Backwater Revival

When an experiment mimics natural low-water conditions on the tamed Mississippi, new life springs up in the backwaters.

By Jason Abraham

Blue-winged teal rose from new stands of arrowhead. Sandpipers and avocets winged across emerging bulrush. Tim Schlagenhaft liked what he saw on the Mississippi River south of La Crescent late last summer.

The Department of Natural Resources Mississippi River and blufflands coordinator was checking the progress of an experiment that uses the Mississippi River's lock and dam system to mimic seasonal low-water conditions that occurred naturally before the river was impounded.

Construction of locks and dams in the 1930s stabilized water levels for commercial shipping and transformed the river into a series of navigable pools. Seasonally flooded backwaters where river bulrush, soft-stem bulrush, arrowhead, and other aquatic vegetation once thrived became permanently inundated. Without annual drying of the plant beds, the vegetation began to disappear. And, over time, so did the fish and wildlife that relied on the plants to provide food, shelter, and dissolved oxygen.

Several years ago Schlagenhaft and his counterparts from the Wisconsin DNR, U.S. Geological Survey, U.S. Fish and Wildlife Service, and U.S. Army Corps of Engineers began looking at temporarily reducing water levels in pools along the Mississippi River. They wanted to replicate periods of natural drying to rejuvenate native vegetation and boost fish and wildlife habitat, without disrupting commercial navigation. After years of planning and a series of public meetings, the group - known as the Water Level Management Task Force - chose navigation pool 8 (between La Crescent and Genoa, Wis.) for the experiment.

For 45 days in 2001 and 90 days in 2002, the corps lowered the water in pool 8 by allowing increased flow through the dam gates at the downstream end. This action exposed about 2,000 acres of river bottom to air and sunlight for the first time in decades.

Although analysis is still in progress, the experiment is showing promise. Arrowhead, rice cutgrass, nutgrass, bulrush, and cattail sprouted from seeds deposited by the river in exposed areas. These native plants slow erosion and furnish high-quality food for wildlife such as tundra swans, which favor arrowhead tubers.

"We're not going to be able to turn back time to pre-lock-and-dam days," Schlagenhaft said. "But I think it is possible to greatly improve vegetation growth and habitat by using water-level management."

With some dredging of the channel in 2001, barges continued navigating with little difficulty despite low water, said Kent Pehler, vice president of Brennan Marine, a barge shipping and fleeting company. No additional dredging was required during the 2002 drawdown.

"Some pools can be managed at lower levels without significant negative impact on navigation," Pehler said. "But each pool has to be evaluated on an individual basis. In this case everyone came to the table, and it was very successful."

The corps also dredged some areas away from the main navigation channel to ensure access to marinas and boat ramps during low water. The corps and riverside communities worked out a cost-sharing agreement for the additional dredging.

Because water levels were lowered gradually, fish, turtles, and other aquatic animals were able to move to deeper water without being trapped. In 2001 volunteers helped relocate mussels in danger of being left dry. The effort was called off during the 2002 drawdown because volunteers found that few mussels were in danger of drying.

This summer (acting on a plan approved by natural resources agencies and the transportation industry, and through public comment) the corps will maintain pool 8 at its usual high level. By keeping this level for several summers, researchers hope to learn how often the water must be drawn down to sustain emergent vegetation.

USGS research wildlife biologist Kevin Kenow said the plants that sprouted during low water could persist for up to five years. "Water-level reductions for wildlife management have been used for years on small lakes and ponds," he said. "In those cases we generally see benefits that last for five to seven years. It's difficult to say what will happen with water-level reduction of this size on a river."

In the meantime, the task force is exploring options for periodic drawdowns to revitalize other Mississippi pools and backwaters.

Jason Abraham is a DNR staff writer and a contributing editor of the Volunteer.