COMPARING BATHYMETRIC MAPS FROM 2011 AND 2016 USING SIDE-SCAN SONAR AND UNMANNED AERIAL SYSTEMS AT MOUNTAIN LAKE, GILES COUNTY, VIRGINIA

Mountain Lake, in Giles County, Virginia, is one of only two naturally formed lakes in the state. Sitting at an elevation of 3,875 feet above sea level, it is a unique natural feature in the unglaciated southern Appalachians. Geophysical investigations by Radford University researchers suggest that the lake formed as a result of colluvial damming of an ancient water gap in the ridge of a breached plunging anticline some 6,000 to 7,000 years ago. Beginning in about 2002, water levels decreased significantly during fall months and recovered only partially during the summer months. In 2008, the lake went completely dry and then nearly so again in 2011.

Additional studies reveal that conduits form periodically through the colluvial dam allowing water and lake sediment to pipe into and through the landslide debris, where it then continues downstream following the natural drainage. In 2013, the owners undertook a massive earthmoving project intended to restore the lake by filling funnel-like depressions at the base of the dam, caused by the piping of lake sediment into the conduits. Using naturally available materials from the site as fill, the effort was successful and water levels rose rapidly until encountering additional side conduits at higher elevations Those now control lake levels, along with changes in water input, the result of land development within the primary channelized watershed feeding the lake.

The purpose of this study was to compare side-scan sonar imagery and bathymetric maps from 2011 and 2016, in conjunction with aerial UAV imagery from 2016, to evaluate changes in the configuration of the lake bottom between those years, and in light of the earthmoving efforts that occurred in 2013. The extent and boundaries of the earth moving are clearly visible in the UAV imagery. The sonar-generated bathymetric maps indicate that the deepest portions of the lake are now farther away from the base of the natural dam, and they are higher in elevation, than in 2011, as would be expected from the 2013 modifications.