Monday, August 11, 2014

The Mystery of Mountain Lake in Virginia

In last Sunday’s Washington Post was a Travel section article about the Mountain Lake Lodge and how that vacation Lodge is faring since the Mountain Lake levels have fallen, went dry in 2008, and recovered moderately. The Travel article was written by Becky Krystal. Mountain Lake is really an amazing and mysterious geological and hydraulic wonder. It is one of only two natural lakes in Virginia and the only natural lake in the Southern Appalachians where there were no glaciers in the last ice age. Mountain Lake is in southwest Virginia only about 15 miles from Blacksburg, the home of Virginia Tech.
From Radford University

The first records of Mountain Lake were from a British surveyor named Christopher Gist who in 1751 surveyed the area and noted a lake that was ¾ of a mile long and a quarter of a mile wide. In 1768 settlers to the area found only a spring and a grassy meadow. The settlers called the area Salt Pond Mountain and questioned the accuracy and ability of the surveyor, Mr. Gist. Recent analysis of sediments by scientists and pulled together by in the PhD dissertation of Jon C. Cawley in 1999 found that Mountain Lake had either been at very low levels or completely dried up at least six times since the lake was formed about 6,000 years ago.

In 1932 G. E. Hutchinson and Grace Pickford were the first to study the lake. They suggested that Mountain Lake was formed from a landslide damming of the valley. Other methods of formation have been suggested over the past century, but all were reviewed and rejected by research done in 1975 by Dr. Parker, who returned to a variation of the landslide hypothesis. This hypothesis is reasonable, but does not fully explain Mountain Lake’s existence, or why there is no residual of the canyon that is thought to have been the source of the landslide. Current thinking is that a seismic or storm event caused the landslide.

Salt Pond Drain, a small stream that flows to the northwest provides drainage from the lake. The stream is very small where it presently leaves the lake. Less than half of the 600-700 gallons per minute that drains from the lake can be accounted for by flow from Salt Pond Drain. Mountain Lake leaks. There are a series of natural holes in the bottom of Mountain Lake. When silt, grave and organic material carried into the holes, they serve to plug the holes, rainfall that averages 55 inches a year exceeds the loss of water through evaporation and reduced leakage and the lake level rises. Once the lake reaches the level of the outlet to Pond Drain, any excess inflow from rain and springs will just flow over the top and down into the New River. In drought years, the level of the lake typically falls, but drought alone does not explain the lake's dramatic and intermittent shrinkage.

In the mid and late 1950s the lake was low enough that the lake-bottom springs were exposed and the lake was a fraction of its “full” size. The lake returned to normal full levels after a local earthquake that registered as 6 on the Richter scale in April of 1959. Mountain Lake is located in a seismically active area of Virginia and the underground loss of water may be a contributing factor to seismic events. After 1959 the Lake remained relatively full dropping only seasonally at an unusually rapid rate during dry portions of the late summer. In 1997 and 1998, largely due to drought conditions throughout the summer, Mountain Lake was nearly 10 feet below the rim. Such a drop was more than three times the water loss expected from evapo-transpiration alone and would be the warning that the lake water loss had increased.

Mountain Lake formed over a fault line and though silt and debris is continually washing into the lake, occasional earthquakes and the scrubbing action of the water continually erode away the silt and debris plug. When the cracks at the bottom are opened wider through earthquake and or scrubbing, drainage through the cracks and holes exceeds the inflow, and the surface level of Mountain Lake drops. Occasionally, the lake dries up completely. This happened in the late 1700’s and in 2008 when the half full lake drained completely in a period of about 6 months.

Southwest Virginia had experienced several years of drought by 2002, when water levels at Mountain Lake had fallen 15 feet from the top and the surface area of the lake decreased from 50 acres in 1997 to just 25 acres. By 2003, the lake was full again. In July of 2008, water levels were 51' feet below full depth of over 100 feet. Even though the area received more rain than normal that year, by October of 2008, the lake was almost completely gone and was reportedly completely dry for several days. Despite heavy rains that restored part of the lake the rate of water loss was too high to refill the lake the drainage had somehow increased. The lake recovered somewhat in the next year.
image from Radford University

In 2011 researchers from Virginia Tech identified four piping holes in the deeper end of what remained of the lake. Scientists and the Mary Moody Northen Endowment that has owned Mountain Lake, the surrounding 2,600 acres and the Mountain Lake Lodge since 1986 decided to investigate where the water was going. One pound of fluorescein dye was placed into each of the four piping holes and then the scientists searched over the course of a year for the dye to appear. By April, 2013, there was still no sign of the dye in the various nearby streams, though there was some trace of the dye in Pond Drain. The dye trace experiment did not determine where Mountain Lake's "leak" is going. The dye may still be coursing through the underground, who knows.

Even if they could not identify conclusively where the water was going, maybe they could “plug the holes.” Radford University engineering geologist Skip Watts, believed that there was a good chance of slowing the leaks enough to allow the lake to refill itself. So with a grant from the Mary Moody Northen Endowment, Dr. Watts and his team filled in the four pipe holes, first with chunks of larger rock, and then with gravel, sand and finally fine clay. All the materials were excavated from the lake bottom to mimic the natural processes. While Dr. Watts was very confident” the lake would rise to full within a year or two, so far there has been only a partial recovery of the lake. However, sooner or later it is presumed that a minor earthquake will restore the lake (I hope). Nonetheless, Mountain Lake is a geological and hydraulic wonder located a short drive from the Jefferson National Forest that is part of the 1.8 million acres that comprise the George Washington and Jefferson National Forest in Virginia, West Virginia and Kentucky. Staying at the Mountain Lake Lodge supports the Mountain Lake Conservancy which acts as a steward for the Lake and surrounding environment.
image from Radford University

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