the need for significant infrastructure and careful operation, hydroelectric power generation is generally compatible with two other watershed benefits—provision of water supplies and flood protection. But hydroelectric power is not always compatible with other watershed objectives. For instance, depending on the design and operation of the facilities, a dam often creates a stretch of river below it with little or no water present until the water is returned from the bypass structures, thus eliminating fish habitat and recreation. Dams also create barriers to the upstream migration of species such as salmon, and fish ladders and other methods for moving populations upstream are less than ideal. Upstream, dams create flatwater reservoirs where moving water species once lived. Downstream, fish populations can change, and native species can be threatened with extinction, because the water released from dams may be much colder or different in other ways from pre-dam conditions.
Dam operation also can have environmental effects. Many dams operate to produce what is called ''peaking power," that is, they produce power only when it is most needed—a practice that causes rapid and sometimes major fluctuations in flows. For instance, the Colorado River below Glen Canyon Dam has seen river level fluctuations up to 13 feet per day in the Grand Canyon, leading to accelerated erosion of beaches and loss of critical habitat along the river corridor. Beach replacement is slow because the sediment that once muddied the river is trapped above the dam; native species like the humpback chub are endangered because of the many changes to the environment (NRC, 1996a). As a result of careful study and societal pressure, in 1996 Glen Canyon Dam operators released an experimental high flow designed to rectify some of these problems, and early indications are that future operation of the dam will be altered to better balance the benefits add costs of the dam.
Scientific studies of terrestrial and aquatic ecosystems during the last two decades have drastically altered our understanding of their composition, structure, function, and complexity (Franklin, 1992), and increased our awareness of the value of biodiversity in ecosystems. As explained by the noted scientist E.O. Wilson (Wilson, 1988):
The diversity of life forms, so numerous that we have yet to identify most of them, is the greatest wonder of this planet. . .. Biological diversity must be treated more seriously as a global resource, to be indexed, used, and above all, preserved. Three circumstances conspire to give this matter an unprecedented urgency. First, exploding human populations are degrading the environment at an accelerating rate, especially in tropical countries. Second, science is discovering new uses for biological diversity in ways that can relieve both human suffering and environmental destruction. Third, much of the diversity is being irreversibly lost through extinction caused by the destruction of natural habitats. . .