Virginia has only two natural lakes. Every other "lake" in Virginia besides Mountain Lake and Lake Drummond is a human-created reservoir. The formation of Lake Drummond is still mysterious, but geologists have finally deciphered the plumbing and origin of Mountain Lake. While less mysterious now, it's still an amazing story.
Like almost all other lakes formed naturally over millions of years in the Appalachians, the traditional assumption has been that Mountain Lake was created originally by a large rockslide. According to this theory, when the layered sediments on Salt Pond Mountain eroded, a layer of soft rock eroded faster than the hard sandstone above it.
The overhanging sandstone ledge grew too large, and finally collapsed in a landslide that blocked the stream we know today as Pond Drain about 6,000 years ago. (The presence of an old forest growth surrounding the lake proves that no recent landslide could have created Mountain Lake in historic times.) Since Giles County is the most seismically-active area in Virginia, an earthquake may have jiggled the rocks loose and triggered the landslide. We can see sandstone boulders that have crashed into the creek valley. The sharp edges show they have not travelled far, and these boulders may have come down during the landslide 6,000 years ago.
Water backing up behind the landslide would have flowed over the new dam of boulders and soil, back into Pond Drain. Today, when Mountain Lake is full, the waters flow over the edge into Pond Drain, down to Little Stony Creek, then go over the large Cascades waterfall, past Tangent Outfitters and Liberty gas station on US460 at Pembroke, and into the New River in Giles County.
However, the overflow into Pond Drain did *not* carve a stream channel through that barrier and drain the lake completely. There's still a lake on the side of the mountain over 100 feet deep, at least when the lake is full. The watershed of the lake is only 320 acres,1 but with the annual 50" of rain2 in this area... plenty of water should have backed up behind the landslide over the last 6,000 years, then flowed over the top and cut a channel 100' deeper through the rock/soil debris. The lake should have become just another valley on the slopes of the mountain, long ago.
Streams have eroded all other natural dams, explaining why (except for Lake Drummond and Mountain Lake) every other natural lake formed in Virginia over the last 200 million years has disappeared. Pond Drain did not cut through the landslide, adding to the network of stream channels throughout the Virginia landscape while eliminating the latest natural lake in the process. Instead, one lake on a mountainside has survived over 6,000 years. Why?
Turns out the sandstone underneath Mountain Lake has a crack in it, and the excess water had another exit other than over the rock at Pond Drain. In fact, the crack is so large that Mountain Lake may have formed without the effects of any major landslide blocking Pond Drain.
Mountain Lake formed at least in part because the sandstone under the lake has a crack that has eroded deeply. Water flowing through the crack has carried bits of eroded sandstone downward, diverting some of the erosive power of the accumulating water. The bottom of Mountain Lake was carried away down the hole, creating a deeper basin over the last 6,000 years. As the bedrock washed away into the crack, the basin deepened and filled with rainwater, creating Mountain Lake.
The lake could have formed just through erosion of the sandstone bottom. A landslide damming up a valley to form a natural lake is not unusual, but the eroding sandstone crack at the bottom of Mountain Lake may be unique in Virginia - and the reason why a stream has not cut a channel on the surface of the land and drained that one lake (yet).
Even more unusual is that Mountain Lake completely disappears at times, leaving behind just a lake bottom that becomes a grassy meadow. When Christopher Gist came through the area in 1751 on his way to explore lands claimed by the Ohio Company, he was the first to record the existence of the lake. He wrote in his journal, with the precision of a surveyor recording directions of travel:
In 1768, settlers recorded just a spring in a valley with a grassy meadow. It was a good location for watering cattle and providing them salt as a dietary supplement, and the settlers named the location Salt Pond Mountain... but the "missing" lake caused the settlers to question the honesty of Gist's records of his explorations.
From the 1950's until more recent times, the lake has been full. From 1998-2002 the surface area shrank by half, down to 25 acres. It recovered to its normal full level in 2003... but then the cycle began again. As of September 2008, the lake had drained almost completely, despite above-average rainfall in the first half of 2008.
The outlet down Pond Drain has not changed; there's no reduction and then increase in the height of the natural dam that holds water on the mountain. So... how could the lake appear and disappear so quickly?
Mountain Lake has a series of leaks, located in the crack which caused the basin to develop in the first place. Normal "leakage" through the natural holes in the bottom of Mountain Lake is about 600-800 gallons per minute.4 When silt plugs the hole, rainfall exceeds the loss of water through evaporation/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 down to the New River. In drought years, the outflow may be reduced - but drought alone does not explain the lake's dramatic shrinkage every 50 years or so.
