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ABSTRACT Impoundment of the Kootenai River in 1972 by Libby Dam altered the aquatic environment in the river downstream from the dam. Flow regimes, temperature patterns, sediment loads and water quality were markedly changed, resulting in changes in periphyton, aquatic insect, and fish populations. Periphyton biomass and productivity increased. Insect densities near the dam increased, but species diversity decreased. Insect diversity increased with increasing distance downstream from the dam, but species diversity was lower than would be expected in a free-flowing river. Biomass of aquatic insects was highest near the dam, but was not significantly different from the Fisher River at two downstream stations. Extensive recolonization of shoreline areas occurred above the 4,000 cfs level when discharge from Libby Dam was maintained at a higher level for two weeks or more. Reduction in the discharge following recolonization resulted in the stranding of large numbers of aquatic insects. Considerable overlap occurs between the food habits of rainbow trout and mountain whitefish. Chironomidae were the most important food item of all sizes of mountain whitefish and for rainbow trout less than 20 centimeters long. Rainbow trout more than 20 cm long fed on Chironomidae, Trichoptera and Ephemeroptera. Water released from the dam sluices and spillways caused gas super- saturation and appeared to limit fish populations in the Kootenai River from 1972 until 1975. From 1975-1981, water was released primarily through the penstocks and rainbow trout and mountain whitefish populations increased over 300 percent. The increased densities were associated with a marked decl ine in ra inbow trout growth rates and a sl ight decl ine in mounta in whitefish growth. Significant spawning runs of rainbow trout have developed in four tributaries above Kootenai Falls and mainstem spawning activity was noted in 1981 and 1982. Mountain whitefish spawn primarily in the mainstem Kootenai, the Fisher River and Libby Creek. The lack of suitable spawning habitat and barrier problems in tributary streams downstream from Kootenai Falls are limiting trout populations. Burbot populations have increased since impoundment, whereas white sturgeon numbers have declined. The rainbow trout fishery in the Kootenai River is comparable to some of Montana's more famous blue ribbon streams. Fishing pressure has increased markedly from 1968 to 1981. The current catch rate during the summer of 0.6 fish per hour of effort and average size of rainbow trout creeled (11.0 inches) make the Kootenai one of the better wild trout fisheries in western Montana. The interaction of many environmental components have produced a favorable environment for rainbow trout downstream from Libby Dam. The single most important factor in maintaining the productivity of the Kootenai River has been the establishment of an adequate minimum flow. Other important environmental components include: 1) improved water temperatures for i i