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.
Mountain Lake is appropriately named, located at nearly 4,000 feet in elevation and over 1/2 mile long (when full). 10,000 years ago, the stream we know today as Pond Drain was at the headwaters of the Eastern Continental Divide, flowing uninterrupted towards the New River and the Gulf of Mexico. On the other side of Salt Pond Mountain, Johns Creek flowed northeast towards the James River and the Atlantic Ocean.
The rock layers near Mountain Lake on Salt Pond Mountain include a thin crust of Rose Hill sandstone at the top, 50-100' of Tuscarora (Clinch) sandstone below it, then a layer of Juniata sandstone/mudstone about 200' thick, with over 1500' of Reedsville-Trenton (Martinsburg) shale at the bottom. Salt Pond Mountain itself is not limestone, but there is karst topography at lowel elevations in older limestone bedrock.1
The bottom of Mountain Lake is Juniata sandstone/mudstone. A contact between a purple shale layer and a white/orange sandstone layer is located below the edge of the perennial vegetation surrounding the lake. The shale crumbles easily when squeezed by hand, while the sandstone is substantially harder to crack. Within the Juniata, four "piping holes" drain water underground at the northern end. These were clearly exposed when the lake drained completely in 2008. A fifth "leak" was discovered in 2014, about 45 feet above the bottom of the lake.2
Pond Drain and Doe Creek are on the opposite side of the Eastern Continental Divide from Johns Creek
Source: US Geological Survey (USGS), National Map
watershed divides at Mountain Lake
Source: US Geological Survey (USGS), National Atlas
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 the landslide theory, Pond Drain eroded though the hard Tuscarora (Clinch) sandstone, then into the Juniata sandstone/mudstone. A layer of soft mudstone eroded so fast that Pond Drain may have flowed through a narrow canyon with sandstone walls on Salt Pond Mountain, and perhaps a waterfall near modern-day Newport House on the northern tip of modern-day Mountain Lake.
The overhanging sandstone ledge finally collapsed in a landslide over 6,000 years ago. Since Giles County is one of the most seismically-active areas in Virginia, an earthquake may have jiggled the rocks loose and triggered the landslide. Another possibility is that a storm comparable to Hurricane Camille in 1969 could have dropped over 30" of rain in one day, loosening the soil and triggering landslides.
The presence of an old forest growth surrounding the lake proves that no recent landslide could have created Mountain Lake in historic times. Native Americans hunting and gathering food in the area, still without the technology of pottery, bows-and-arrows, or agriculture, would have witnessed the change. Sediments at the bottom of the deep end of the lake have been dated to 6,100 years ago.3
Sandstone boulders that crashed down into the valley of Pond Drain created a highly-visible cluster near the Newport House, where the lake overflows into Pond Drain (when the lake overflows...). The sharp edges on the Tuscarora (Clinch) boulders show they have not traveled far, after they came down during the landslide. The boulders continue to move through slow mass wasting.4
Right after the landslide, water backed up behind the new dam of boulders and soil. The new lake may have over-topped the dam, and water may have flowed across the top of the dam on its way downstream. Today, when Mountain Lake is full, water flows over the edge into Pond Drain, on down to Little Stony Creek. It then goes over the large Cascades waterfall, past the gas station on US 460 at Pembroke, and into the New River in Giles County.
However, the overflow into Pond Drain did *not* carve a stream channel completely down through that landslide barrier and drain the lake. On the side of Salt Pond Mountain, there's still a lake over 100 feet deep when the lake is full. The watershed of the lake is only 320 acres, but with the annual 55" of rain in this area.5
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 been temporary, and become just another valley on the slopes of the mountain after a channel was cut through the landslide.
where Mountain Lake empties - in high water years - into Pond Drain
(arrow shows direction of flow towards Little Stony Creek)
Source: Microsoft Research Maps
color infrared digital orthoquarterquad of Mountain Lake
(when it was "full" in 2000, and showing direction of flow)
Source: Radford University GIS Spatial Data Server
Streams have eroded all other natural dams in Virginia, draining every natural lake except for Lake Drummond and Mountain Lake. Rivers chisel through the bedrock like liquid sandpaper, over time, so lakes formed in Virginia over the last 300 million years has disappeared.
At Mountain Lake, Pond Drain did not cut through the landslide dam. Mountain Lake should have been temporary. Over time, a stream channel should have eroded through the colluvium that slid down the mountain and drained the lake. Instead, that unusual lake on a mountainside has survived over 6,000 years. Why?
