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positions in the boundary layer on the surface of rocks as their qills form a suction cup which assists in maintaining their positions on rock surfaces. Caddisflies (T~Qhopt~a) often show compositional changes in regulated areas (Henricson and Muller 1979). The species present are often determined by the composition and particle sizes of the available food in the seston. The distribution of filter feeding caddisflies is often determined by the prevalence of food particle sizes. Various species spin nets of different mesh sizes and thus utilize only a specific range ~f particle size (Wallace and Merritt 1980). Seston from Lake Kooca~~sa is abundant in the Kootenai River and is utilized by certain filter feeding caddisflies and influences their abundance (Hydnop~YQhe o~l~, B~aQhYQen~~ ~p.) (Table 9). Caddisflies which spin nets with larger mesh sizes (A~cto- p~YQhe g~an~) and smaller mesh sizes (Cheumatop~YQhe ~p.) than the medium sized mesh of Hydnop~YQhe nets are more abundant in the Fisher River (Table 10). Periphyton biomass is high in the Kootenai River due to reduced turbidity and scouring. Caddisflies which graze on the peri- phyton (Hydnoptiia ~p.) are found in higher densities in the Kootenai River than in the Fisher River (Table 10). Allochthonous material (leaves and woody material from the terrestrial sphere) are much more available in the Fisher River. Caddisflies which shred leaves (Lepido~toma ~p.) are found in much higher densities in the Fisher River (Table 10). Blackflies (Diptera) are also filter feeders which have much higher densities in the Kootenai River than in the Fisher River (Table 9). Midges, worms and snails are more abundant in the Kootenai River. A species list (Table 11) was compiled from all of the insects collected during the study. The Odonata, Hemiptera, Chironomidae and some other groups of Diptera were identified only to family. Adult collections were likely incomplete for the Fisher River, so that the species list based on our samples is incomplete. Some of these species may have been incidental in the Kootenai River, arriving there in the drift from tributary streams. The relative abundance of species is indicated: rare ~ 1 or 2 specimens collected during the entire study; infrequent = less than 10jm2jyear; common = >10 but <1,OOOjm2jyear; abundnt = >1,OOpm2jyear. A total of 53 species were collected at the Dunn Creek site, 73 at Elkhorn, 89 at Pipe Creek and 105 in the Fisher River. A number of species were collected only in the Fisher River, while others only in the Kootenai River (Table 12). Many were collected rarely or only as adults, so may also occur in the other river. It is probable that a number of the stonefly species found in the Fisher River no longer occur in the Kootenai River. It is also possible that caddis species like B~aQhYQentn~ am~Qan~ have been eliminated in the Kootenai River, and that many of the leaf shredding species of caddis do not occur within the study area of the Kootenai River. Certain dipteran species were collected only in basket or drift samples in the Kootenai River, collection methods which were not used in the Fisher River. Some species differences would be expected based on the difference in the size of the river where -26-