channels of actively prograding distributaries in the delta display bifurcated patterns both upstream and near their mouths. This type of pattern normally is associated with extremely low offshore slopes and low wave energy. Situated between the channels are interdistributary bays displaying a variety of sizes and shapes. These bays are usually extremely shallow (generally less than a few meters) and contain brackish to normal marine water during periods of low flooding and fresh water during periods of high flooding. Sedimentation rates are relatively low. The bays receive sediment only during periods of overbank flow associated with floods.
Immediately seaward of the actively prograding distributaries are the turbid river-mouth effluent plumes (Coleman and Wright, 1971). Deceleration of a turbid plume as it spreads laterally allows the coarser sediment being transported to be deposited, forming the distributary mouth-bar and delta-front environments. The finer grained sediments remain suspended and spread laterally over broad distances, forming a turbid plume that fronts the entire offshore delta; as these fine-grained sediments are deposited, they form the prodelta platform as the distributaries build seaward at rates of 100 to 150 m per year. Wave energy is relatively low offshore of the Mississippi River delta and Figure 64 illustrates, by month, the average wave power along the coast. Wave power is highest during the low discharge months and this often results in excessive coastal erosion.
Wetland loss in Louisiana is extremely rapid and has been documented by numerous investigators (a good regional review is by Barras, et al, 1994). Since the 1930s, 2,800 sq km of wetlands have been converted to open water. This loss is primarily the result of the high subsidence rate that is taking place in the delta region. Rising sea level, dredging, and conversion of wetlands for agricultural and industrial uses have also played a major role in this wetland loss. Because of the size of the delta, satellite images of only the active delta were acquired in 1983 and 1995 and were utilized to detect changes in open water and industrial modification of the delta wetlands in the active Birdfoot delta. The area of the delta analyzed comprised some 1,904 sq km. In a twelve year period some 252 sq km of wetlands had been converted to new open water, at an annual rate of 21 sq km/year (Table 1). Most of the land loss occurred in the numerous bay fills, especially in the West Bay area (South of the Mississippi River channel). In isolated regions, some new marsh was created. Much of the wetlands of the active delta have been modified by man and converted into industrial use. In the twelve year period, a total of 112 sq km of wetlands had been destroyed by man’s activity. A total net wetland loss by conversion to open water or by industrial use was 364 sq km during this 12 year period.
Niger River. The source of the Niger River is in the pre-Cambrian West African Shield region of interior Africa. The central part of the basin, where the "inner delta" is located is in the Central Tertiary Basin. The drainage basin has an area of 2,117,700 sq km and the drainage density is quite high(drainage density of 0.20 km stream length per 500 sq km), especially in the northern part of the basin. The main course of the Niger river has a length of 4,350 km from its headwaters to the mouths of the delta. Average basin elevation is relatively low, some 431 m with a maximum of 1,693 m and a minimum of 157 m (in the inner delta region). Annual average rainfall is 672 mm per year, with a maximum of 2,247 mm and a minimum of only 8 mm. The wet season occurs in July through September and average monthly rainfall generally exceeds 120 mm. The dry season commences in mid-October and lasts through the month of May, where monthly rainfall rarely exceeds 10 mm per month.
The alluvial valley is generally well-defined, especially in the region below the inner delta. River discharge averages some 1,045 cu m/sec annually, with a peak discharge of 1,424 cu m/sec in October and a minimum discharge of 750 cu m/sec in March. A very pronounced feature within the alluvial valley is the presence of an "inner delta". The total area covered by the inner delta, which is a network of tributaries, channels, swamps and lakes, can reach about 30,000 km2 in flood season. The delta area is swampy and the soil sandy. This inner delta has formed in a large cratonic basin, the E1-Djouf or Taoudene.
The delta of the Niger displays a relatively smooth lobate pattern. The delta covers an extremely large area, some 19,135 sq km in area. The subaerial delta is nearly eight times larger than the subaqueous delta, probably as a result of the relatively high near-shore wave action and the presence of strong littoral currents. As a result of the extremely high density of active distributaries, only about one-half of the delta is in a inactive state. Wave energy is relatively high along the delta front. Wave energy is relatively uniform along the entire length of the delta shoreline and the highest wave action occur in the months of June through October. The lobate nature of the delta generally results from the alongshore wave energy gradients. Root mean square wave height is 1.11 m.
