Control structures may affect fish populations

The Middle Mississippi River, the stretch of river located between St. Louis and Cairo, Ill., has two general types of side channels -- open and closed.

Open side channels have both ends connecting with the main river channel, while closed side channels have only one end connecting with the main river channel. Closed side channels can function as both types, depending on river elevation.

Historically, side channels in this area were created and maintained by large-scale floods that removed colonized plant assemblages. The removal of this vegetation kept side channels accessible. Side channels served as corridors to backwater habitats, and both side channels and backwater habitats were used by various fish and other aquatic organisms for feeding, reproducing and rearing young.

Today, manmade control structures, such as wing dikes and closing structures, direct water away from side channels and into the main river channel. These diversions do not allow for the maintenance or creation of side channels and other low-velocity backwater habitats.

As a result, side channels are being isolated from the main river channel. This transforms what was once a backwater habitat into a terrestrial habitat. In turn this may cause loss of habitat to fishes and other aquatic organisms.

Loss of backwater areas may affect reproduction and recruitment of threatened and/or endangered aquatic organisms, such as the Ohio shrimp, pallid sturgeon, paddlefish and sturgeon chub. Common fish requiring low-velocity waters for spawning, such as sunfish, may decline within the Middle Missouri River because of reduced availability of these habitats.

Valerie A. Barko is a systems ecologist with the Missouri Department of Conservation.