Because large reservoirs behind major dams are areas where water flow velocity is slowed (also often called “slackwater”), sediments are deposited where rivers enter the water body (Figure 5). Sedimentation in reservoirs behind dams has several consequences. Sediment deposition reduces reservoir water storage capacity and therefore limits the useful lifetime of the dam for flood control, water supply, and hydropower generation. Recent detailed studies of storage capacity and sedimentation rates for reservoirs in the U.S. suggest that average annual storage losses range from less than 0.5% to more than 2% (see supplemental reading: Graf et al., 2010; “Sedimentation and sustainability of western American Reservoirs, Water Resources Research”). The highest rates of storage loss are occurring in the American West, and the lowest in the Northeast.
For example, almost 20 million tons of sediment are deposited annually in reservoirs along the Mississippi River (UNESCO, 2011). China’s Three Gorges Dam alone (one of several along the Yangtze River) traps 34 million tons of sediment per year, or 31% of the river’s sediment load (Hu et al., 2009). Globally, the amount of sediment trapped behind dams is estimated to be 73 km3 (Syvitsky & Kettner, 2011). This sediment accumulation slowly reduces reservoir capacity behind dams and is one factor that limits their useful life expectancy. Recent studies of sediment accumulation suggest that the life expectancy of Lake Powell is ~300-700 yr, and that of the Three Gorges Reservoir in China is ~150 yr.
The concomitant reduction in sediment delivery to downstream areas also has several consequences (Figures 5-6). Ultimately the decreased sediment supply to the river mouth translates to net erosion of beaches and loss of land in coastal regions, as natural coastal erosion by currents and subsidence caused by compaction of delta sediments is not offset by delivery of sediment. For example, prior to construction of the Aswan High Dam began in 1960, the annual sediment flux to the Nile Delta was ~100 million tons. This sediment supply was enough to offset erosion and natural subsidence.