The distributary pattern in the delta is highly complex in nature and approximately eleven active river mouths exist. Distributary density is quite high, averaging some 0.48 km stream length per 500 sq km, one of the highest of the deltas analyzed. Most of the interior part of the delta is densely vegetated and mangroves dominate the vegetation along the fringes of the delta. One of the more prominent features of the delta is the relatively large and complex tidal channels that front most of the delta. Tidal range is moderate, with an average tidal range of 1.43 m. Intricate tidal channels extend from the shoreline into the interior part of the delta. The active distributaries are tidally dominated and often display typical bell-shaped river mouths. The presence of strong coastal currents is indicated by the deflection of the river mouths in a downdrift direction. Because of the relatively high wave action, most of the coast displays active sandy beaches and barriers along the coast.
Georegistered satellite images acquired in 1987 and 2002 were analyzed to detect changes in open water and agricultural and industrial use during this 15 year period. The entire delta was not analyzed, but 1,110 sq km of the lower delta was included in the analysis. Some 97.7 sq km of open water existed on the 1987 image and increased to 179.0 sq km by 2002, a total net increase of 81.3 sq km in this 15 year period (Table 1). This represents an average rate of conversion to open water of 5 sq km/year. Much of the lower delta consists of mangrove tidal swamps and agricultural and industrial usage is quite low. In 1987, some 97 sq km of the delta plain was occupied for agricultural usage and by 2002, this had increased to 104 sq km, an increase of 7 sq km/year, an average rate of 0.5 sq km/year. Thus during this 15 year period, some 88 sq km of wetlands had been converted to open water or converted to agricultural usage representing an average rate of 6 sq km/year.
Nile River. The Nile River system is the largest river system in Africa and the drainage basin covers an area of 3,038,100 sq km. From its major source, Lake Victoria in east central Africa, the White Nile flows generally north through Uganda and into Sudan where it meets the Blue Nile at Khartoum, which rises in the Ethiopian highlands. From the confluence of the White and Blue Nile, the river continues to flow northwards into Egypt and on to the Mediterranean Sea. The southern part of the drainage basin consists of Pre-Cambrian rocks (East African Rift) and the remainder of the basin consists of a variety of Mesozoic, Tertiary and Quaternary sediments. The southern and central basin is sparsely vegetated, consisting of subtropical savannas and grasslands, while the northern part of the basin consists mainly of xeric shrubs and deserts. The tributary density in the in the drainage basin is quite dense (0.20 km stream length per 500 sq km). From its headwaters, the main channel is some 3,878 km long. The average elevation in the basin is 737 m while the maximum is 2,900 m and the minimum is only 40 m. Average annual rainfall is 664 mm with a maximum of 2,703 mm and a minimum of 1 mm. Rainy months occupy the months of April through October and the dry months are November through March, during which the monthly rainfall rarely exceeds 20 mm per month.
The alluvial valley of the main river channel is well-defined and is approximately 1,100 miles long and has an average width of 50 km. The average annual river discharge is 2,778 cu m/sec with a maximum of 7,692 cu m/sec in the month of September and a minimum of 979 cu m/sec in the month of April. The river is predominantly meandering in nature and numerous abandoned meander loops are found within the valley. Population within the alluvial valley is quite high, average density being 30 people per square kilometer.
The term "delta" was first applied by the Greek historian, Herodotus, approximately 450 B.C., to the triangular alluvial deposits at the mouth of the Nile River. The delta displays the classical triangular shape characteristics of numerous large world-wide deltas. The area of the subaerial delta is 12,512 sq km and the abandoned delta is 8.68 times larger than the active delta. In ancient times, the Nile had seven distributaries; the Pelusiac, the Tanitic, the Mendesian, the Phatnitic (Bucolic), the Sebennytic, the Bolbitine and the Canopic. There are only two today; the Damietta and the Rosetta. Tides offshore are extremely low, averaging only 0.43 m range. Wave energy is relatively high, with an average wave power at the shoreline of 10.25 x 107 ergs/sec/m coast. The shoreline wave energy is highest during the months from November through April. The highest wave energy is concentrated at the protruding mouths of the two distributaries. The root mean square wave height is 1.53 m. Subsidence is relatively high in the delta region, averaging 1.2 mm/yr. A considerable part of the delta coast lies below 1m elevation and some parts are below sea level and with continued eustatic sea level rise and continued subsidence, much of the delta will be inundated in the next century. Severe beach erosion is occurring along the coast and will continue and increase in future especially at the Rosetta and Damietta headlands. The delta was continuing to prograde at the mouth of the two main distributaries until construction of the Aswan High Dam, when severe coastal erosion commenced.
