Geologists still plumbing the secrets of Mountain Lake and its mysterious fluctuations

Virginia, Land of Two Natural Lakes, can still claim that title, but just barely. Mountain Lake in Giles County, 50 acres when full, is down to a couple of acres. After attempts to plug holes in 2013 failed to stabilize water levels, managers of Mountain Lake Lodge turned their focus to hiking trails, the magnificent views and other fresh-air attractions of the resort at nearly 4,000 feet elevation.

Meanwhile, geologists at Radford University continue to study the lake, with the next land and air survey planned for January. Most likely the results won’t radically change their understanding of the lake’s geology, but Mountain Lake has surprised people in the past.

Skip Watts is a professor emeritus of geology at Radford University. The work of Watts and other geologists and students solved the long-standing mystery of Mountain Lake’s fluctuating levels.

According to Mountain Lake Lodge’s website, Christopher Gist of the Ohio Land Surveying Company was the first known European to lay eyes upon the lake in 1751. Early settlers “salted” their cattle in the basin of the lake, then known as Salt Pond. The mountain is still known as Salt Pond Mountain.

A pleasure resort has stood on the shore since the 1850s. William Lewis Moody of Galveston, Texas, bought the property in 1930. In 1936, he built the large sandstone lodge that stands today. After his death, the hotel was purchased by his daughter, Mary Moody Northen.

After her death in 1986, trustees of her estate established the Mary Moody Northen Endowment, which owns and operates the resort. Parts of the 1987 film “Dirty Dancing” were filmed there.

In 2008, as the water level dropped, guests discovered the remains of Samuel Ira Felder, an unfortunate angler who fell overboard in 1921 and failed to resurface.

While distressing to management, the lake’s disappearance was not unprecedented. According to geography website virginiaplaces.org, the lake has gone through cycles of filling and draining since before recorded history. In 1768, settlers found just a spring in a valley with a grassy meadow. 

Mountain Lake was once a shallow valley drained by a stream now called Pond Drain, which still flows through a gorge north of the lake. 

Some 6,000 years ago, a landslide dropped thousands of tons of rocks and soil into the gorge. The landslide may have been triggered by an earthquake; Giles County is in one of the state’s most seismically active areas.  

Native Americans were in Virginia then, although it’s unknown whether any saw the tumbling boulders and felt the earth shaking beneath their feet.

Near the lake’s north end is a natural rock city called the Garden of the Gods, composed of huge blocks deposited by the landslide.

The gorge-filling landslide created Mountain Lake. The natural dam is not a vertical wall but rather a horseshoe-shaped bank, sloping at around 30 degrees, that curves around the lake’s north end. When the lake was at full pond it served as a natural spillway. 

“One of the characteristics of a natural landslide dam is that landslides don’t make really, really good dams, compared to a manmade dam,” Watts said. “They’re basically just huge blocks of rock.  And then the blocks don’t fit together perfectly like a 3D puzzle. So you have openings between the blocks. And those openings when the landslide originally happened were all filled with smaller, finer debris. And so it starts off making a pretty good dam. And then as the thousands of years roll by, sometimes some of that fine material between the large landslide blocks is eroded out in a process called piping. The definition of piping is internal erosion. And so as the water works its way into the dam it can create a conduit that goes through the dam and takes out some of the sediment.

“It can go through cycles where sometimes the conduits are open, and the dam doesn’t hold water. Sometimes they’re closed and the dam does hold water. Sometimes these conduits can shift around. One will fill up with sediment and block the flow of water, but another one nearby might open up, so it’s kind of like a game of Whack-a-Mole.

“Through the history, the sedimentological evidence indicates that it has sometimes been full of water, and sometimes it’s been empty completely. So  it goes through long-term cycles. It may not fill back up naturally within our lifetimes, or it might. It kind of depends on whether or not those conduits get plugged back up again.

“And that could happen, if there was an earthquake in the Giles County seismic zone that shakes everything up. The material that’s all loose, that makes up this landslide dam could readjust itself, and close some of those conduits, in which case, an earthquake could cause the lake to come back. Could also be that an earthquake shaking up the dam could open up conduits, even more so than they are now.”

Watts and other academic geologists and agencies, including the United States Geological Survey and the Virginia Department of Mines, Minerals and Energy, have used a number of high-tech and low-tech methods to gather data about the lake. In 2014 Watts and colleagues from Radford used a remote-controlled submersible to shoot underwater video of water and sediment leaking through holes in the dam. Thrusters on the submersible stirred up silt and sediment, producing vivid footage of water swirling down a hole. “You can watch it just kind of go ‘pfft,'” Watts said.

Further evidence in support of the permeable dam hypothesis came from dye studies. Dye released in the lake showed up in Pond Drain on the other side of the dam.

When Heidi Stone became general manager of Mountain Lake in 2013, “really everything was about the lake, the success of the resort was based on the lake,” she said. 

Mountain Lake LLC board chairman Bob Donovan, quoted on the lake’s website, said the resort was run-down and neglected, and the endowment was considering closing the property. 

Stone, who’s now Mountain Lake LLC’s president and CEO, tried several strategies to turn it around. 

Using Watts’ data, the resort hired a contractor with earth-moving equipment to plug the holes. Stone declined to disclose the price of the geological plumbing job.

The contractors succeeded in fixing the holes. Lake levels rose, only to drop again when other holes opened up. 

Another factor is recent construction in the lake’s watershed, which had the unintended consequence of reducing runoff into the lake. Watts stressed that proper construction procedures were followed.

“We had hoped that the lake would return after all that work, but it didn’t work,” said Betty Massey, executive director of the Mary Moody Northen Endowment, which owns the lodge and the 2,600-acre Mountain Lake Conservancy. “And when that became clear, I think to Heidi’s credit, to the Mountain Lake board’s credit, to the endowment’s credit, we were honest about it. We didn’t pretend something was there, that you’re going to get up there and you’re going to be disappointed, and we kind of turned our head and said, ‘There is so much more here. Let’s talk about what we have, not what we don’t have.'”

Asked how the fluctuating lake levels have affected business at the lodge, Stone replied: “Mountain Lake Lodge is about getting outdoors and a great place to just ‘get away’ – the mystery of the lake and its fluctuating levels are no longer the focus of the lodge guest. It (the lake) has become a natural, interesting phenomenon at Mountain Lake Lodge.”

During the pandemic year of 2020, the resort enjoyed its best year ever, according to its website.

Among the attractions are an outdoor pool, archery, fire pit, target shooting, and guided hikes. The 22 miles of hiking trails are open to the public. 

Guests in January may see a quad copter and a remote-controlled fixed wing aircraft buzzing the lake. A student will be gathering imagery for compilation into various types of maps. “I’m supposing that most of the large blocks will probably still be in place,” Radford geology professor Beth McClellan said. “But some of the sediment around the edges or the smaller blocks around the edges may have shifted some.”

The Commonwealth’s other natural lake, Lake Drummond in the Dismal Swamp, also holds interest for students of nature, but the swamp’s humidity and clouds of mosquitoes make it an unappealing location for a waterfront lodge, leaving Giles County with the one natural lake in the state where the yearly cycles of the hospitality industry intersect with the eons-long processes that are the subject of geology.