International Journal of Speleology 38 (2) 103-109 Bologna (Italy) July 2009 Available online at www.ijs.speleo.it International Journal of Speleology Official Journal of Union Internationale de Spéléologie Sediment flushing in Mystic Cave, West Virginia, USA, in response to the 1985 Potomac Valley flood James J. Van Gundy1 and William B. White2 Abstract: Van Gundy J.J. and White W.B. 2009. Sediment flushing in Mystic Cave, West Virginia, USA, in response to the 1985 Potomac Valley flood. International Journal of Speleology, 38 (2), 103-109. Bologna (Italy). ISSN 0392-6672. The great November 5, 1985 Potomac Valley flood was responsible for the release of 1800 m3 of alluvial and colluvial sediment from the walls of the entrance doline of Mystic Cave. Flood waters were sufficiently powerful to flush the entire mass of sediment not only into the cave but through the cave. Remnants of the sediment mass in the form of sand bars and a few cobbles wedged in speleothems were the only evidence in the cave that the huge mass of sediment had moved through. The sediment moved as a suspended mass in water moving at peak velocities of many meters per second. Present day cave sediments must be interpreted with the understanding that entire sediment fillings can be transported or rearranged by single extreme events. Keywords: Mystic Cave, Floods, Clastic Sediments, West Virginia Received 4 November 2008; Revised 21 January 2009; Accepted 10 February 2009 INTRODUCTION Karst springs often become turbid following storms. Many caves contain deposits of clastic sediments Collection and measurement of the transported ranging in size from clays and silts to gravels, cobbles particles reveals particle sizes in the clay to fine silt and occasionally boulders (Bosch and White, 2004). size range (Atteia & Kozel, 1997; Mahler & Lynch, An extreme example was the 1 – 2 meter sandstone 1999; Drysdale et al., 2001; Massei et al., 2003; boulders apparently flushed out of a sump in Aqua Herman et al., 2007). It is apparent that moderate Cave, Highland County, Virginia, USA (Palmer & discharge, high frequency, storms generally do Palmer, 2005). These deposits are usually composed not provide sufficient energy to move the coarse- of quartz sand and sandstone as well as other non- grained sediment. The effects of high discharge, low carbonate rocks clearly derived from locations distant frequency storms are rarely observed. Measurement from the caves in which they are found. It is generally is difficult and direct observation would be extremely agreed that clastic sediment transport is an episodic hazardous. event with distinct thresholds for the movement of An opportunity for observation was provided when particles in a given size range (Herman et al., 2008). an extreme storm in the Potomac River drainage in Fine-grained sediments can be transported either as West Virginia, USA, flushed a measurable volume bedload or in suspension by moderate storm flows of sediment through a cave system. The present (Dogwiler & Wicks, 2004). Transport of cobbles and paper describes the event and its aftermath. It was boulders requires extreme storms and such conditions a rare instance in which reasonable estimates of occur only rarely. One documented example of the both hydraulic behavior and sediment loading can effects of extreme storms was the sediment scouring be inferred for an extreme storm event in a well- of Cave Springs Cave, near Lexington, Virginia, by defined small karst drainage system. It is an even Hurricane Camille in 1969 (Doehring & Vierbuchen, more interesting example because a large volume of 1971). In this example, the storm flow transported sediment was flushed entirely through the system, sediment in the sand to cobble size range and scoured leaving only a few traces of its passage. the cave walls of existing sediment coatings. GEOLOGIC SETTING 1. Department of Environmental Science, Davis and Elkins The study site is in the Appalachian Mountains College, Elkins, WV, USA of Pendleton County, West Virginia, in the Potomac 2. Materials Research Institute and Department of River drainage (Fig. 1). The site is on the boundary Geosciences, The Pennsylvania State University, University between the Valley and Ridge Province to the east with Park, PA 16802, USA, E-mail: [email protected] strongly folded and faulted older Paleozoic rocks and 104 James J. Van Gundy and William B. White Fig. 1. Drainage map for West Virginia showing location of study area. Map adapted from the U.S. Geological Survey National Atlas of the United States of America. the Allegheny Plateau Province to the west with less end of the cave also receives a small stream. A third deformed younger Paleozoic rocks. The two provinces (West) entrance