places such as the Grand Canyon, dams have prevented or slowed sediment transport downstream, causing erosion of beaches in the canyon (NRC, 1987). The combination of dams on the upper Mississippi River and levees along the lower Mississippi has reduced replenishment of the Mississippi delta by sedimentation during the annual floods and thereby contributed to the problem of land subsidence, shoreline erosion, and loss of coastal marshes (Keown et al., 1981; Penland, 1982; Penland and Boyd, 1985). More than half of the nation's rivers have fish communities adversely affected by turbidity, high temperature, toxins, and low levels of dissolved oxygen. Almost 40 percent of perennial streams in the United States are affected by low flows, and 41 percent by siltation, bank erosion, and channelization (Council on Environmental Quality, 1989).
The problems affecting aquatic resources cannot be solved without examining the deleterious land management practices that contribute to those problems. For example, failure to control wind and water erosion and destruction of forested riparian areas has produced heavy silt loads. Increased sediment delivery resulting from forestry practices has also increased sedimentation and turbidity in downstream channels, lakes, and reservoirs, with attendant loss of capacity for water storage and conveyance, recreational and aesthetic values, and quantity and quality of habitat for fish and wildlife. Low or nonexistent dry season flows are one result, leading to water shortages, elimination of river biota, and the increased potential for flash floods. Annual sediment loads in major rivers range from 111 million to 1.6 trillion metric tons, three-fourths of which is deposited in riverbeds, on floodplains, or in reservoirs. One of the major items in the budget of the U.S. Army Corps of Engineers is the cost of dredging, particularly of the lower Mississippi River (Brown and Wolfe, 1984).
Although there have been measurable improvements in stream quality over the last 20 years in the United States, these are associated primarily with improvements in municipal wastewater discharges (Smith et al., 1987). River sediments remain contaminated with toxic substances in many areas, flash floods are common and occasionally lethal, costs to treat water prior to its use have increased, and streambeds remain covered with silt. Vast stretches of rivers and streams have been channelized, a practice that destroys wetlands; increases sediment, nutrient loss, and bank erosion; and often eliminates streamside vegetation that is essential to maintain cool stream temperatures and to stabilize banks. Thousands of miles of rivers and streams are affected by acid mine drainage. Eight percent of the samples of 59,000 stream segments (21,000 km) examined in the National Surface Water Inventory between 1984 and 1986 were