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