One possibility is that the Juniata sandstone/mudstone underneath Mountain Lake has cracked, and the excess water has exits other than over the landslide-caused dam at Pond Drain. Another possibility is that the dam blocking Pond Drain is porous, with a series of channels eroded at the bottom of the dam.
Before a landslide blocked flow, Pond Drain was a free-flowing stream starting at the watershed divide with Doe Creek, near the modern-day lodge, and flowing through a sandstone-walled canyon that may have included a waterfall. The waterfall would have been smaller than the modern Cascades downstream on Little Stoney Creek. Still, there may have been enough energy in the water to carve narrow channels into the Juniata sandstone/mudstone bedrock at the lip of the waterfall.
After the landslide, boulders could have bridged those channels, leaving them unblocked. Water seeping through the unconsolidated sediments right after the landslide could have continued to flow through those channels, or through new paths cut through the boulders and soil that crashed down, or through cracks formed in the Juniata sandstone/mudstone lake bottom by the impact of the landslide or an earthquake that triggered the event - or through another process.
Cracks in the bedrock at the bottom of the lake could also have formed by collapse of solution channels in calcium-rich layers of the Reedsville-Trenton (Martinsburg) shale, fracturing the overlying Juniata sandstone/mudstone. The Juniata formation at Mountain Lake does not have enough carbonate itself to develop karst-like features, but cave collapse at depth in the Reedsville-Trenton (Martinsburg) shale could affect the overlying Juniata sandstone/mudstone, creating a "caprock sinkhole" that causes the water to leak out of Mountain Lake.6
Water flowing through the cracks in the bedrock, or through the landslide dam, has diverted some of the erosive power of the accumulating water. As a result, the dam has not been overtopped with a high-energy stream, so a channel has not been eroded into the surface of the dam over the last 6,000 years. The energy of the water exiting the lake has been diverted through four holes (currently) etched out of the Juniata sandstone/mudstone in the lake bottom, allowing the landslide dam to withstand erosion - so far.
A landslide damming up a valley to form a natural lake is not unusual, but the continued existence of the dam is unique in Virginia now. Normally, a stream would cut a channel on the surface of the dam and drain the lake, often within weeks of a landslide. On Salt Pond Mountain, however, the water trapped behind the dam drained into the cracked bedrock, or into natural channels or pipes through the dam. The basin behind the dam deepened and filled with rainwater to create Mountain Lake, which at times has grown to 100 feet in depth.
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:8
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.
Analysis of the sediments at the bottom of the lake, completed for a PhD thesis published in 1999, indicated that Mountain Lake has been at very low levels or dried up at least six times since it formed. The dry periods, when the lake became a Sphagnum bog or dry meadow, were 100, 400, 900, 1200, 1800, and 4200 years ago. In 1655 AD (plus or minus 80 years), a yellow pine grew for at least 30 years on what was later the bottom of Mountain Lake. Water levels would have been affected by changes in climate, with lower rainfalls associated with changes in the sunspot cycle, as well as by changes in water "leakage" through the landslide dam and bedrock.9
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. By September 2008, the lake 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 43 liters/second (roughly 700 gallons per minute).10
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 and intermittent shrinkage.
Silt and leaf litter is continually washing into Mountain Lake, but occasional earthquakes help the water draining through the natural crevices at the lake bottom erode away the silt plug. There's still a layer of gooey silt at the bottom as the lake drains, but at times it is not watertight. When the cracks at the bottom are open wider, 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.
After the lake drains, presumably one of the constant minor earthquakes in Giles County is required to adjust the way silt seals up the cracks at the bottom. A quake may realign the rocks so the natural silt can seal the crevices between them again, and the lake re-fills 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 within less than 2 years, 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.11
Still remaining to be discovered: where the water goes, once it leaks out of Mountain Lake.
In January, 2012, one pound of fluorescein dye was placed into each of the four piping holes (after cutting though the ice on top of the frozen remnant of Mountain Lake). By April, 2013, there was still no solid recovery of the dye in the various streams nearby. The most likely candidate for a "receiving stream" was Pond Drain, and some evidence of dye was recovered at the confluence with Hunters Branch. When Mountain Lake is full and overflows, the surface water goes over the lip near Newport House and down Pond Drain to Little Stony Creek. However, the dye trace experiment did not determine conclusively if Mountain Lake's "leak" also ends up in Pond Drain.12
Subsurface drainage could ignore the topographic divides at the surface; there is no evidence yet that Mountain Lake's water really doe flow over Cascades waterfall on Little Stony Creek. The dye may still be coursing through the underground counduits before emerging at a spring - but is still unclear which springs, in which stream drainage(s), are fed by Mountain Lake. If the piping holes are connected to carbonates in the Reedsville-Trenton (Martinsburg) shale, then the roughly 700 gallons per minute leaking into the bedrock could be diffusing though an extensive network of underground fractures rather than flowing to just one or two outlets.