The delta area is heavily populated, with population densities of 3,000 per sq km being common in some parts of the delta. The region is under heavy agricultural use. Although heavily modified by man, the remnants of the former distributaries can still be discerned. Saline salt and algal flats are common behind broad coastal barrier islands and dune fields.
Satellite images from 1984 and 2001 were analyzed to detect changes that have occurred in the delta plain during this 17 year period. Conversion of delta wetlands to open water has been very low in the delta and most of the change has resulted from shoreline erosion. During the 17 year period, only 2.4 sq km of the shoreline has been lost to erosion, a relatively low rate of 0.2 sq km/year (Table 1). Conversion of delta wetlands for agricultural use, however has been quite high. By 1984, some 135 sq km of the delta plain was in agricultural usage; this increased to 147 sq km by 2001, an average annual rate of 0.7sq km/year. Thus the total change in the delta plain during the 17 year period has been 14 sq km representing an average rate of 0.8 sq km/year.
Shatt el Arab River. The Shatt el Arab river rises in the Tertiary and Mesozoic northwest-southeast trending Zagros fold belt. The river is formed by the confluence of the Tigris and Euphrates Rivers, which flow through central and eastern Iraq. A third river, the Karun River, which rises in west-central Iran and drains the Zagros mountains joins the Shatt el Arab just north of the modern delta. The Tigris/Euphrates Basin, as well as its extension, the Persian Gulf, occupies a zone of subsidence flanked by mountains and/or desert. This elongate depression was formed during an era of mountain building initiated early in the Tertiary that continues with the movement of the Arabian plate against the stable landmass of Asia. The drainage basin of these three rivers covers an area of 793,600 sq km. The main channel is some 2,658 km long and debouches into the Persian Gulf. Tributary density is quite low and the average tributary density is 0.01 stream length per 500 sq km. The average elevation in the basin is 1,090 m, with a maximum of 2,450 m and a minimum of 60 m. Most of the drainage basin is covered with desert and xeric shrubland, with some temperate coniferous forests in the northeastern part of the basin. The average annual rainfall is 920 mm, with a maximum of 1,921 mm and a minimum of 599 mm. The wet months begin in June and end in November, but the monthly average rainfall during this period rarely exceeds 100 mm. During the dry months, December through May, there are many months that have no rainfall and the average monthly rainfall rarely exceeds 50 mm.
The alluvial valley of each of the three rivers are fairly well defined and the channels display a meandering tendency, with stretches of braiding where tributaries enter the main channels. The average annual river discharge is 1,966 cu m/sec with a maximum of 3,299 cu m/sec and a minimum of 849 cu m/sec. The delta exists today in an arid climate, with extremely high rates of evapotranspiration and notable fluctuations in temperature and wind, controlled mainly by topographic variations outside the delta. The Shatt el Arab delta is located at the northern end of an elongate shallow sea where semidiurnal tidal variations reach about 2.5 m. Although much of the delta is made up of broad marshes and associated lowlands that are valuable as agricultural lands, most coastal regions are tidal flats and sabkhas devoid of extensive vegetation where salts are deposited. The river mouths are bell-shaped and prominent broad natural levees flank the distributary channels. They support a growth of salt-tolerant vegetation (mainly blue-green algal mats). Freshwater wetlands just north of the active delta support fresh water vegetation; this area is actively subsiding, but receives a large percentage of the sediments of the Tigris and Euphrates Rivers. The marshlands contain broad expanses of floating cane marsh and bullrush and are inhabited by a unique group of people commonly referred to as the Marsh Arabs.
