Effects of stream regulation on density, growth, and emergence of two mayflies (Ephemeroptera: Ephemerellidae) and a caddisfly (Trichoptera: Hydropsychidae) in two Rocky Mountain rivers (U.S.A.)

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Abstract
The influence of altered temperature and food regimes on the life histories of three univoltine species of aquatic insects were compared in two regulated river systems in northwestern Montana, U.S.A. Temperatures in both regulated rivers were more moderate than below many deep-release dams as a result of the operation of a selective withdrawal system on the dam on the Kootenai River and of major input of waters from unregulated forks in the Flathead River. Population densities of these species were generally highest in the regulated Kootenai River where the greatest biomass of periphyton and seston were measured. Differences in growth rates between regulated and reference stations were observed for Drunella flavilinea and Hydropsyche oslari but not for Serratella tibialis. Temperature had little effect upon total developmental time for H. oslari but altered growth rates and time spent in late instars. Peak emergence times were 2 to 4 weeks later in regulated areas where summer water temperatures were cooler, and the duration of emergence periods was extended in regulated areas.