UNDERWATER REMOTELY OPERATED VEHICLE INVESTIGATIONS OF A LEAKY LANDSLIDE DAM, MOUNTAIN LAKE, GILES COUNTY, VIRGINIA

Seismic refraction and electrical resistivity studies suggest that Mountain Lake, in Giles County, Virginia, formed as a result of landslides that blocked a gorge approximately 6,000 years ago. The buried gorge appears to have developed in alignment with a tear fault in the Narrows thrust sheet and shows up clearly in seismic cross sections. A dye study revealed a possible hydrologic connection to an area downstream along the fault and the ancient gorge. Sediment analyses indicate that the lake has drained and refilled numerous times over its history.

The lake went completely dry in September of 2008, during which time four funnel-shaped depressions became visible in the lake sediment at the base of the landslide dam. Water and sediment continued to flow in streams across the lake bed disappearing into the depressions where they were apparently piped through the landslide. The largest of the funnel depressions was about 20 feet deep and 90 feet across.

In 2013, the owners undertook a massive earthmoving project intending to restore the lake by filling the depressions with colluvium. Leaks through the base of the landslide were stopped and water initially rose rapidly. Since that time, SCUBA divers, side scanning sonar, and underwater remotely operated vehicle (ROV) investigations have failed to find signs of what were once major leaks.

Nevertheless, the lake has not returned to full pond and now appears unable to rise higher than 3830 feet above mean sea level. That is approximately 50 feet below full pond and approximately 35 feet above the deepest part of the newly shaped lake bed.

Suspecting the presence of additional conduits in the side slopes of the landslide dam, new investigations were undertaken using the ROV, a SCUBA diver, and a drone for tracking silt plumes. A boulder/cobble field was discovered through which water and silt are disappearing into the colluvium above one of the most troublesome of the former depressions. It is believed that voids between boulders and cobbles are sometimes clogged while at other times they open and connect with other voids creating a network of connections through the landslide.

Video ROV imagery of the leaky area will be presented and options for reducing water loss will be discussed. Also introduced will be changes to the watershed that may have unintentionally reduced water flow into the lake.