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
1U.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.  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 actual drawdown was delayed from a target date of 15 June due to spring 
flooding and protracted high river flows.  Consequently, the pool elevation at 
Lock and Dam 8 did not reach normal pool elevation until 30 June and the 
drawdown was not effectively completed (1.5' reduction at L&D 8) until 6 July.  
The drawdown was maintained near the target level at L&D 8 for 40 days, until 14 
August, or about 47% of the prescribed 85-day period.  The drawdown effectively 
persisted throughout the mid portion of the pool through 15 September. The 
extent of substrate exposed on 21 July was determined to be 1,954 acres (8.2% of 
the 23,721 acres that were evaluated).

We detected a reduction in the distribution and biomass of SAV during 2000 and 
2001 relative to 1999, and expect that drawdown-related effects on SAV will not 
be measurable until 1-2 years post-drawdown.  We identified 43 taxa of moist 
soil (26), emergent (6), rooted floating aquatic (2), submersed aquatic (1), 
non-rooted aquatic (4), and tree species (4) growing on substrates exposed 
during the drawdown.  Rice cut grass (Leersia orizoides), broadleaf arrowhead 
(Sagittaria latifolia), water stargrass (Heteranthera dubia), nodding smartweed 
(Polygonum lapathifolium), chufa flatsedge (Cyperus esculentus), false pimpernel 
(Lindernia dubia), and teal love grass (Eragrostis hypnoides) were the dominant 
species that developed on exposed substrates.  Growth progressed well, despite 
the later-than-scheduled water level reduction 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.

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