CHANGES IN SEDIMENT PENETRATION DUE TO A DRAWDOWN AT LIZZY PAULS
POND ON THE UPPER MISSISSIPPI RIVER.  

Greg J. Egan and Joseph H. Wlosinski.   U.S. Geological Survey,
Upper Midwest Environmental Sciences Center, 2630 Fanta Reed
Road, La Crosse, WI 54603.  

Water levels were lowered at Lizzy Pauls Pond during the summer
of 1997.  One of the objectives of the project was to dry and
consolidate bottom sediments.  The U.S. Army Corps of Engineers
performed the drawdown as part of a Habitat Rehabilitation and
Enhancement Project for the Upper Mississippi River.  Lizzy Pauls
Pond is in Pool 5, cover 21 hectares, and is connected to the
River by a culvert.  Sediments were measured with an in situ
penetrometer in the summer of 1996 (pre-project) and again in the
spring of 1998 (post-project).  A total of 131 sample sites were
established on a 40 X 40-m grid.  The pond was divided by 40 m
transects both north/south and east/west with a sampling site
being established at each intersection.  Sediment penetration was
measured in triplicate for each of three penetrometer cones of
different weights and diameters.  An adjoining backwater area was
also sampled as a control.  It had 30 sample sites on a similar
grid.  The drawdown commenced on June 24, 1997, and continued
until mid-October.  The maximum drawdown amount was 0.42 m.  We
performed four paired t tests for each cone at each site sampled. 
The four tests were a combination of drawdown or control and
areas sampled in less than or greater than 0.42 m.  None of the
four test showed conclusive results.  Either the difference for
all three pins had different signs or the tests were not
significant.  We also performed a regression analysis on the
difference in penetrometer readings for the 131 sites between the
two years using depth as the independent variable.  That analysis
showed a significant trend (à = 0.05), with penetration being
greater with an increase in depth.  However, the regression for
each cone had an R2 value below 0.24.  Also, two transects across
Lizzy Pauls Pond, that measured the elevation of the
sediment/water interface in 1996 and 1998, showed little
change.  Results may have been inconclusive for a number of
reasons.  The drawdown was not as great as originally planned. 
Based on sampling depths, we estimated that only 15 of the 131
sample sites were dried for any portion of the drawdown period. 
The summer of 1997 was also wetter than normal.  A total of 62 cm
of rainfall fell on 51 days during the drawdown, causing some
dewatered areas to again be flooded.  A number of springs also
fed the pond.  In addition, all sites were vegetated which may
also have had an effect on penetrometer results.  

Keywords:  drawdown, penetrometer, sediment, Upper Mississippi
River