Spatial patterns and variation of suspended sediment yield in the upper Indus River basin, northern Pakistan

Much of the flow of the Indus originates in the mountainous regions of the Karakoram and Himalayas, resulting in a high sediment yield that creates a number of operational and maintenance problems for downstream water use in Pakistan. Analysis of the available hydro -climatological data indicates that the upper Indus basin can be subdivided into three characteristic regions based on whether runoff production is controlled by temperature (Region 1, upper, glacierized sub-basins), precipitation caused by the monsoon and western disturbances (Region 3, lower sub-basins), or a combination of the two (Region 2, middle reach sub-basins). The runoff and sediment transport regimes for the Shyok, Gilgit, Hunza and Indus Rivers in Region 1 show the role of snow and ice melt in generating high runoff, and 80-85% of the annual sediment load is transported in July and August. The Astore River at Doyian in Region 2 shows an early rise of the sediment yield starting in May due to the interaction of rainfall and snowmelt at lower elevations, and a comparatively lower sediment yield in August indicating a lesser influence of glacier-melt. Region 3 experiences two types of rainfall: summer monsoons and the western disturbances in late winter and early spring. Consequently, the Gorband River at Karora in Region 3 exhibits two separate sediment yield peaks of equal height in April and July. The runoff and sediment transport regime along the main stem of the Indus River is dominated by the melt of snow and ice in its headwaters, and consequently is similar to the regimes of the Region 1 sub-basins. Contrary to the conventional model, the specific sediment yield increases markedly with drainage area along the Indus River, from 355 t km^-2 yr^-1 at Kharmong to 1197 t km^-2 yr^-1 at Besham Qila, likely because of the large number of small and relatively steep catchments discharging straight into the Indus River. Analysis of the magnitude-frequency characteristics of sediment transport for the tributaries shows that the effective discharge in the basin ranges from 1.5-2.0 to 5.5-6.0 times the average discharge, and decreases downstream. The main Indus River, however, shows a consistent effective discharge of 2.5-3.0 times the average discharge.