The modern delta has only two active distributaries, and in its lower course, the natural levees are covered with mangrove vegetation. The interdistributary areas consist predominantly of salt flats. The delta area is some 18,497 sq km in area and much of the modern delta plain is inactive or abandoned. Immediately to the north of the modern delta is a large tidal basin displaying intricate tidal channels and broad unvegetated tidal flats. The wave energy along the delta shoreline is extremely low, the average wave power being 0.014 x 107 ergs/sec/m coast and the average root mean square wave height is 0.99 m. Due to low wave energy, only narrow beaches and small dune systems lie along the leading edge of the delta. Mudflats and sandbars dissected by tidal channels dominate the prograding delta front. Where seawater is trapped during very high (storm) tides, salt pans consisting of gypsum/anhydrite are present. Cultivated areas in the lower delta generally follow the Shatt el Arab and Karun channels. Offshore from the river mouth are broad elongate subaqueous tidal ridges that form in response to tidal fluctuations.
Man has profoundly affected the Shatt el Arab and its delta. The network of irrigation ditches in the delta region appears to be responsible for a nearly 64 percent water loss after contributing sources reach the main channel. Most loss is accounted for by evapotranspiration in the irrigated fields of the lower basin and the Hawizeh marsh. Comparisons of satellite images acquired in 1984 and 2000 allow an evaluation of the changes that have taken place in the delta during the 16 year period. It should be mentioned that tidal range is quite high in the Shatt el Arab and part of the large amount of open water analyzed on the two images could be the result of tidal differences. Most of the changes have occurred in the marshes to the north of the delta, the tidal basin east of the delta and on the seaward edges of the southwestern part of the delta. In the sixteen year period, some 1,610 sq km or 23 percent of the former wetlands have been converted to open water (Table 1). This represents an annual rate of loss of wetlands of 101 sq km/year. The most profound change, however, has been conversion of wetlands to agricultural and industrial use. In 1984, some 2,760 sq km of the delta plain had been converted to agricultural use, but by the year 2000, some 7,849 sq km had been converted. Thus, in this sixteen year period, some 5,089 sq km marsh and tidal basin regions had been converted to agricultural and industrial use. This represents nearly a 72 percent loss of wetlands. Total change from natural wetlands to either open water or agricultural or industrial uses has been 6,699 sq km or 36% of the total delta area in the sixteen year period. Thus, the average annual rate of wetland loss in the Shatt el Arab delta is 419 sq km/year, a very significant landloss rate.
Volga River. The Volga River, the largest river system in Europe, rises in the Valdai Hills northwest of Moscow at an elevation of 225 m and flows through its 2,365 km length to discharge into the Caspian Sea. It has a drainage area in excess of 1,553,900 sq km. Over much of this drainage area, the river traverses a broad, often swampy basin, surrounded by low morainic hills. Within its basin lives nearly 25 percent of the total population of the U.S.S.R., and the river and its tributaries carry about two-thirds of all the riverborne freight in the country. The eastern boundary of the drainage basin lie in the Ural-Novaya-Zembya fold belt, while the central part of the basin contains the Moscow basin. The western edge of the drainage basin is bounded by the Baltic shield. Most of the basin drains Paleozoic and Mesozoic sediments. Earthquakes are common along the eastern edge of the drainage basin. Tributary density is quite and numerous small tributaries enter the main channel of the river. The average elevation in the drainage basin is 161 m, with a maximum of 783 m and a minimum of 30 m. Relief is quite low within the basin, averaging only 32 m. The average annual rainfall is 626 mm, with a maximum of 839 mm and a minimum of 395 mm. The wet months are July through August and the basin has some 240 days with temperatures that are below freezing. Discharge during these periods is extremely low. Boreal forests and taigas cover most of the drainage basin.
The alluvial valley of the Volga River is well-defined and displays a meandering pattern. More than 200 tributaries merge with the main river. Today, much of the flow is regulated through a series of dams and reservoirs. The Volga is fed mainly by snowmelt. Average annual river discharge is 8,103 cu m/sec with a maximum of 24,022 cu m/sec and a minimum of 3,918 cu m/sec. High discharge is in May and June and low discharge is from August through March. Prior to damming, the river delivered 25.5 million tons of suspended sediment and an unknown quantity of bedload to the Caspian Sea (Zenkovich, 1967).
