Vegetation Response to Pool-Wide Drawdowns on the Upper Mississippi River

In an effort to enhance aquatic plant production and habitat diversity on the Upper Mississippi River (UMR), the U.S. Army Corps of Engineers St. Paul District conducted summer-time water level reductions (drawdowns) of Navigation Pools 5 (2005), 6 (2010), and 8 (2001 and 2002) of the UMR.  The water level reductions were expected to improve conditions for seed germination and growing conditions for emergent and submersed aquatic vegetation (SAV).  We assessed vegetation response to the drawdowns through (1) interpretation of high-resolution aerial photography, (2) field measures of the distribution of SAV, and (3) field measures of the composition and productivity of emergent perennial and moist soil vegetation on exposed substrates.

A wide variety (>80 taxa) of moist soil, emergent, rooted floating aquatic, shrub, and tree species colonized substrates exposed during the drawdowns.  Pool-wide drawdowns likely contributed to increases in deep and shallow marsh annual, submersed aquatic, and shallow marsh perennial plant communities.  While increases in SAV were documented, it is difficult to attribute this response solely to drawdown effects when system-wide variability in growth is considered.  Long-term persistence of emergent perennial and rooted-floating aquatic plant beds reestablished as a result of the drawdowns is also addressed.