Predevelopment freshwater heads in the glaciofluvial, Saginaw, and Marshall aquifers in the Michigan Basin

Maps showing predevelopment freshwater heads were developed for the major aquifers in the Michigan Basin. The Michigan Basin is an isolated, regional ground-water-flow system consisting of a glaciofluvial aquifer and three bedrock aquifers separated by confining units. The bedrock aquifers investigated are the Saginaw, Parma-Bayport, and Marshall aquifers. Fractured sandstones within the bedrock aquifers comprise the principal water-bearing zones. Withdrawals from supply wells in the glaciofluvial aquifer, and more so from the Saginaw and Marshall aquifers, have lowered the potentiometric surface in several areas. The Marshall aquifer is a highly developed natural resource. Locally, extensive withdrawals of brine and natural gas from the Marshall aquifer have lowered the potentiometric surface to below pre-1890's predevelopment levels.

The maps of freshwater heads were prepared from hydraulic-head measurements made from the late 1800's through 1976. Lake and river stages also were used to prepare a water-table map.