Just because dissolved and particulate materials enter a river does not mean that they reach the ocean; modern reservoirs have had a tremendous impact on the hydrologic cycle. Starting about 50 years ago, large dams were seen as a solution to water resource issues, including flood control, hydroelectric power generation, and irrigation. Now, there are more than 40,000 large dams worldwide (World Commission on Dams 2000). This has resulted in a substantial distortion of freshwater runoff from the continents, raising the "age" of discharge through channels from a mean between 16—26 days and nearly 60 days (Vorosmarty et al. 1997). Whereas erosion has clearly increased the mobilization of sediment off the land, the proliferation of dams has acted to retain those sediments. Vorosmarty et al. (2003) estimates that the aggregate impact of all registered impoundments is on the order of 4—5 Pg y-1 of suspended sediments (of the 15— 20 Pg y-1 total that he references). Stallard (1998) extrapolates from a more detailed analysis of the coterminous United States to an estimate of about 10 Pg y-1 worldwide (versus 13 Pg y-1 efflux to the oceans), for a storage of about 0.2 PgC y-1 (which he includes as part of his overall calculation of continental sedimentation).
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