100 YEARS OF CHANGE IN ISLAND MORPHOLOGY AND BACKWATER
BATHYMETRY IN POOLS 6, 7, 8, 9 & 10, MISSISSIPPI RIVER.

Anne J. Jefferson[VG2C1]1                            1 and Carol A. Jefferson2.  
1 Department of Civil Engineering, University of Minnesota, Minneapolis, MN 55455 
2Department of Biology, Winona State University, Winona, MN 55987.

Islands and backwaters in the lower reaches of navigation pools in the Upper Mississippi River
have changed substantially since the 1890's and the installation of locks and dams in the 1930's. 
The Mississippi River Commission maps of the 1890's and the USDA Flowage maps of the
1930's contain depth soundings for the navigation channels, side channels and sloughs, as well as
the elevation of emergent portions of the floodplain.  During 1996 through 1999, we duplicated
numerous historical depth sounding transects and analyzed maps dating from the 1870's and
aerial photography dating from 1926.
In the lower portions of Pools 6 and 10, islands are enlarging, new islands are appearing, and
islands are migrating downstream. Morphology of islands along the main channel approximates
the 1870's configuration, while backwater islands and sloughs are reestablishing configurations
that existed in the 1920's. Island area in lower Pool 6 increased 36% between 1962 and 1994,
with a gain of 27 ha. A new island that emerged enlarged 2.2 ha from1993-1998. Since 1993,
several other islands have formed and are expanding rapidly.  In Pool 6, the river bed rose 3-6 m
since the 1890's. In Pool 10, 2-3 m of sediment cover land inundated by impoundment, and
former sloughs have filled 6-7 m.  Side channels passing over spillways rose 3 m in Pool 6 and
downcut 4.5 meters in Pool 10.  
In lower Pools 8 and 9, islands have eroded and backwater sloughs have filled, resulting in a
nearly level river bed. In lower Pool 8, islands have lost about 1 meter in height, and slough beds
have risen 0.5-4.0 m.  In Pool 9, the highest islands of 1894 eroded 1-3 m, and the low relief
islands elevated 1 m. Backwater sloughs filled 1-3 m.  
Lower Pool 7 presents a different picture. Erosion and deposition in Lake Onalaska have
produced a mostly level bed, with the higher parts of islands losing 0.5 meter and the lower parts
rising 2 meters. French Slough, which flows over the spillway, has deepened 4 meters. Along the
main channel of lower Pool 7, islands are shrinking.  The shape of an island, or island group, is
related to its stability. Horseshoe-shaped islands are more stable than linear islands. Where
islands are emerging, horseshoe island configurations are forming.  Side channels that pass over
spillways and have closing dams at their heads are filling. These same channels are downstream
of a major bend in the navigation channel.  The probable reason for leveling of the river bed is
trapping of sediment by wing, closing, and navigation dams. The lower reaches of pools may be
attaining a new equilibrium, but the eventual results are unclear.  The Mississippi may become
confined to the navigation channel, and be surrounded by a low relief floodplain in former
backwater areas. The leveling of the river bed is a loss of environmental heterogeneity, with
implications for biotic diversity.

Keywords: islands, backwaters, Mississippi River, sedimentation, erosion
[VG2C1]