Effects of a Reduced Fall Drawdown on Benthos Abundance in Lake Francis Case

Lake Francis Case, a 356-km2 Missouri River reservoir, is drawn down each fall to hold water released from upstream for winter power generation. The customary drawdown of 10-12 m in 1953-70 was reduced to 6-7 m in 1971-73. Benthos samples collected from 1966 to 1973 indicated that abundance of benthic organisms in May along two transects (to depths of 10.5 and 15 m) increased more than threefold after the drawdown was reduced; abundance in September did not increase. May increases were most evident for five burrowing forms: chironomids, Hexagenia, Caenis, oligochaetes, and ceratopogonids.