In the past 25 years, Jack Beranek has made many travels to duck blinds and fishing holes in the Weaver Bottoms of the Mississippi River. Over two decades, he saw a decline in this once fertile backwater's vegetation, ducks, and fish.
But this year was different. As he piloted his boat past stands of arrowhead, rice-cut grass, and bulrush in places that had been open water for decades, more mallards, teal, and coots flushed. The reason: In the summer of 2005, the U.S. Army Corps of Engineers lowered the water in pool 5, which includes Weaver Bottoms, exposing about 1,000 acres of mud flats to air and sunlight for the first time in some 75 years. Another drawdown occurred this summer.
This new vegetation was predictable for biologists who saw similar results after lowering pool 8, near Brownsville, in 2001 and 2002. In both pools 5 and 8, the locks and dams were manipulated to mimic seasonal low water, but still allow commercial navigation. Although pool 8 has been at its usual flow for the past four years, vegetation that took root during drawdown persists; and biologists note more migrating tundra swans, ducks, and other shorebirds.
Kevin Kenow, a vegetation specialist with the U.S. Geological Survey in La Crosse, Wis., said early results from pool 5 look positive. In 2005 Kenow identified 72 plant species on exposed mud flats. He also noted more submersed vegetation such as water star-grass and Canada waterweed.
The new vegetation attracts ducks and duck hunters to Weaver Bottoms. "Migrating ducks used to race right through Weaver. But last year they seemed to stick around longer," says Beranek, co-chair of the citizens committee on pool 5. "We had better shooting than we've had in years."
Data from the U.S. Fish and Wildlife Service support his observations. The number of puddle ducks observed in October 2005 on pool 5 increased to 2,153, compared with 1,121 in 2004. Counts of diving ducks, which arrive in November, increased to 1,669, compared with 1,225 in 2004.
One drawback of the drawdown was high mortality among mussels exposed by the reduced water level. A comparison with mussels in pool 4, where water levels were not reduced, revealed that 28 percent of mussels in pool 5 experimental plots did not survive the 2005 drawdown. To increase mussel survival for the 2006 drawdown, mussels in areas known to contain rare species or high population densities were relocated to deeper water.
Tim Schlagenhaft, who coordinated the drawdown for the Minnesota DNR, says biologists remain apprehensive about the impact to mussels. A more comprehensive mussel survey will be conducted to provide a better estimate of the mussel population in pool 5. More water-level reductions will still be scheduled in the future.
"We learn a ton from these, and we will continue to look for ways to minimize the impact to mussel species," says Schlagenhaft. "Pool-wide drawdowns are an important tool in preserving the Mississippi River's habitat."
DNR staff writer