Subsurface Erosion, Scarp Retreat, and Sedimentation at Mountain Lake, Virginia, USA: Groundwater Geomorphology in a Flow-Through Lake with Subsurface Drainage
Cawley, Jon C. and Irwin, Rossman P., III. 2018. "Subsurface Erosion, Scarp Retreat, and Sedimentation at Mountain Lake, Virginia, USA: Groundwater Geomorphology in a Flow-Through Lake with Subsurface Drainage." Earth Surface Processes and Landforms, 43, (8) 1663–1676. https://doi.org/10.1002/esp.4345.
The complete natural drainage in 2008, 2011, and 2012 of Mountain Lake in Giles County, Virginia, allowed detailed observations of the only natural lake basin in the southern Appalachian Mountains. Here we use these observations to support geomorphic analysis and develop a model of basin evolution, which may advance the understanding of rare flow-through lakes with subsurface drainage elsewhere. Key features included a) an angle-of-repose slope with a smoothly concave planform across the entire 260 m width of the north end of the basin, b) an arc of steep-sided depressions along the deep northern margin of the basin floor, and c) an abrupt transition between colluvial and finer-grained sedimentary deposits on the floor. Our geomorphic analysis suggests that subsurface erosion has enabled long-term northward scarp retreat in the basin by removing water and sediment. Mountain Lake formed on the northern limb of a breached anticline along the Eastern Continental Divide, where strong-over-weak stratigraphy and a small watershed have enabled the basin to evolve generally as follows. 1) Pond Drain, a first-order tributary of the New River, incised north-dipping sandstones and underlying shales on the northern limb of the anticline. The valley floor subsequently accumulated meters to tens of meters of mostly late Pleistocene colluvial fill. 2) Subsurface drainage developed likely along the contact between the sandstones and shales, facilitated by pre-existing fractures. 3) Ongoing subsurface erosion has progressively undermined the sandstone, causing scarp retreat along the northern margin of the basin while a surface stream intermittently incised the shallow southern end. Sedimentary deposits indicate that only the deeper northern portion of the basin is usually flooded under Holocene conditions. Our basin evolution model suggests slow development of the basin over hundreds of thousands of years rather than sudden damming by a catastrophic landslide.
