HYDROLOGIC CONNECTIVITY AND NITROGEN DYNAMICS IN BACKWATERS OF
THE UPPER MISSISSIPPI RIVER.

Dennis M. Wasley1,2 and David M. Soballe1  
1U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, WI 54603 
2River Studies Center, University of Wisconsin-La Crosse, La Crosse, WI 54601.

We found that nitrogen dynamics in off-channel areas of the Upper Mississippi River (UMR)
system are influenced by the degree of hydrologic connection to the main channel.  Our earlier
mass-balance calculations revealed that net longitudinal transport of total nitrogen in the UMR
was approximately conservative across several reaches; even though we found strong evidence of
nitrogen transformation in backwater areas.  Subsequent investigations show that areas relatively
isolated from the main flow of the river often have low nitrate concentrations (< 0.01mg/L) from
June through November, while nitrate in the main channel ranges from 1 to 4 mg/L. We found
that fluctuations in river stage (< 1m) deliver main channel water (and nitrate) into semi-isolated
backwaters (e.g., Lawrence Lake near La Crosse, Wisconsin), and the nitrate in this inflow is
rapidly removed by assimilation or denitrification within the backwater.  Backwaters that are
well connected to the channel exhibit seasonal nitrate concentrations that closely follow those in
the main channel.  We speculate that increased delivery of main-channel water to off-channel
areas could result in additional removal of nitrate from the UMR; however, the overall effect on
downstream transport of nitrogen would be small due to overwhelming inputs from the
surrounding drainage basin. 

Keywords:  nitrogen, Mississippi River, backwater, transport, connectivity