Human Alterations to Natural Flows

Human activities and uses of water have substantially altered river flow regimes in the U.S. Today, less than two percent of U.S. rivers remain relatively free-flowing and undeveloped. One of the more obvious forms of flow alteration results from the construction of dams and reservoirs. The operation of dams for flood control, water supply, barge navigation, hydroelectric power generation, or recreation affects the timing and amount of water releases from a dam. More than 5,500 large dams and 100,000 small dams have been built in the U.S.

Less obvious is the degree to which land uses for various human activities, such as agriculture, timber harvest, urbanization, or grazing have affected aspects of natural flow regimes in rivers that drain human-altered landscapes. Hydrologic systems can be quite sensitive to changes in soil infiltration capacity and evapotranspiration rates, which explains why some intensive land uses can change flood peaks and low flow conditions by orders of magnitude.