RATES OF BACKWATER SEDIMENTATION BETWEEN 1997 AND 2002 IN POOLS 4, 8, AND 13 OF 
THE UPPER MISSIPPIPPI RIVER

James T. Rogala and Peter J. Boma.  U.S. Geological Survey, Upper Midwest 
Environmental Sciences Center, La Crosse, WI 54603.

Net rates of sediment accumulation were measured annually between 1997 and 2002 
as changes in bed elevation along randomly selected backwater transects.  
Weighted mean accumulation rates (centimeters per year) in aquatic portions of 
the transects were -0.01, 0.22, and 0.76 for Pools 4, 8, and 13, respectively.  
Near-shore, terrestrial portions of the transects accumulated sediment at rates 
of 0.59, 0.51, and 1.09 for Pools 4, 8, and 13, respectively.  Rates differed 
for the 4-year period prior to the severe flood in 2001.  Rates of accumulation 
were higher in the aquatic portions and lower in the terrestrial portions of the 
transects during 1997-2000.  Regardless of the time period, the rates observed 
in the aquatic portions of the transects were much lower than previously 
reported for backwaters of the Upper Mississippi River.  The lower rates perhaps 
can be explained by the high variability along and among transects observed, 
which can provide highly biased results depending on the sampling design.  It is 
also likely that accumulation rates have decreased as trapping efficiency in the 
backwaters has diminished.  Although prediction of future backwater 
configuration in the Upper Mississippi River System is difficult, the 
documentation of the rates over this 5-year period, and the better understanding 
of causal mechanisms, can be used to better predict future conditions.

Keywords:  sedimentation, erosion, Mississippi River, backwaters, sediment 
transport