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Human activities impact an estimated 98% of rivers in the United States. This chapter summarizes impacts associated with pioneer societies, commercial activities, and public works. In pioneer societies, individuals or small groups undertake activities such as timber harvest, agriculture, or navigation improvements. Nineteenth-century placer mining of gold along rivers of California's Sierra Nevada is used as a case study. Commercial activities are conducted by groups of people seeking profit through industry, commerce, or agriculture. Commercial activities impacted rivers much more extensively than most activities of pioneer societies. Impacts to water quality, and particularly the U.S. Geological Survey National Water Quality Assessment (NAWQA) program, provide a case study. Results from the 1991–1995 NAWQA program indicate that approximately half the sites sampled in urban areas have surface-water contamination exceeding levels at which adverse biological effects can occur in aquatic biota. Public works such as dam and levee construction undertaken by local and federal governmental agencies caused massive alteration of river systems. Channelization is used as a case study of the impacts of public works on rivers; more than 56,000 km of waterways were channelized by the Corps of Engineers and the Soil Conservation Service after 1940.

The net effect of human activities in the United States has been to disconnect rivers from adjacent hillslopes, floodplains, and valley bottoms, underlying hyporheic and groundwater zones, and from headwaters and downstream processes. Because a connected river is a functioning ecosystem, rather than simply a canal for moving water and sediment, disconnection simplifies and impoverishes the ecosystem. This has resulted in widespread loss of biological diversity in rivers of the United States.

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