Historical Analysis of Lake Surface Area Change and Implications for Holocene Lake Stratigraphy
For this study, we chose a northern transect of the State of Minnesota. In order to compare lakes with similar hydrologies, the study was focused on small (< 1km2), closed basin lakes, within 5 km of the boundaries of watersheds. Historic photographs, from the late 1930s, were obtained at the John R. Borchet Map Library at the University of Minnesota. We chose to study the late 1930s because this period represents the end of a decadal scale drought. Recent digitally orthorectified aerial photographs (2003) were obtained from the U.S. Department of Agriculture Farm Services Agency. We recognized 197 lakes on the 75 photographs collected. The late 1930s photographs were digitized, registered to the modern orthophotos, and lake shorelines were traced using ESRI ArcMap. We calculated the percent change in lake surface area from the historic photographs to the recent ones. We found an increasing trend in percent surface area change from east to west. From the aerial photography study, 16 lakes were chosen for field work with the Ground Penetrating Radar (GPR). In addition of covering the entire east-west transect, the selection criteria included road accessibility. The analysis of the GPR profiles indicated the evolution of basin form, from a complex, multibasinal morphology, to a less complex form. The interpretation of the GPR data revealed an increasing frequency of erosional events towards the west. However, this trend it was not as pronounced as expected. The erosional surfaces were most likely caused by fluctuations in lake levels. The preservation of lake stratigraphy appears to increase eastward, with thicker units between unconformities, consistent with trends in the aerial photo analysis.