HISTORICAL CHANGES IN UPPER MISSISSIPPI RIVER WATER AREAS AND ISLANDS

Periodic surveys of the upper Mississippi River since 1866, and a discharge record of nearly equal length, provided an opportunity to learn more about the magnitudes and rates of geomorphic processes at work in large stream systems. Furthermore, since considerable land use changes and small-scale climate fluctuations in the watershed marked the period of record, geomorphic and hydrologic adjustments could be evaluated in relation to environmental changes. This study quantitatively compares historical changes in mapped land and water phenomena in the upper Mississippi River Pool 10, located along southwest Wisconsin’s border, using GIS. Modest channel widening and decreases in island area throughout the study reach are detectable since the last century. Flood magnitudes and frequencies have also varied during this time, and since the 1940s stages and low flow discharges have increased. The latter hydrologic change appears to be closely associated with the reach’s geomorphic adjustments. Results are representative of a valley reach where a large sand bedload is contributed. In other settings, the rates and magnitudes of historical evolution of island, floodplain, and river morphology have probably differed.