The Volga River flows into the Caspian Sea, the earth’s largest landlocked water body, and its isolation from the world's oceans has enabled the preservation of several unique animal and plant species. The Volga provides most of the Caspian's fresh water and nutrients, and also discharges large amounts of sediment and industrial waste into the relatively shallow northern part of the sea. The delta has a classical "delta pattern" and comprises an area of 27,224 sq km. It has both a well developed subaerial and subaqueous delta components. The ratio of the subaerial delta to the subaqueous delta is 1.97. The river system can be described as an erratically discharging river, flowing into a receiving basin whose water level has varied consistently during the Recent. Within the receiving basin, wave and current energy is extremely low. The level of the Caspian Sea has been fluctuating significantly, and in the last 150 years, water level has fluctuated over 6 m; during the period 1930 to 1963, water level dropped 2.6 m. This water-level fluctuation has led to three zones in the delta proper. The higher areas of the first zone are referred to as "Behr's mounds," linear ridges of clayey sands ranging from 400 m to 10 km in length and averaging 8 m in height. Between the ridges are elongated depressions (ilmens in Russian) that fill with water and become either fresh or saline bays. It is believed that these ridges and swales represent coastal banks now stranded by the falling level of Caspian Sea. The delta proper, comprising the second zone, displays low relief (generally less than 1 m) and is the site of active and abandoned channels, interdistributary regions (often containing saline water), small dunes and algal flats, and small, partially vegetated eolian dunes that derive their sediment from the exposed dry channel courses. The third zone is the submarine part of the delta, which forms a broad platform extending 30 to 60 km offshore.
The main eastern distributary displays a complex anatomizing channel patterns consisting of numerous dry and abandoned channels, as well as active channels. Flow in the channels is so erratic that, for much of the year, little or no water flows in the channels. Strong winds erode the channel floors and form linear dunes on the overbank areas. Adjacent to the main channels are well-developed natural levees, capped with small eolian dunes; the source of the sand is the adjacent channel. The active channels that contain river flow are ice bound during the period December through March. Before the construction of dams, these complex channels constantly shifted their position with each flood. North and west of the delta are broad coastal dunes, many of which have been stranded inland by the falling level of the Caspian Sea. Many of these show little or no orientation and are generally devoid of substantial vegetative cover. A similar extensive system bounds the western flank of the delta, in which interdune areas enclose elongate, oriented lakes. The sand ridges have been stranded by the falling level of the Caspian Sea and consist of marine sands reworked by eolian action; they generally contain a high shell content.
In the lower delta, the small distributaries display well-developed, complex, bifurcated channel patterns, and because of this process, the few major distributaries that enter the head of the delta have split, producing more than 80 active river mouths in the delta. At the river mouths, many shoals and triangular river-mouth bars are the most common geomorphic landform. Relief is very low, rarely exceeding 0.5 m. Immediately offshore is another complex system of subaqueous channels and shallow shoals that forms the delta-front platform. Because of their small size, most of these are barely visible in the image. All along the front of the delta, mudflats, coquina banks, and muddy sand shoals are present, associated with the rapid progradation of the channels before damming. On the lateral margins of the delta are algal flats and salt pans which have accumulated in those parts of the delta that are no longer active or in depressions that have been stranded by the falling level of the Caspian Sea.
Because of the changes in the level of the Caspian Sea, the delta has grown significantly in the past century. In 1880, the delta had an area of 3,222 sq km and by 1920, an additional 2,970 sq km had been added as a result of the falling level of the Caspian Sea (Alekseevskiy, Aibulatov, and Chistov, 2000). From the period 1920 to 1991, an additional 6,580 sq km of subaerial delta had been deposited.
In order to determine the short-term changes in open water and conversion of wetlands to agricultural and industrial use, images acquired in 1984 and 2001 were analyzed for change detection. In 1984, a total of 1,610 sq km of open water existed in the Volga River delta plain. This represented 10.6 percent of the delta plain that contained open water. Some 17 years later, in 2001, a total of 1,710 sq km of open water existed in the delta plain, a loss of 100 sq km of wetlands and now nearly 12 percent of the delta plain is open water (Table 1). During this same period of time, new wetlands were being formed, but only 78 sq km were deposited during the 17 year period. Thus subsidence and other natural factors resulted in a net loss of wetlands on the order of 100 sq km. The rate during this period of time averaged 6 sq km/year. Industrial and agricultural modification to the delta plain has also resulted in significant wetland loss. In 1984, 931 sq km of delta plain were used for agricultural or industrial use. This represents roughly 6 percent of the original wetlands that existed in the delta. By 2001, a total of 1,108 sq km of the delta plain had been modified by man, an increase of 177 sq km during the 17 year period. Thus, in a 17 year period, a net loss of delta plain wetlands by natural causes and man-induced causes has been 277 sq km and the loss is occurring at a rate of 16 sq km/year.
