Why Does the 'Dirty Dancing' Lake Keep Draining?

Mountain Lake is one of only two natural lakes in the state of Virginia and the only natural lake in the southern Appalachian Mountains. It's unlike any other lake we know about because it empties and refills almost completely every few hundred years. Even during the intervening periods, the lake experiences large fluctuations in water volume.

While Watts and I are sitting in the lodge, he tells me a legend about when the water level got so low during the first half of the 1900s, hotel workers put mattresses over the holes at the bottom to stop the water from draining.

Because that is another thing that makes Mountain Lake special: It has small holes on the bottom — lots of holes — that continually drain the water out of the lake and, according to a dye test Watts's team did, deposit the water around a mile away. These holes have been there since the lake's formation at least 6,000 years ago, according to the sediment record.

For most of the 20th century, the lake looked like a lake. But beginning in 2002, the amount of water became visibly lower.

"Water levels always go up and down," says Watts. "But the lows were getting lower and lower for several years." In 2005, the numbers jumped back up thanks to a wet year but by 2008, the lake was nearly completely empty. It remained mostly empty from 2008 to 2012.

When Jon Cawley did his doctoral thesis on Mountain Lake in 1999, he warned that Mountain Lake, which had had just four completely empty periods in the past 6,000 years according to his research, was due for another low time. Just three years later, he was proven right.

"So very seldom in geology can you make a prediction like that and then have it happen, not only during your lifetime, but really soon after you make the prediction," says Cawley, a biologist and geologist who has studied the lake for more 20 years. He is currently a research geologist at the Smithsonian Institution.

Scientists flocked to the lake, trying to figure out what was happening. The managers of Mountain Lake Lodge scrambled; their hotel's namesake having been reduced to a mud puddle.

Of course the holes were part of it; the drains had become easily visible with nearly no water in the lake. But the holes had always been there. People wondered why the lake had emptied out so completely this time and if there was anything that could be done about it.

According to Watts, the timing of the lake's emptying can be explained by its water budget. He says to think about the water budget like a bank account. "You want your bank account to get bigger and bigger," he says. "You have your losses, the outflow of the lake in this case, but you also have an inflow. So as long as your inflow is bigger than your outflow, it's going to get bigger until it's at full pond." But in 2008, the outflow was clearly outpacing the inflow.

There are a few reasons for this. One is that between 1997 and 2002, there was a drought in the area, which produced around 30 percent less average rainfall than the period between 1982 and 1996. This drought was likely caused at least in part by climate change, according to both Cawley and Watts.

Back when he was doing his work on Mountain Lake, Cawley would say that "Mountain Lake is the canary in the coal mine in this area of Appalachia. You will see climate change there before you see it in a lot of other places" because the Appalachian Mountains have one of the most diverse populations of plant and animal species per-square-mile than any other place in the world.

At the same time, Mountain Lake Lodge was building a conference center on its property above the lake. In order to comply with state regulations, the Lodge constructed stormwater management basins. This was standard practice and probably not a lot of thought went into it at the time, Watts explains. When he learned of it, he assumed that the water would still get to the lake as groundwater. But a few years later when his students did a project modeling Mountain Lake's groundwater profile, they determined the water being diverted by the basins wasn't going into the lake after all. Much of it had been flowing underground to a nearby creek instead.

Given that this construction was in 2001, right before the water levels began to drop, Watts says that it's likely that the basins are at least partly responsible for the lake shrinking.

Cawley also points to other local components: houses, the University of Virginia biological station and the hotel. It could also be due to the sediment cycle, he says. Sometimes the sediment can get stuck in the system and make the "pipe" where the water drains out smaller, and sometimes it washes away, and the water flows out more quickly. That sediment difference changes over time.

"All these things are complex," Cawley says. "You can't say the lake disappeared because of x, y or z."

In 2013, the hotel decided to take action and patch up the holes in Mountain Lake where water was draining. Watts and his team provided maps for the project, but in the end, he says, they were left out of the process, which was completed differently than he would have recommended.

"What we were pushing for was for it to be done in stages, because you don't know exactly what's going to happen," Watts says. "You could block this part of it and then the water's going to come up and then you get ready for the next stage."

Instead, the patching was completed all at once, with a mix of dirt and materials scraped from the side of the lake along with some bentonite, which is a swelling clay. While the lake did fill quickly after the patch-up job, new holes formed higher up from which water still escaped. Ultimately, the water level never returned to full pond, which was a disappointment to the hotel. While there is currently water in the lake — certainly much more than there was in 2008 — guests can no longer swim in, or even paddleboat in, the water.

If the hotel was still interested in trying to refill the lake, Watts says that their best bet would be to redirect some water from another local watershed using a gravity feed pipeline. This would hopefully bring back the inflow that was lost when the conference center was built in 2001.

"I'm fairly confident that would work," Watts says. "It would have to be carefully done though; there would be environmental concerns."

As of now, however, Mountain Lake Lodge has rebranded itself. It now focuses on terrestrial activities and incorporates the lake's unique hydrology and ecology into its tourism experience. It's not currently focusing on bringing the lake back up to full pond as far as Watts knows.

Which is just fine with Cawley, who likes the lake just the way it is. "The lake will take care of itself," he says. "It's not broken. It's doing exactly what it's supposed to do."