EROSION RATES ON INDIAN MOUNDS IN THE UPPER MIDWEST

Barring complications such as excavation or reconstruction, simple measurements on Indian mounds provide reasonable estimates of historic erosion rates. Surveys from the late nineteenth century, document dimensions that can be combined with modern observations to estimate rates of erosion. This study is based on more than 50 published data pairs from mound groups in Iowa, Minnesota, and South Dakota; about half are from the Blood Run National Historic Landmark in northwestern Iowa.

The calculated erosion rates average .022 ft/yr (6.71 mm/yr) and range from .001 ft/yr (.305 mm/yr) to .081 ft/yr (24.7 mm/yr). These values are very similar to those observed in eroding agricultural fields and are larger than most estimates for rates of geologic erosion. As might be expected, erosion rates on the mounds are directly proportional to the initial measured height. Cross plots of erosion rate and initial height also show discrete clusters for cultures from different geographic areas and times of occupation. Within the Blood Run site, distinctive patterns can be mapped using erosion rates and mound volumes.

Estimates of erosion rates and summaries of geometry provide a useful tool for non-invasive characterization of Indian mounds.