AQUATIC VEGETATION IN THE UPPER MISSISSIPPI AND ILLINOIS RIVERS:
CHANGES FROM 1991 TO 1998 IN THIRTY-TWO BACKWATER AREAS.  

Yao Yin 1, Heidi Langrehr2, and Sara Rogers3.   1 University of
Tennessee, Knoxville, TN 37996; 2 Wisconsin Department of Natural
Resources, Onalaska, WI 54650; 3 U.S. Geological Survey, Upper
Midwest Environmental Science Center, La Crosse, WI 54603.  

Aquatic vegetation data, including submersed and floating-leaved
species collected under the Long Term Resource Monitoring Program
of the Upper Mississippi River System, were analyzed.  The data
were collected from 32 backwater areas in five separate stretches
named after the U.S. Army Corp of Engineers navigation pools,
Pools 4, 8 and 13 on the Mississippi River and La Grange and
Alton pools on the Illinois River.  Aquatic vegetation was
surveyed twice a year in May-June and July-August, respectively,
from 1991 to 1998.  During each survey the occurrence of aquatic
species was recorded at individual sites at 15- or 30-m intervals
along transects in the 32 backwater areas.  Most of the transects
were established in 1991 and a few were added in later years. 
Transects in each backwater area are usually parallel lines of
equal distance (50 to 200 m) apart.  Our objective was to
identify important changes, especially the trend(s) of the
changes.  We have identified several distinct trends.  In upper
Pool 4 (above Lake Pepin), aquatic vegetation has been declining
since 1991.  In lower Pool 4 (below Lake Pepin), aquatic
vegetation was declining from 1991 to 1995, and the trend of
declining was broken in 1997 and 1998.  In Pools 8 and 13,
aquatic vegetation has been stable or increasing since 1994.  As
of 1998, aquatic vegetation in Pools 8 and 13 are at or near
their best conditions recorded since 1991.  In La Grange and
Alton Pools, aquatic vegetation was rare in contiguous backwater
areas as a rule; and the detrimental effects of rare floods were
obvious.  

Keywords:  aquatic vegetation, trend, Mississippi River, Illinois
River, LTRMP