Limnology of the Middle Mississippi River. III. Mayfly Populations in Relation to Navigation Water-Level Control

Naiad populations of the mayfly Hexagenia rigida were studied in an isolated channel of the Mississippi River. Egg-laying by adult females was concentrated at the upper end of the channel. Numbers of naiads were greates at the upstream end of the channel during most of each summer. The naiads moved downstream prior to emergence. Following a winter in which the water level was drawn down about 4 ft for 6 weeks, the numbers of naiads were reduced at all stations. Populations near shore were reduced more than those in deeper water.