The annual cycle of fluctuation in water levels has been modified on the Upper Mississippi River, primarily by the system of dams installed by the U.S. Army Corps of Engineers in the 1930s to enhance commercial navigation. The navigation pools that resulted from the artificially maintained high water levels were initially diverse in structure and supported a rich variety of fish and wildlife. Over time, the high water levels have contributed to a degradation in the habitat quality of these pools and large expanses of open water have developed that provide little benefit to fish and wildlife resources. In an effort to enhance the production of aquatic vegetation and improve fish and wildlife habitat, the U.S. Army Corp of Engineers conducted a pilot drawdown on Navigation Pool 8 in summer 2001. The drawdown involved reducing pool water levels by 1.5 ft at Lock and Dam 8 during the growing season.

The 2001 drawdown was scheduled for mid-June, but was delayed because of spring flooding and protracted high river flows. Consequently, the pool elevation at Lock and Dam (L&D) 8 did not reach normal pool elevation until June 30 and drawdown was not effectively achieved until July 6. The drawdown was maintained near the target level at Lock and Dam 8 for 40 days, until August 14, or about 47% of the prescribed 85-day period. On July 21, 2001, during the period of maximum drawdown, a total of about 2,000 acres were exposed.

We monitored the development of vegetation on exposed substrates along 26 transects throughout Pool 8 (south of Root River). A variety of moist soil and emergent plant species important to wildlife grew on substrates exposed during the drawdown. The plants that developed at a particular site depended on a number of variables, including the available seedbank (viable seeds contained within the substrate), soil moisture, and timing and duration of substrate exposure. We found more than 50 species of moist soil, perennial emergent, and aquatic species. Growth progressed well despite the later-than-scheduled drawdown and hot, dry conditions during much of July. Plant density was related to the duration of substrate exposure, with higher plant densities and more plant development occurring on substrates exposed for a good portion of the growing season (i.e., mid-pool sites that remained exposed through mid-September) and low plant density on those substrates that were re-inundated in mid-August.

The project will be completed in September 2007.

Principal Investigator: Kevin Kenow