VEGETATION RESPONSE TO A DEMONSTRATION DRAWDOWN ON POOL 8 OF THE UPPER 
MISSISSIPPI RIVER.

Kevin P. Kenow, James E. Lyon, Randy K. Hines, and Larry R. Robinson
U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, 
WI 54603. 

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 a pilot water level reduction on Navigation Pool 8 of the UMR 
during summer 2001 and again in 2002.  The water level reduction was expected to 
dry and consolidate bottom sediments and, thereby, increase the area of emergent 
and submersed aquatic vegetation by natural seed germination.  We assessed 
vegetation response to the water level reduction during the drawdown through (1) 
use of high-resolution aerial photography and land cover data generated from 
that photography, (2) field measures of the distribution and biomass of 
submersed aquatic vegetation (SAV), and (3) field measures of the composition 
and productivity of moist soil and emergent perennial vegetation on exposed 
substrates.  The drawdown and/or spring flooding in 2001 likely contributed to 
an increase in deep marsh annual, shallow marsh perennial, wet meadow, sand bar, 
submersed aquatic vegetation, wet meadow shrub, shallow marsh annual, and mud 
communities in Pool 8.  Plant communities that developed on exposed substrates 
in response to the Pool 8 drawdown in 2001 were dominated by arrowhead 
(Sagittaria spp.), false pimpernel (Lindernia dubia), water stargrass 
(Zosterella dubia), teal love grass (Eragrosits hypnoides), rice cutgrass 
(Leersia oryzoides), and flatsedge (Cyperus spp.).  We observed a shift from a 
plant community dominated by annuals to one dominated by perennials in 2002. 
Arrowhead tuber production increased 16-fold (( = 3.4 g/m2 in 2001 vs. 55.3 g/m2 
in 2002) across transects we examined during the two years.

  In general, submersed aquatic vegetation did not appear to be negatively 
effected by the drawdown.  Submersed aquatic vegetation standing crop biomass 
was significantly lower in 2000 and 2001 (( <20 g/m2) from 1999 levels (35 g/m2) 
and rebounded to 32 g/m2 in 2002.  In the near future, we will determine the 
persistence of emergent perennial plant beds that were reestablished as a result 
of the drawdown and continue to assess the distribution and abundance of 
submersed aquatic vegetation. 




Keywords:  drawdown, moist soil, Navigation Pool 8, vegetation response, water 
level management