Silt and leaf litter is continually washing into Mountain Lake, but the water draining through the natural crevice in the rocks at the bottom of lake erode away the silt plug occasionally. There's still a layer of gooey silt at the bottom as the lake drains, but it is not watertight. When the cracks at the bottom are open, drainage exceeds the inflow. More lake water flows underground into the water table, and the surface level of Mountain Lake drops. Occasionally, the lake dries up completely - as occurred in 2008, and apparently between 1751-1768.
Salt Pond Mountain itself is not limestone, so the lake is not related to the nearby karst topography. The rock at the bottom of the lake is composed of three separate formations, Juniata Sandstone, Martinsburg Shale, and Clinch sandstone. You can see the contact line between the purple shale and the white/orange Juniata sandstone, just below the edge of the vegetation surrounding the lake. The shale crumbles easily in your hand, while the sandstone is substantially harder to crack.
The cracks at the lake bottom are in the Clinch Sandstone. Those cracks open and close because of earthquakes more than due to the silt flowing into the lake, apparently. After the lake drains, presumably one of the constant minor earthquakes in Giles County is required to adjust the sandstone at the bottom. The quake could realign the rocks, so the natural silt can seal the crevices between them again.
In 1959, one quake cracked the mantle above the fireplace in the Mountain Lake Hotel, and the lake re-filled soon afterwards. Since it appears the water in the lake drains out (emerging as springs in the valley of Pond Drain) within less than 2 years,5 the lake levels can change rapidly. Any increase in the porosity of the holes at the bottom could trigger rapid decline in the lake levels - as demonstrated in 2008, when the half-full lake completely disappeared in about 6 months.
| Question: Why isn't there a "Mountain Bog" instead of a "Mountain Lake"? Why hasn't the lake bottom become filled with organic material over the last 6,000 years as needles, leaves, branches, and soil in the small watershed above Mountain Lake have washed down into the lake?
Answer: Because the underground crack was wide enought to carry sediment, little organic material has accumulated in the lake bottom. If for some reason the hole at the bottom of the lake were to be sealed completely, then sediments would accumulate and Mountain Lake would gradually become a bog (or carve a deeper channel through the Pond Drain outlet). Soil would develop in the bog, as bedrock decayed and organic nutrients were deposited each Fall. Trees would grow in the soil, and a forest would replace the bog. That's what has happened to the very small depresssions in the Virginia mountains that did not have enough water to carve a discharge outlet, draining their water to the Atlantic Ocean/Gulf of Mexico. |
The disappearance of the lake is more than a geologic detective story. It has economic consequences now. Mountain Lake is privately owned, and the owners (Mary Moody Northern Foundation) have been particularly sensitive to the natural systems in the area... but they care about how the lake impacts tourism. The owners have been protecting the lake, only to see their investment in conservation literally disappear.
The landowners in the drainage area of Mountain Lake managed it basically as wilderness, excluding the developed property of the Mountain Lake Hotel, some dirt roads, and the University of Virginia's biological field station.
The hotel has sought to minimize lake pollution. The lake has been especially clear because the soils are low in calcium, natural sources of nitrogen and phosphorous are limited, and there are few sources of nutrients from human development (in contrast to the Chesapeake Bay). Extra algae in the lake would reduce the clarity and beauty of the view.
However, man-made sources recently threatened to shift the ecology of Mountain Lake from nutrient-poor to nutrient-rich. To maintain both the vista and recreational facilities on the edge of the lake, the owners have constructed artificia wetlands between the lawn in front of the hotel and the lake. The wetlands intercept fertilizer and pesticides used to maintain the grass and shrubs. Plants in the wetlands absorb the excess nutrients, and cleaner water is discharged into Mountain Lake.
In addition, wastewater produced in the cabins and hotel rooms is treated and sprayed onto the forest soil above the lake. Bacteria and plant roots are expected to absorb nutrients in the soil, long before the water eventually trickles into Mountain Lake.
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constructed wetlands downhill between hotel and lake | |
Still, tourism at the hotel depends in part upon the clear blue water in front of the hotel. Loss of the lake, with its swimming/boating recreational pleasures and scenic vistas, will reduce revenues at the hotel.
In the drought of 2002, the lake's owners considered supplementing the natural supply of water by pumping groundwater from a well into the lake.6 In 2008, the owners tried to plug the holes with sandbags. On July 28, "An audacious project to stuff sandbags into the eighteen or so known holes in the bottom were cancelled due to zero visibility just 15 feet beneath the surface." 7 As the lake drained in the summer of 2008, business dropped by 20% at the Mountain Lake Resort. One concern of hotel management: at least once in the six times the lake has dried up in the last 4,500 years, Mountain Lake stayed dry for decades.8
Mountain Lake in September, 2008
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