The Observed Functionality of Dams and Reservoirs

Here we investigate the observed impact of dams and reservoirs on streamflow and water availability. We combine information from river discharge records at 4853 gage stations from the Global Runoff Discharge Centre (GRDC; 4853) database with information on 6454 dams and reservoirs from the Global Reservoir and Dam (GRanD) database. Changes in annual streamflow statistics (annual minimum, mean, and maximum flows) due to dam and/or reservoir constructions have been assessed as a function of changes in residence time of water. The analysis confirms that existing dams and reservoirs generally function as intended by altering the seasonality of streamflows (e.g., increasing annual minimum flows and/or decreasing in annual maximum flows). The GRDC stations including multiple dams and reservoirs within the drainage areas have been analyzed in terms of seven function types, and the primary function is assessed at the catchment scale. In general, dams and reservoirs add a surface water storage and thus increase the residence time of water in river systems, which has implications for the trapping of sediments in the flow. In closing, we discuss dam and reservoir effects on sediment and nutrient surface flux in a river system.