Rivers carry silt and other particulate matter in suspension. Normally, this material is deposited in natural basins, such as lakes, river deltas, and continental shelves. However, dams intercept these sediments, which then accumulate at the bottom of the man-made lakes. Sediment accumulation lowers the storage capacity of reservoirs, which in turn decreases the volume of water that otherwise would have been withheld from sea level rise. Although sedimentation rates vary widely, the overall effect is comparatively small. The extent of sediment settling depends on the sediment load of the river, the size and settling velocity of the sediments, and the trapping efficiency, which in turn is related to the reservoir capacity and annual volume of inflow.
Around 0.2% of U.S. reservoir storage capacity is lost annually to sedimentation (Gleick, 1992; Dendy et al., 1973). Lake Mead, for example, lost 0.3%/yr of capacity to siltation in the 1940s (Smith et al., 1960). Table 5.6 summarizes data on siltation rates from many reservoirs around the world. If the global mean annual loss of reservoir capacity due to siltation is no more than 1%, global reservoir storage would decrease by 45.1 to 53.3 km3, or 1.13-1.33 km3/yr since the 1950s, which corresponds to a SLR of 0.0030.004 mm/yr. Dam construction may have decreased the sediment load of rivers delivered to the world's oceans by 885 million metric tons/yr (based on Milliman and Syvitski, 1992). Most of this sediment has been trapped behind dams. Given a mean rock density of 2.7 g/cm3, this sediment diminishes the reservoir capacity by 0.33 km3/yr, which is negligible in terms of its effect on SLR (0.001 mm/yr).
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