multipurpose reservoirs. Until recently, dams were viewed as good, clean ways to generate hydroelectric power (Abelson, 1985; Bourassa, 1985), control floods, and provide storage for water supplies. Hence, their environmental impacts were not understood, evaluated, or monitored. Recently, society has become more aware of these impacts and the economic value of resources, such as fisheries, that have been damaged by dam construction.

Limnologists, along with hydrologists and fisheries biologists, have been involved in documenting how dam building affects river ecosystems in several ways (National Research Council, 1987):

• Temperature alterations: Because the water passing through reservoirs often originates from points near the middle or bottom of the water column, it may be much colder during the summer months than natural flows would have been. Sustained low temperatures during the warm months may support cold-water fisheries for species such as trout in streams and rivers that otherwise would not provide appropriate temperature regimes for these fish. At the same time, however, temperature alteration may suppress important native fish (Minckley, 1991) and other aquatic animals (Ward and Stanford, 1979).

• Changes in dissolved oxygen, nutrient, and suspended solids concentrations: Dams may affect the amounts of oxygen, suspended solids, and nutrients in water flowing downstream (Gordon et al., 1992). The concentrations of dissolved oxygen in the lower water column of reservoirs may be low or zero in some instances, hampering the development of fisheries or altering the native fauna below dams (Petts, 1984). Further, water released from dams is likely to have a lower sediment content than water entering a reservoir (Andrews, 1991), causing substantial biotic changes such as enhanced growth of algae (Blinn and Cole, 1991) as well as physical changes in the downstream sediment balance (Simons, 1979).

• Hydraulic modifications: Dams may stabilize the natural variation in the flow of rivers, alter seasonal extremes, or induce entirely new patterns. In addition, hydropower production facilities associated with some dams may establish a regular daily pulse in stream discharge and mean depth. These hydraulic peculiarities in turn can have significant biological effects. For example, decreased variability in streamflow below dams may cause habitat losses for fish and other aquatic organisms (Kellerhals and Church, 1989). In addition, wetland areas can suffer massive losses of important habitats (Baumann et al., 1984). The Atchafalaya Delta of the Mississippi River and the Peace-Athabasca Delta in northern Alberta are important examples. Aquatic scientists have estimated that the latter delta, which supports many unique species of wildlife and several hundred indigenous people, will disappear in fewer than 50 years unless Bennett Dam is decommissioned (see Box 3-1).