Sinkhole Distribution in Winona County, Minnesota

Winona County, located in southeastern Minnesota, is part of a karst region in the upper Mississippi Valley. The karst is developing in flat-lying dolomitic Ordovician rocks. As part of a Minnesota Geological Survey county atlas program, we have systematically field located sinkholes, and prepared a 1 to 100,000 scale map showing sinkhole locations and sinkhole probability. We located 535 sinkholes in Winona County (~1600 [sq-km]). Most of these relatively small, geomorphically young sinkholes are not included in the USGS 7.5 minute topographic maps and cannot be readily detected on air photos. The sinkhole density, while low compared to many karst regions was much greater than local, regional, and state land use planners anticipated. New bedrock, surficial and hydrogeology maps of Winona County were used for correlation with the geographic distribution of the sinkholes. The primary control on the distribution of sinkholes appears to be the bedrock stratigraphy. The secondary controls, not necessarily in order of importance include slope of the land surface, and composition of surficial materials. The depth to the water table does not appear to have an important [effect] on sinkhole development. Age data indicate that the rate of sinkhole formation has [significantly] increased in recent years.This study was funded by Winona County, the Minnesota Geological Survey, and a grant from the Legislative Commission on Minnesota Resources