HUMAN IMPACTS ON RIVERS

Human activities have been impacting rivers around the world for several thousand years. Indirect impacts come from activities within a drainage basin that alter water and sediment yields to a river, and water quality within the river. The most widespread indirect impacts result from agriculture (grazing and cropping), deforestation, urbanization, industrialization, mining, and road building. Direct impacts arise from activities within the channel that alter the movement of water and sediment, channel geometry, or the stability of the channel bed and banks. Such in-channel activities include flow regulation, mining of placer deposits and aggregate, construction of levees, channelization, and removal of large woody debris. Recent work by Graf indicates the extent of human impacts on rivers within the United States, where more than 80,000 dams affect approximately 98% of the nation's 5.8 million km of rivers. Case studies of river metamorphosis and the impoverishment of river ecosystems come from placer-mined rivers in California's Sierra Nevada, severely polluted drainages of the Great Lakes region in Ohio and Illinois, and the incised channels of northern Mississippi.