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32 RESTORATION OF AQUATIC ECOSYSTEMS pairs their capacity to control flooding, and constitutes a severe eco- nomic loss. Siltation also remains a serious problem in the United States; 1.7 billion tons of topsoil are lost to erosion every year (U.S. Department of Agriculture, 1982). Pollution abatement alone will not return many lakes and reser- voirs to their former condition because nutrients and toxic materials are recycled from lake sediments. These processes maintain eutro- phic conditions or continue to contaminate food webs and associated fisheries, even though loading has been reduced or eliminated. Inva- sions and planned introductions of nonnative species have become serious problems, impairing fisheries or recreational use (see Chapter 4 for further details). The extent of lake damage in the United States is substantial. A recent survey by the- U.S. Environmental Protection Agency (1990) indicates that about 2.6 million acres of lakes are impaired (relative to suitability for intended uses), and this most likely is a significant underestimate of the acreage that is ecologically degraded and po- tentially restorable. By far the most common source of stress leading to impairment is agricultural activity (almost 60 percent of impaired acreage is attributed to this source); nutrient and organic enrichment and siltation problems are the most common causes of impairment. It must be noted, however, that survey information regarding some problems such as exotic species and toxic metals is grossly inad- equate. These lakes and reservoirs, and others like them, require active restoration and subsequent protection and management, in part because sites for new reservoirs are rare or absent in most areas of the United States (Brown and Wolfe, 1984). Acidification of lakes by acid rain is widespread in the northeastern United States and Canada, and in Norway, Sweden, and the United Kingdom (NAPAP, 1990}. Acidified lakes will recover only slowly after cessation of sulfur deposition and may require significant restorative efforts (Schindler, 1988; Schindler et al., 1989). Conditions of Rivers and Strums Streams and rivers perform numerous ecological and economic func- tions. They are conveyances; diluents; sources of power generation; sources of potable water, water for industrial uses, and water for irrigation; and recreation sites. Unfortunately, multiple problems af- flict many U.S. rivers today. Our rivers have been diverted, dammed for navigation and hydropower (FERC, 1988; Benke, 1990), channelized, polluted, their wetlands removed, their basins silted in from soil and bank erosion, and their sediments contaminated with toxins. In