The City of La Crosse, Wisconsin, a city of 51,000 with a metropolitan and county population of about 107,000 (US Department of Commerce 2000), is located on the bank of the Mississippi River across from the Minnesota-Wisconsin border (Figure 1). In the vicinity of La Crosse (longitude 91[degrees]12' W and latitude 43[degrees]50' N), the Mississippi River flows through a postglacial alluvial floodplain, 2-10 km wide, which is bordered on both sides of the valley by a series of steep-sided Cambrian Lower Magnesian limestone/dolomite bluffs, 70-200 m high (Martin 1965, 171). With such steep bluffs, a gorge-like appearance and relatively narrow floodplains, this section of the valley is considered a part of the Upper Mississippi Basin. Most of the urban development in the La Crosse area has been confined between the bluffs and the left bank of the Mississippi, a distance of 2-4 km with an average slope of about 2-3 m/km. Most of the residential districts in the city have been built in flood-free areas on higher ground closer to the bluffs. Yet, there are at least three flood zones in La Crosse that experience flooding from the Mississippi (Figure 1).
First, in the northern section of the city there are major flood-prone areas on French Island and the township of Campbell. French Island is a large channel bar of the Mississippi with several low-lying places along its edges. Second, the most extensive flood-prone zones are located in the middle section of the city between the La Crosse River and the Black River, the two most important tributaries of the Mississippi in the region. In the lower section of the La Crosse River, a major structural depression is occupied by an extensive floodplain and backswamp. During high-magnitude floods, the marsh acts as a flood detention basin, although several low-lying areas adjacent to the marsh experience flooding. Third, additional flood-prone zones are located in the southern section of the city in a narrow strip of land along the river bank.