ELECTRICAL RESISTIVITY SURVEY OF MOUNTAIN LAKE, VA

Mountain Lake, located in Giles County in SW Virginia, is one of only two natural lakes in Virginia. It is well known as the filming location for the movie Dirty Dancing and more recently for its extraordinary fluctuations in water level. The lake emptied completely during the fall of 2008 and nearly completely during the fall of 2011. The lake is hypothesized to have formed from a combination of landslide damming of a water gap on the northwest limb of a plunging anticline at the northern end of a structurally controlled basin and from possible sinkhole subsidence. In the fall of 2011, students from Radford University had the rare opportunity to study the lake bed during that period of extremely low water levels. Reconnaissance field surveys were first performed to collect photographs and to characterize four known depressions, approximately 80 feet in diameter, acting as drain holes. Electrical resistivity studies were then carried out on the dry lake bed using both a SuperSting array and an OhmMapper resistivity meter with the goal of better understanding the relationship between the lacustrine sediments, the landslide colluvium, and the underlying bedrock. These data were expected to provide insight into the subsurface flow of water at the lake. Based on the locations and characteristics of the depressions it was hypothesized that a significant amount of water is lost through the base of the natural dam. Cross sections developed from the electrical resistivity surveys suggest a complex interaction between the sandstone colluvium and the lacustrine sediments. In addition, there are distinct areas where subsurface flow appears to be diverted around colluvial blocks. Piping holes through the lake sediments that overlie the base of the dam support the idea that water loss is due to the development and growth of conduits through the natural dam.