Huang He (Yellow) River. 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. 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, redirecting the river to the Bohai. The present delta is 36,272 sq km in size. Although there are several distributary channels, the southern branch is the presently most active. This distributary has changed considerably in the past twenty years. The delta grew nearly 400 sq km between 1989 and 1995, then began eroding back (Evans, 2002). Between 1995 and 1997, the delta area eroded back about 255 sq km. 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 sq km. 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. 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 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. 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.
Satellite images from 1989 and 2000 were used to determine changes in the delta wetlands. The total open water in the 1989 image was 45.1 sq km and by 2000, the total open water had increased to 53.1 sq km, a relatively small increase of 8 sq km in this 11 year period (Table 1). Conversion of delta wetlands resulting from agricultural and industrial use, however, has much more significant. In 1989, some 350 sq km were in agricultural use, while in 2000, this usage had increased to 1,077 sq km, an increase of 727 sq km; the average rate of change being 66 sq km/year. Thus, during this 11 year period some 735 sq km of delta wetlands had been converted to open water or used for agricultural purposes; this represents an average rate of change of 67 sq km/year.
Yukon River. The Yukon River rises in PreCambrain, Paleozoic, and Carboniferous sediments of Alaska and Canada. The drainage basin covers some 829,700 sq km and the river flows some 3,219 km west and empties into the Bering Sea. The average basin elevation is 740 m, with a maximum of 2,797 m and a minimum of 50 m. The basin is frozen for nearly one-half of the year, and little or no flow is experienced in the channel. Average annual rainfall is 502 mm, with a maximum of 2,041 mm and a minimum of 77 mm. Drainage density of the tributaries is quite dense.
The alluvial valley of the river is well defined and controlled primarily by the structural grain of the region. Although some meandering is displayed by the main channel within the alluvial valley, it is braided most of its distance. The average annual discharge is 6,115 cu m/sec with a maximum of 12,988 cu m/sec and a minimum of 895 cu m/sec. Discharge peaks in May/June, following thawing in the basin and declines over the next few months until November when the basin again freezes. During the winter months, discharge averages less than a 1,000 cu m/sec.
The delta has an area of 5,280 sq km and protrudes into the Bearing Sea. Much of the delta plain is inactive and active deposition takes place mostly at the three active river mouths. Much of the interdistributary region consists of small lakes and contains channel scars of former active distributaries. Much of the delta plain is covered by these small lakes. Pleistocene terrace remnants are found just north and south of the active delta. Along the shoreline are ice-pushed beach ridges that contain a large volume of woody debris in the form of large logs. For some 180 days, the entire delta plain is frozen solid and permafrost occurs almost throughout the delta.
Since there is very little agricultural or industrial use in this remote delta only changes in open water were calculated. In addition, this was one of the few arctic deltas in which georegistered images were available for analysis and since no industrial use is present in the delta plain, any changes in open water would be solely caused by natural processes. Satellite images were obtained for 1985 and 1992 and comparisons were determined by analysis in ArcView. The 1985 image contained 1,210 sq km of open water or roughly 26 percent of the delta plain consists of lakes and open water channels. Some seven years later, in 1992, the open water in the delta plain was calculated to be 2,310 sq km or an increase in new open water of 1,100 sq km or an increase of nearly 48 percent of new open water (Table 1). Only a small percentage of this loss resulted from shoreline erosion, the remainder being lost by enlargement of the numerous small lakes within the delta plain. During this same interval of time, some 316 sq km of new wetlands were formed at the mouths of the rivers and by infilling of some of the open water lakes. Thus in this seven year period, there was a net loss of delta plain wetlands of 1,100 sq km or the annual rate of wetland loss is 157 sq km/year. As there is virtually no influence by man in the delta plain, this significant loss is solely the result of natural processes. Since a high percentage of the loss is along the shoreline of the lakes that exist on the delta plain and in the creation of numerous small new lakes, it is interesting to speculate that the global warming trend might be the most significant contribution to this wetland loss.
