Enhancement of benthic macroinvertebrates by minimum flow from a hydroelectric dam

The biological consequences of instituting a summer minimum flow from the Conowingo Hydroelectric Dam were investigated by comparing invertebrate abundance in the Susquehanna River during 1980, when flow shutdowns were frequent, to that in 1982, when the minimum flow (142 m3 s−1) was maintained. The minimum flow was only maintained from April 15 to September 15, and additional investigations compared invertebrate abundance before and after the minimum flow was terminated in the fall of 1982 and 1983. There was a large difference in summer invertebrate density in the between-year comparison, with almost a 100-fold increase in the year when the minimum flow was maintained. Most of this difference was exhibited by two taxa, chironomids and net-spinning trichopterans. These results were consistent with findings from the fall of 1982 and 1983, when cessation of the minimum flow in September led to declines of more than three orders of magnitude in both of these taxa. The rate of decline was faster in the shoal habitat (areas dewatered at low flow, but submerged when the minimum flow is released), suggesting that dewatering was the predominant mechanism of effect. Declines were also evident in the channel habitat (always submerged), suggesting that maintenance of current velocity is also an important beneficial component of the minimum flow.