WATER LOSS AT MOUNTAIN LAKE, GILES COUNTY, VIRGINIA: LIKELY EFFECTS OF A LEAKY COLLUVIAL DAM IN CONJUNCTION WITH DRAINAGE BASIN MODIFICATIONS ON THE WATER BUDGET

Mountain Lake, in Giles County, Virginia was the principal filming location for the 1987 movie Dirty Dancing at a time when the lake was full. Starting in about 2002, water levels decreased significantly during the fall months and recovered only partially during the summer months. In 2008, the lake went completely dry and then nearly so again in 2011. Mountain Lake is one of only two naturally formed lakes in Virginia. At an elevation of 3,875 feet above sea level, it is a truly unique feature in the Valley and Ridge Province within the unglaciated southern Appalachians. Recent geophysical studies confirm that the lake owes its existence, at least in part, to colluvial damming of an ancient water gap in the breached limb of a dissected plunging anticline approximately 6,000 years ago.

It seems that major conduits form periodically within the colluvial dam allowing water and lake sediment to pipe through the debris until such time as the conduits become sufficiently clogged to again hold back nearly 100 feet of water. The colluvial deposits are likely never completely free of leaks, however it does appear that they have varied in severity somewhat over the thousands of years. In 2013, the owners undertook a massive earthmoving project intended to restore the lake by filling depressions at the base of the dam, caused by the piping of lake sediment, with naturally available materials from the site. The effort was successful and water levels rose rapidly until encountering additional side conduits at higher elevations that now appear to control lake levels.

Observations indicate that the leaks overall are greatly reduced and that precipitation is nearly normal for this region, raising the question of whether changes within the watershed may also play a role by decreasing the inflow side of the water budget equation. In 2002, a part of the drainage basin was modified by the development of new cottages, parking lots, and storm water retention basins. Runoff modeling using the rational method reveals that annual surface flow to the lake has decreased from that area. Groundwater modeling reveals that infiltration beneath these stormwater retention basins lies outside of the groundwater divide for the system that provides base flow recharge to the lake, hence surface water captured by the retention basins appears permanently lost to the lake.