Effects of Dams on Rivers

Dams and reservoirs significantly affect rivers in the northern third of the world, and hundreds of new dams are proposed elsewhere. Dams fragment the channel network and block the natural flux of sediment and organic matter. Reservoir operations typically seek to transform the natural flow regime to provide flood control, a relatively constant water supply, or to allow production of hydroelectricity, thereby reducing the natural diversity of flow regimes. Most large reservoirs thermally stratify in summer and release relatively cool water in summer. The changes in stream flow and sediment supply cause downstream changes in sediment mass balance and may lead to bed incision and disconnection of the post-dam flow regime from adjacent floodplains. Change in thermal regime allows proliferation of non-native species often introduced into reservoirs and tailwaters. Flood control typically leads to an expansion of riparian vegetation.