Zambezi River. The Zambezi River arises in central Africa and the western portion of the drainage basin is located in the Quaternary and Tertiary sediments, while the central and eastern portion of the basin drains the PreCambrian Luffillian Arch and the East African Rift. The drainage basin has an area of 1,388,200 sq km and is sparsely vegetated with grasslands and savannas and flooded grasslands. The main channel of the Zambezi flows for some 2,650 km before entering the Indian Ocean. The drainage density is quite high and numerous dams have been constructed that impound large freshwater lakes. The average elevation in the basin is 1,058 m with a maximum of 1,847 m and a minimum of 333 m. The average annual rainfall is 955 mm, with a maximum of 1,518 mm and a minimum of 533 mm. The rainy months commence in November and last through mid April with an average monthly rainfall of 140 mm. The dry period is from mid-April through October and the monthly average rainfall rarely exceeds 20 mm. Thus there is distinct wet and dry periods within the basin.
The alluvial valley is well-defined in its lower reaches and the channel displays a predominantly braided pattern. Average annual discharge is 3,341 cu m/sec, with a maximum of 4,558 cu m/sec and a minimum of 1,954 cu m/sec. The delta protrudes slightly into the Indian Ocean and has an area of 2,705 sq km. Wave energy is quite high and numerous beach ridges are found abandoned within the delta plain, alongshore away from the delta, and flanking the river mouths. Tidal influence is best expressed south and north of the delta, where broad tidal basins exist. Within the delta plain, abandoned channels are present, indicating rather active changes in the distributary channels. Extensive mangroves are present in the lower delta and in the tidal basins. Salt flats are common in the interdistributary basins and vegetation is sparse. The river carries a substantial silt and clay suspended load and the distinct sediment plumes are present offshore.
The lower delta plain is virtually absent of any agricultural or industrial land use. Comparison of 1986 and 2000 satellite images indicates that coastal erosion is quite high along the delta front. Some 46 sq km of wetlands and coastal barriers have been lost in the fourteen year period between the satellite images. The only significant land gain is along some of the tidal channels and at the mouths of the main channels. In the fourteen year period, only 22 sq km of new delta plain sediments has been formed, thus there has been a net loss of delta plain wetland of 24 sq km (Table 1). This represents an average annual rate of wetland loss of 2 sq km/year.
As mentioned there is virtually no agricultural or industrial use in the lower delta plain, but at the head of the delta (in the upper delta plain, some agricultural use is made of the delta plain, especially the areas adjacent to the active channel. In 1986, a total of only 22 sq km of land were under cultivation. But by the year 2000, a total of 347 sq km were utilized for agricultural purposes, an increase of 325 sq km. This represents an annual rate of delta plain loss of 25 sq km/year. As agricultural practices increase and levees are constructed to block tidal water incursions, it is highly probable that land reclamation will continue to expand into the lower delta.
In the fourteen deltas analyzed, all deltas showed wetland land loss, but at varying rates. The change in open water areas within the delta plain, and hence wetland loss, ranged from 8 sq km to 1,610 sq km (Table 1) and the total wetland loss by conversion to open water was 5,104 sq km and this loss took place within the last fourteen years. Loss of wetlands caused by conversion of the delta plain to agricultural and industrial use was even greater than the loss experienced by natural causes. The range of wetland loss caused by man’s intervention was 7 sq km to 5,089 sq km. The average annual rate of loss ranged from less than 1 sq km/yr to 318 sq km/yr. Total wetland loss caused by man’s modification of the delta plain was 10,786 sq km in roughly fourteen years. Total wetland loss by both natural causes and man’s intervention was 15,845 sq km with average annual rates ranging from slightly less than 1 sq km/yr to 419 sq km/yr.
In the fourteen deltas analyzed, approximately 52.4 percent of the total delta plain area was irreversibly lost by natural causes and conversion by man for agricultural and industrial use. Quantitative examination of lower resolution browse satellite images tended to show that this trend is not unique to the deltas analyzed in detail, but is occurring in all of the deltas studied. The total delta plain area in all of the forty-two deltas studied is 694,600 sq km. If the wetland loss calculated for the deltas analyzed, 52.4 percent of the total delta plain is correct, and if a similar loss of wetlands is occurring in the other forty-three deltas, a total loss of wetlands would be on the order of 363,970 sq km in the past fourteen years or an average annual rate of loss of nearly 26,000 sq km/yr in the forty two deltas.
