Mountain Lake

Mountain Lake is a freshwater lake located in Giles County near Newport, Virginia, in the United States. Along with Lake Drummond in the Great Dismal Swamp, it is one of only two natural lakes in Virginia. It was originally known as Salt Pond, as it was where cattle were given salt.

Mountain Lake typically covers about 50 acres, and its level was largely constant at an elevation of 3,875 feet during the 19th and 20th centuries. Since 2002 it has been subject to dry-season level drops of as much as 15 feet. The last such level fluctuations occurred between 1751 and 1804 as historical accounts give widely different accounts of the lake size.

Natural lakes are common well to the north, where the Appalachian Range was subject to geologically recent glacial activity. But the basis on which this lake—the only natural one in the southern Appalachians—exists has been the source of much speculation. Recent scientific studies indicate that an unusual combination of natural processes created the lake, which is maintained by a fissure at the bottom that provides an outlet for both sediment and water and prevents the lake from otherwise simply becoming a bog. Replenishment of the water lost depends on rain levels, and apparent washing-out of sediment from the fissured bedrock bottom is causing the unstable lake levels.
The lake is estimated to be about 6,000 years old, and geologists believe it must have been formed by rock slides and damming. Cold underground springs feed the lake and rarely allow the temperature to rise above 70 F (22 C) on the surface and 46 F (5 C) 50 feet (15 m) below the surface. Because of narrow channels and openings in the lake bottom, the level has a history of changing dramatically depending on the water flow through these channels. It is more than 100 feet (30 m) deep when full. Close to the lake is one of Virginia's few virgin forests, including a rare virgin spruce bog—Mann's Bog—with an unusual array of northern disjunct species. The lake drains into Little Stoney Creek, which passes over a spectacular waterfall -- "The Cascades"—before reaching the New River.
During a severe cold wave in January 1985, Mountain Lake set the statewide record low for Virginia: -30 F (-35 C).