WATER LEVEL DRAWDOWN: IMPACTS ON SEDIMENT CHARACTERISTICS AND POREWATER NITROGEN 
IN MISSISSIPPI RIVER POOL 8.

J. C. Cavanaugh, D. A. Bruesewitz, L. A. Bartsch, E. A. Strauss, W. B. 
Richardson, D. M. Soballe, A. M.  Mahan.  US Geological Survey, Upper Midwest 
Environmental Sciences Center, 2630 Fanta Reed Road, La Crosse, WI, 54603.

Pool 8 of the Mississippi River was drawn down in July of 2001 to stimulate 
aquatic plant growth and compact sediments.  Sediment and sediment porewater 
characteristics (i.e. sediment moisture content, bulk density, ammonia, and 
ammonium) may be affected by water level fluctuation and duration of dry 
periods.  We examined these variables at 25 backwater sites before, during, and 
after the water level drawdown to observe changes resulting from desiccation and 
re-wetting of sediments.  Sediment moisture content, bulk density, ammonium, 
exchangeable ammonium, and ammonia did not vary among the sites prior to the 
drawdown (p>0.1).  Some of the sites dried during the drawdown, resulting in 
lower sediment moisture content and greater sediment bulk density.  There was 
also a decrease in ammonium, exchangeable ammonium, and ammonia in sediment 
porewater at the dry sites.  This could be due to a decrease in mineralization 
of organic nitrogen and assimilation, while nitrification rates remain constant.  
When sites were re-wetted, these variables again do not vary among sites 
(p>0.05), and appear to be returning to pre-drawdown conditions.  These results 
suggest that desiccation of sediments has an immediate impact on sediment and 
porewater nitrogen, with rapid return to pre-dried conditions.