Mountain Lake and the land around it are privately owned. The owners (Mary Moody Northen Endowment) have been particularly sensitive to the natural systems in the area. The restaurants and overnight facilities (lodge/cabins) are operated to generate funding for the foundation, but revenue generation is not the only management objective. The landowners in the drainage area of Mountain Lake manage much of their privately-owned 2,600 acres as wilderness, creating the Mountain Lake Conservancy to oversee activities outside the developed area.
In particular, the private landowner has sought to minimize lake pollution. Mountain 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. Scientists warned in 1994 that excessive amounts of phosphate and nitrate had caused the lake to shift from oligotrophic in 1985 towards eutrophic status, with new algae species and even a "dead zone" of anoxic bottom water. By 1998, however, phosphate levels had dropped back to those measured in the early-1980's, most nutrients entering the lake again came from rainwater, and the trend towards eutrophication had stopped.13
To maintain both the vista and recreational facilities on the edge of the lake, the private owners have constructed artificial wetlands between the lawn in front of the lodge 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 lodge rooms is treated and sprayed onto the forest soil into a different watershed, so surface runoff should not reach Mountain Lake. On the surface, bacteria and plant roots should absorb nutrients in the soil. In major storms, whatever material might wash downhill should end up flowing towards Pond Drain or Little Stoney Creek downstream of Mountain Lake. Whatever might seep underground could flow like the surface watershed pattern, and never seep into Mountain Lake.14
The disappearance of the lake is more than a geologic detective story; it has economic consequences. The owners have been protecting the lake, only to see their investment in conservation literally disappear.
Tourism at the lodge depends in part upon the clear blue water in the lake. Visitors used to spend more than one week on family vacations, since there were so many recreational opportunities involving boating and swimming. Loss of the lake, with its active recreation opportunities and scenic vistas, reduced revenues at the resort substantially. After the lake dried up in 2008, average occupancy dropped from 60% to 30%.15
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.16
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."17
As the lake drained in the summer of 2008, business dropped by 20% at the Mountain Lake Resort. One concern of Mountain Lake management: at least once in the six times the lake has dried up in the last 4,500 years, it stayed dry for decades.18
Without the lake, efforts to increase revenue included highlighting Hollywood history at the site. Mountain Lake and the old stone hotel were featured (especially in the first 15 minutes) of the 1987 movie "Dirty Dancing." The movie described events at the fictional Kellerman's Resort in the Catskills, but the producers needed outdoor shots set in late summer. The cast and crew went south to a forested setting where the leaves were not turning colors in September, 1986. Mountain Lake - and Lake Lure in North Carolina - got 15 minutes of fame imitating a New York resort, providing a excuse for "Dirty Dancing"-themed events at both locations.19
After the Mountain Lake Hotel closed for the season in November 2012, the owners of the resort initiated a $2.5 million dollar makeover to restore its allure as a vacation destination. The Mary Moody Northen Endowment upgraded the rooms, revised restaurants, expanded outdoor recreation facilities such as zip lines, and tore down structures that no longer met the building code. To highlight the changes, the resort was retitled Mountain Lake Lodge (vs. "hotel").20
Mountain Lake is near the watershed divide between the Chesapeake Bay (James River watershed) and Gulf of Mexico (New River watershed)
Source: ESRI, ArcGIS Online (with Chesapeake Bay Watershed Boundary)
In addition to the cosmetic makeover and facility upgrades, the four piping holes in the bottom of the lake were plugged in 2013.
Despite the cold weather and snow in February, 2013, contractors with heavy construction equipment moved dirt and rocks from the colluvium on the lake edges, dumping the material directly into the holes. Using local material mimics the natural process of siltation, though there were discussions about sealing the lake bottom with asphalt pavement and concrete. Paving the bottom would have made Mountain Lake more like a swimming pool, and damaged its reputation as a natural lake. By the end of 2013, the lake size had doubled. Engineering geologists at Radford University anticipate that if the plugs are successful, Mountain Lake could return to full pool in less than two years.21
|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 cracks have been wide enough and currents fast enough to carry away sediment, organic material and silt has not plugges the piping holes. 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. Over time, the lake bottom would fill up, reducing the volume of water. 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.
exposed muddy bottom with water remaining only in deepest portion of Mountain Lake, October 2011