A 3-YEAR SUSPENDED SEDIMENT BUDGET FOR TWO NAVIGATION POOLS OF THE
UPPER MISSISSIPPI RIVER SYSTEM.

Robert F. Gaugush.  
U.S. Geological Survey, Upper Midwest Environmental Sciences Center, La Crosse, WI 54603.

The suspended sediment budget for Pool 13 of the Mississippi River and the La Grange Pool of
the Illinois River has been examined over a three year period (October 1995 through September
1997).  Pool 13 output between 3.07 - 3.64 million metric tons of suspended sediment annually
during the study period.  Loads to Pool 13 were dominated by those delivered by the Mississippi
River (76%, 74%, and 66% in the 1995, 1996, and 1997 water years, respectively).  Pool 13
exhibited sediment export (output > input) in 1995, a balance in 1996, and marked sediment
trapping (output < input) in 1997.  Within the three study years, Pool 13 received it's highest
water and sediment loads during the spring (March through May) but this period was marked by
sediment export.  Suspended sediment loads were trapped during the remainder of the year.  The
La Grange Pool output between 3.71 - 5.04 million metric tons of suspended sediment annually
during the 1995-1997 water years.  The Illinois River accounted for less than 50% of the
suspended sediment load to the pool (19%, 25%, and 45% in the 1995, 1996, and 1997 water
years, respectively.  The La Grange Pool exhibited considerable sediment trapping in 1995 while
the 1996 and 1997 water years were much closer to a balance where output was within 5% of
input.  Generally, the La Grange Pool received it's largest water and sediment loads during the
late spring - early summer period (April through June), but in 1997 the late winter loads
predominated.  In all years, the La Grange Pool trapped the sediments during these high
discharge, high loading periods and exported suspended sediments for the remainder of the year. 
The distinct differences in the manner in which these pools process suspended sediment loads
may be related to the considerable differences between the morphometry of these pools.

Keywords:  sediment budget, sediment load, Mississippi River, Illinois River