is located at the extreme downstream are separated by the Allegheny Front, a 500 m-high end of the cave and consists of a tight opening that escarpment. Mystic Cave is formed in Timber Ridge, can only be negotiated with difficulty (Fig. 2). a segment of the Allegheny Front, separated from The cave stream reaches the surface as a large the main escarpment by the incised valley of Brushy spring about 60 m distant from the West Entrance, Run. Brushy Run is a strike-oriented stream flowing and 30 m below it. In total, the cave contains about northeast as a tributary of Seneca Creek which in 2,500 m of passages (Dasher, 2001). The streamways turn cuts across strike to join the North Fork of the in Mystic Cave have moderate to high gradients. The South Branch of the Potomac River. The site is in the tributary stream from the South Entrance falls 15 m headwaters of the Potomac Drainage. Spruce Knob, over a linear course of about 735 m for a gradient the highest point of West Virginia, at an elevation of of 20 m/km. Downstream from the North Entrance 1482 m and a major drainage divide, lies 14 km to the the gradient is much steeper with several waterfalls. southwest. The fall is 50 m to the passage leading to the West Mystic Cave is a well-known cave (Davies, 1958; Entrance and another 30 m to the spring over a total Dasher, 2001) located approximately 3 km. west of the length of 588 m giving a gradient of 137 m/km. village of Seneca Rocks, West Virginia, at an elevation The apparent watershed area for Mystic cave as of 685 m (U.S. Geological Survey Onego 7.5 minute determined from the topographic map is approximately quadrangle). The cave has three entrances, the largest 180 ha (Fig. 3). Given the nature of karst drainages, of which is the North Entrance located in a 1 km- this must be considered a minimum figure, but the long compound doline containing a small permanent true figure is probably not much larger because Mystic stream that sinks at the cave entrance. About 1 km Cave underlies the western margin of its surface to the south, a second entrance near the upstream drainage basin and Blowhole, another sizable stream International Journal of Speleology, 38(2), 103-109 Bologna (Italy). July 2009 Sediment flushing in Mystic Cave, West Virginia, USA, in response to the 1985 Potomac Valley flood 105 Fig. 2. Map of the downstream segment of Mystic Cave between the North Entrance and the West Entrance. The cave also extends 735 m to the south. The full detailed map of the cave from which this segment was extracted was prepared by Bob Gulden in 2001 and was published by Dasher (2001). International Journal of Speleology, 38(2), 103-109 Bologna (Italy). July 2009 106 James J. Van Gundy and William B. White Fig. 3. Segment of U.S. Geological Survey Onego 7.5 minute quadrangle showing the drainage basin boundaries for Mystic Cave (dashed lines). This catchment includes water draining to both the South Entrance and North Entrance streams. The cave is shown as the heavy dark line. cave system, lies immediately to its west. The study area was in the zone of highest rainfall from Mystic Cave is formed in the Mississippian Greenbrier the storm. Officially, 26.7 cm of rain was recorded Limestone. The Greenbrier is a major cave-former at Seneca Rocks in the valley of the North Fork (US in West Virginia but the formation thins from south Weather Bureau from the US Forest Service gage to north along the main karst belt in eastern West at the Seneca Rocks Visitor Center) on November Virginia. Near Mystic Cave the thickness is about 120 5th, the day of heaviest rainfall during the period of m (Tilton et al., 1927). interest. Private rain gauges located closer to Mystic Cave and at about the same elevation recorded 43 cm THE NOVEMBER, 1985, POTOMAC VALLEY of rainfall during the same period (Ms. Priscilla Teter, FLOOD cave owner, private communication). A technical In early November of 1985, heavy rains totaling account of the storm (Clark et al., 1987) claims that nearly 45 cm fell over a 3 day period and produced the floods brought on by the storm were in excess of floods of record on many streams in western Virginia a 100-year return period and in excess of a 500-year and eastern West Virginia. This flooding caused return period near the study area. 16 deaths and property losses in the hundreds of millions of dollars in the narrow valleys of West THE SEDIMENT FLUSH Virginia’s rural Pendleton County. Not surprisingly, Sediment injection during the flood came from many of the area’s numerous caves experienced different locations within the closed depression at high water and unusually heavy sediment loading the North Entrance (Fig. 4). Three large sections of during this period. According to Clark et al.