FLOODWATERS BEYOND FLOODPLAINS: WATER TABLE RISE AND GROUNDWATER-INDUCED FLOODING AT SPRING GREEN, WISCONSIN

In June 2008, overland flow of storm water and a rise in groundwater levels contributed to flooding 4,380 acres in the town of Spring Green, Wisconsin. The affected area, which is located over a mile from the Wisconsin River, remained flooded for five months.

Spring Green is on the higher of two outwash terraces along the north side of the Wisconsin River in southwest Sauk County. This broad, high terrace lies about 25 feet above the river. Extensive deposits of wind-blown sand create local relief of up to about 20 feet. The terrace is flanked to the north by 200-foot-high bluffs composed of sandstone and dolomite. The lower parts of secondary stream valleys that cut the bluffs are floored with fine-grained sediment deposited in late-glacial outwash-dammed lakes.

Groundwater elevations in the shallow sand and gravel aquifer on the upper terrace indicate a rapid water table rise following spring snowmelt and rainstorms. This region experienced record snowfalls the previous winter, receiving twice as much precipitation as normal between December 2007 and February 2008. The water table rose by over 5 feet following spring snowmelt. Intense rainstorms—15.2 inches in 15 days—in late May and June contributed additional recharge to the groundwater system, resulting in an additional 3.4 feet of water table rise.

We evaluated the role of groundwater in the flooding of this region using conceptual and numerical hydrogeologic models. The geologic setting results in enhanced groundwater recharge from runoff at the base of the fine-grained lake-basin sediments and along the base of the bedrock bluffs. Transient simulations of increased recharge following spring snowmelt and June rainfall indicate a 12-foot rise in water table elevation may have occurred at some locations on the upper terrace. This is consistent with monitoring well data and explains the extensive and long-lasting flooding of topographic depressions far from the floodplain of the river.

These results suggest that a shift in climate in the Midwest towards increased frequency and magnitude of precipitation may cause significant rise in water table elevations. Long-term or frequent increases in water table elevations that contribute to local flooding may require adaptations in infrastructure and land use.