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9.38 Flow Regulation by Dams

2013; Elsevier BV; Linguagem: Inglês

10.1016/b978-0-12-374739-6.00265-7

Francis J. Magilligan, Keith H. Nislow, Carl E. Renshaw,

Hydrology and Watershed Management Studies

Flow regulation associated with dams has led to profound changes to hydrologic, ecologic, and geomorphic systems with significant upstream and downstream effects. Although numerous metrics have been developed to represent the magnitude and direction of hydrologic alteration, these various measures paint a common picture of major changes to hydrologic regimes following large-scale global development of dams over the twentieth century. In general, the dominant hydrologic response has seen significant reductions in both flow magnitude and frequency as well as extensive changes in the timing and duration of flows. These hydrologic changes have led to a range of geomorphic responses. Major channel incision and bed armoring commonly occur directly downstream of dams due to sustained sediment-free releases. Dam distal effects are complex and difficult to predict because fundamental pre-dam data are often lacking as well as the inherent complexity of fluvial systems. However, recent research underscores the importance of considering tributary impacts, differential responses of gravel bed and sand bed systems, and having an appropriate timescale to better predict longitudinal impacts. Because of the pervasive sediment trapping by reservoirs, the global sediment delivery to oceans has been reduced by ~26%, commonly leading to coastal erosion and beach loss. Current work suggests that in gravel bed rivers, channel responses to flow regulation and sediment trapping will require upward of a century to establish new equilibrium profiles and channel widths, with the geomorphic effects of the dam attenuating progressively downstream as drainage area increases. These new metrics capturing the geomorphic effects of impoundment may prove to be useful analogs in predicting the geomorphic responses to dam removal.