SIMULATED VARIATIONS IN THE QUANTITY AND DISTRIBUTION OF SUBMERSED
AQUATIC VEGETATION ON POOL 8 OF THE UPPER MISSISSIPPI RIVER SINCE 1940.

Yao Yin1, James Rogala1, and John Sullivan2.  
1U.S. Geological Survey, Upper Midwest Environmental Science Center, La Crosse, WI 54603 
2Wisconsin Department of Natural Resources, La Crosse, WI 54601.

Submersed aquatic vegetation (SAV) prospered in Pool 8 after Lock and Dam 8 was constructed
in 1938.  Over the past 6 decades, SAV in Pool 8 experienced sometimes drastic fluctuations,
such as the crash following the drought in 1987'-89.  The fluctuations are revealed in a time
series of historical aerial photos and survey maps.  The photos and maps alone, however, did not
provide the information we were seeking - the relative importance of flow, transparency, and the
erosion of islands in the long-term dynamics of the SAV in Pool 8.  To pursue that goal, we used
a statistical model to simulate the variations of the quantity and distribution of SAV in Pool 8
under a variety of observed or hypothesized physical conditions since 1940.  

The statistical model, based on the following parameters, calculates the probability of having
SAV at a site: (1) the number of days that a sufficient amount of sunlight in the water column
was available to allow growth, (2) the number of days the site was dewatered, (3) the current
velocity on the site when river discharge was 90,000 cubic feet per second and, (4) effective wind
fetch at the site.  Sunlight penetration through the water column was calculated from light
extinction coefficient or total suspended solids, site elevation, and river stages on a daily basis. 
The number of days the site was dewatered was calculated from records of daily river stages and
site elevation.  Data of the third and fourth parameters were obtained from output files of two
models developed by other researchers.

Based on the performance of the model we put forward the following hypotheses for discussion:

Hypothesis one: The dynamics of the SAV in Pool 8 since 1988, which consisted of a crash
between 1988 and 1991 and a subsequent rebound, were driven primarily by changes in water
transparency.

Hypothesis two: Several water level management changes in pool 8 that raised the control-point
water level at Lock and Dam 8 have contributed to the loss of SAV in the deep areas behind the
dam, as well as an increase of SAV in the middle section of the pool.  As a result of these
management changes, there was a net gain in the whole.

Hypothesis three: The loss of islands in the lower section of Pool 8 from 1940 to 1989 had an
equivalent effect of approximately 15'-20% reduction of SAV habitat.  However, this reduction
had been offset by the gain from the raised control-point water level at Lock and Dam 8
(Hypothesis two).  As a result, there was a decline of approximately 5% of SAV habitat since the
mid 1970s.

Keywords: modeling, submersed aquatic vegetation, dynamics, Upper Mississippi River