Alekseevskiy, N.I., D.N. Aibulatov, and S. V. Chistov, 2000, Shoreline dynamics and the hydrographic system of the Volga Delta, in Dynamic Earth Environments: Remote Sensing Observations from Shuttle-Mir Missions (K.P. Lulla and L.V. Dessinov, eds.), John Wiley & Sons, New York, pp. 159-169.
Barras, J A; Bourgeois, P E; Handley, L R;1994, Land loss in coastal Louisiana 1956-1990. Open File Report 94-01, Lafayette, LA, U.S. Department of the Interior, National Biological Service, National Wetlands Research Center, 4 pp
Coleman, J. M., Brahmaputra River: Channel processes and sedimentation, Sedimen. Geol., 3, 129-239, 1969.
Coleman, J.M., O.K. Huh, D. Braud, Major World Deltas: A Perspective from Space, research report submitted to NASA (2003).
Coleman, J.M. and L. D. Wright, 1971, Analysis of Major River Systems and their Deltas: Procedures and Rationale, with Two Examples; Technical Report # 95; Coastal Studies Institute, Louisiana State University, 125 pages.
Evans, C.A., 2002, Changes in the Yellow River Delta, 1989-2000. American Association of Petroleum Geologists Annual meeting, paper # 25681.
Fisk, H. N., 1944, Geological investigation of the alluvial valley of the lower Mississippi River: U.S. Army Corps of Engineers, Mississippi River Commission, Vicksburg, MS.
Fisk, H. N., and E. McFarlan, Jr., Late Quaternary deltaic deposits of the Mississippi River, in Crust of the Earth, A Symposium, Spec. Paper 62, edited by A. Poldervaart, pp. 272-302, Geol. Soc. Amer., 1955.
Fly River Delta, www.vims.edu/margins.
Hearn, Jr., P., T. Hare, P. Schruben, D. Sherrill, C. Lamar, and P. Tsushima, 2000, Global GIS DataBase, Department of the Interior, U.S. Geological Survey. Digital Atlas of Central and South America – DDS-62-A; Digital Atlas of Africa – DDS-62-B; Digital Atlas of South Asia-DDS-62-C; Digital Atlas of South Pacific-DDS-62-D; Digital Atlas of North Eurasia-DDS-62-E; Digital Atlas of North America-DDS-62-F; and Digital Atlas of Europe-DDS-62-G.
Kolb, C. R., and J. R. Van Lopik, Depositional environments of the Mississippi River deltaic plain, southeastern Louisiana, in Deltas, edited by M. L. Shirley and J. A. Ragsdale, pp. 17- 62, Houston, Texas Geological Society, 1966.
Pannatier, E. Graf, 1997. Sediment accumulation and historical deposition of trace metals and trace organic compounds in the Mackenzie Delta (Northwest Territories, Canada. Ph.D. thesis n°2952, University of Geneva, Switzerland, 222 p.
Qian, N. and D.Z. Dai, 1980, The problems of river sedimentation and the present status of its research in China. Chinese Society of Hydrologic Engineering, Proc. Int. River Sedimentation, v. 1, pp. 1-39.
Samajlov, I. V., Die Flussmundungen, 647 pp., Veb Hermann Haack, Germany, (translated from the Russian original Ust'ia Rek), 1956.
Stutz, M. L. and O.H. Pilkey, 2002, Global distribution and morphology of deltaic barrier island systems, Journal of Coastal Research, ICS 2002 Proceedings, pp. 694-707.
Vorosmarty, C.J., B. Fekete, and B.A. Tucker. 1998. River Discharge Database, Version 1.1 (RivDIS v1.0 supplement). Available through the Institute for the Study of Earth, Oceans, and Space; University of New Hampshire, Durham NH (USA) at http://pyramid.unh.edu/csrc/hydro/).
Wells, J. T., and J. M. Coleman, 1984, Deltaic morphology and sedimentology, with special reference to the Indus River Delta, in Marine Geology and Oceanography of the Arabian Sea and Coastal Pakistan, Van Nostrand Reinhold.
Yang, Z.S., J.D. Milliman, J. Galler, J. P. Liu, and X. G. Sun, 1998, Yellow River’s water and sediment discharge decreasing steadily, Transactions, American Geophysical Union, vol. 79, pp. 589-592.
Zenkovich, V.P., 1967, Processes of coastal development: New York, John Wiley, 738 pp.