We studied the effect of severe fluctuations in flow on the distribution of bottom faunaof the upper Kennebec River. During the years 1964–1970, discharges below Wyman Damranged from 8.5 m3/sec to an average daily high of about 170 m3/sec. Slow currents resultingfrom low flows appeared to limit the diversity and abundance of swift-water aquatic insects on theriver-bottom below the dam. Sampling stations above the impoundment averaged 19 aquaticinsect genera, while those below the dam averaged 11. About 19 genera were found at stationswhere the current near bottom fluctuated from 0.5 m/sec to 0.9 m/sec while only 4 genera werefound at stations where the fluctuations were from 0.1 to 0.5 m/sec. Aquatic insects adapted forswift water such as Rhyacophila, Chimarra, Iron, Blepharocera, Acroneuria, and Paragnetinawere more abundant above the impoundment than below, and were absent from those stationsbelow the impoundment with the lowest current velocities.