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RESTORATION OF AQUATIC ECOSYSTEMS commodated by channelization, resulting in further loss of stream habitat. This has led to aquatic organisms becoming extinct or im- periled in increasing numbers and to the impairment of many benefi- cial water uses, including drinking, swimming, and fishing. Although public and private decisions to manage aquatic ecosys- tems have enhanced water transportation, developed sources of hy- droelectric power, reduced flood hazards, and provided water for municipal, industrial, and agricultural purposes, these activities have also altered the physical, chemical, and biological processes within aquatic ecosystems. This committee is convinced that U.S. public opinion strongly supports an increased level of attention to environ- mental protection. The nation's investment in different types of en- vironmental programs has been considerable but piecemeal and has not always been effective. An accelerated effort toward environmen- tal restoration and preservation is needed. The committee believes that a comprehensive and aggressive restoration component should be the centerpiece of such an effort. The premise of this report is that ecological restoration of aquatic ecosystems is possible. Restoration means returning an ecosystem to a close approximation of its condition prior to disturbance. Accom- plishing restoration means ensuring that ecosystem structure and function are recreated or repaired, and that natural dynamic ecosystem pro- cesses are operating effectively again. At times, however, restoration may be impractical or undesirable, as when a body of water that is naturally without fish is successfully transformed through stocking into a valuable trout fishery or when important urban developments have been situated on wetlands. In such cases, the committee recog- nizes that the economic value of these developments may preclude any attempt to restore preexisting natural systems at these locations. The committee also recognizes that preventive measures to protect aquatic ecosystems are important and that priority should be given to preventive measures that benefit more than one portion of the hydrologic cycle. Had environmental protection been adequate in the past, many expensive restoration projects would not be necessary today. Naturally, restoration of aquatic ecosystems may be accomplished in stages, and particular ecosystem functions and characteristics- such as potable water-may be restored even when other ecosystem characteristics deviate from natural conditions. Thus, in certain situ- ations, partial ecological restoration may be the operant management goal and may provide significant ecological benefits even though full restoration is not attained. Therefore, since the loss and impairment of aquatic ecosystems is