Movement and Storage of Sediment in River Systems

Abstract

River sediments play two major roles in geochemical cycles. First, the river sediment fluxes represent about three-quarters of the total denudation of the continents under present-day conditions. Second, river sediments adsorb and transport a number of aqueous ionic constituents. Two recent compilations of the available data have estimated that the total discharge of river sediment to the coastal zones of the world under present-day conditions is 15 to 16 × 1012 kg/yr (Milliman and Meade, 1983; Walling and Webb, 1983). This number, however, cannot be taken at face value to compute a world-wide mean rate of long-term mechanical denudation. In the first place many of the measurements on which the estimate is based are probably inaccurate. Among the large rivers of the world, accurate records that span several decades are available only in China, Europe, and North America. Second, the present-day sediment loads of many of the world’s major rivers have been increased or decreased markedly by human activities, such as deforestation, crop farming, surface mining, and construction of dams and reservoirs. Third, the direct linkage between upland soil-erosion rates and downstream river-sediment discharges is so tenuous in large river systems that the common practice of dividing a river’s sediment load by its drainage area to obtain a mean rate of erosion or denudation may yield a number that has little significance. Relations between soil erosion and river-sediment discharge change so markedly in time and space, and the time lags between upland causes and downriver effects are so large relative to the rates of these changes, that any assumptions of equilibrium, or even steady state, must be carefully considered in the context of whatever time scale is relevant (Schumm, 1977; Tricart, 1962; Trimble, 1975, 1977).