THE WORLD DELTA DATABASE

THE INDUS DELTA

Delta ID  # 15.

DISCHARGE WATER 55000M3/S, SEDIMENT 100X106TONS/YR,

DRAINAGE BASIN AREA 9.63X105 KM2

The 1,487 km long Indus River rises in the Himalaya Mountains of western Tibet at an elevation of about 5,700 m. [15-i06]. 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 (15-i04). This image  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 [15-g01]. 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 [15-i14 and 15-i07]. 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 15-i14. 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 15-i14), 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 fluvial 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.

15-i14 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 15-i14 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 15-i14. 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,[15-i02] 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 [15-i01]. 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. 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. Thus in an eight year period, there has been a net loss of wetlands of 162,583 acres. 15-i03 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 15-i03 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.