The Nations aquatic resources are among its most valuable assets. Although environmental protection programs in the United States have successfully improved water quality during the past 25 years, many challenges remain. Significant strides have been made in reducing the effects of discrete pollutant sources, such as factories and sewage treatment plants (called point sources). But aquatic ecosystems remain impaired, mostly because of complex problems caused by polluted runoff, known as nonpoint source pollution. Every 2 years the U.S. Environmental Protection Agency (EPA) reports to Congress on the status of the Nations waters. The 1998 National Water Quality Inventory (USEPA, 2000) reports that the most significant source of water quality impairment to rivers and streams and lakes, ponds, and reservoirs is agriculture, and the most significant source of impairment to estuaries is municipal point sources of pollution. Other important sources of impairment or alterations that can impair water quality include hydrologic modifications like dams and channelization (a leading cause of impairment to rivers and streams and lakes, ponds, and reservoirs), urban runoff and storm sewer discharges (leading sources of impairment to all surface waters), and pollutants deposited from the atmosphere (a leading source of impairment to estuaries). The five leading pollutants impairing the Nations waters are siltation, nutrients (from fertilizers and animal waste), bacteria, toxic metals, and organic enrichment that lowers dissolved oxygen (USEPA, 2000). Siltation is the leading cause of water quality impairment to rivers and streams and the third leading cause of impairment to lakes, ponds, and reservoirs. Nine states list silviculture as a leading source of impairment to rivers and streams.

"April 2005." "EPA-841-B-05-001"--Spine. Print format not distributed to depository libraries. Includes bibliographical references (pages R-1-R-26).