HR: 0830h
AN: B11C-0704 INVITED     [PDF]
TI: Use of Dendrochronology for Determining the Chronology of Landslide Activity Along Meadow Run, Shenandoah Valley, Virginia USA
AU: * Eaton, L S
EM: eatonls@jmu.edu
AF: James Madison University, Department of Geology and Environmental Sciences, Harrisonburg, VA 22807 United States
AU: Yanosky, T M
EM: tyanosky@usgs.gov
AF: US Geological Survey, MS 430, Reston, VA 20192 United States
AU: Wieczorek, G F
EM: gwieczor@usgs.gov
AF: US Geological Survey, MS 430, Reston, VA 20192 United States
AB: The growth rings of living trees were examined to determine the chronology of landslide activity along Meadow Run, a low-order ungaged stream that emerges from the western flank of the Blue Ridge Mountains of Virginia and flows into the South River in the eastern Shenandoah Valley. At the study site, Meadow Run meanders over a Pleistocene-Quaternary alluvial fan comprised of fresh and weathered sands and cobbles derived from quartzites and sandstones of the Antietam Formation. The alluvium is underlain by the Shady Dolomite, which has limited exposure as saprolite in some stream cuts and slope failures. Stream migration and undercutting of topographically higher fan surfaces creates bluffs up to 20 m in height that show signs of past and recent slope instability. Tilting of trees and slope surfaces indicate that rotation is the dominant type of slope movement; hence, these landslides are termed earth slumps. Two of these slides are contiguous and were investigated in detail. The geomorphology of the downstream slide suggests episodic movement since the mid 1930s. A white oak near the main scarp of the slide shows an abrupt and sustained decrease in ring width beginning in 1937, suggesting that the tree was tilted and root pruned; a sprout from the base of the tilted trunk started to grow no later than 1940. A nearby sprout that began growth no later than 1941 from a tilted black gum supports the general dating of the initial slide. Along the South River at Harriston, floods were recorded in March 1936 (12,600 cfs) and April 1937 (11,700 cfs), suggesting that slope instability along Meadow Run resulted from the undercutting of bluffs by flood waters or from over-saturation of soils during heavy rains. The synchronous formation of reaction wood beginning in the early 1970s in the oak and gum sprouts, and in the parent trunk of a nearby Virginia pine, suggests an episode of slope movement triggered by the peak flow (21,300 cfs) following Hurricane Agnes in June 1972. The proximal, upstream slide shows fresh scarp exposures of very recent landslide reactivation. The germination of seedlings on disturbed soils and growth responses of previously established trees suggest episodic slope instability in approximately 1993, 1997, and 1999. For example, a 50-year old Virginia pine formed reaction wood beginning in 1993 followed by a virtual cessation of radial growth after 1997. Changes in the quality and quantity of growth in this tree seemingly are related to two episodes of slope movement that first tilted the tree and then severely pruned its roots. During the period of record (1924-2002) for the South River, recent floods occurred in April 1992 (9,840 cfs), March 1993 (7,360 cfs), September 1996 (28,900 cfs), and February 1998 (10,000 cfs). In addition to the correlation between the dendrochronology and the peak-discharge data, geomorphic evidence suggests that the main channel of Meadow Run has a history of realignment during large storms. The base of the downstream slide is in direct line with the trajectory flow of a paleo channel of unknown age. Additionally, the base of the upstream slide is being removed by the modern channel. Preliminary evidence suggests recent stream avulsion of this channel, as the remnants of an upstream debris jam has redirected flow, and the active channel that impacts the slide appears underfit compared to upstream and downstream reaches. The multiple and quasi-braided paleo channels in Meadow Run suggest a long history of realignment in many locations during floods, further contributing to slope instability.
DE: 1815 Erosion and sedimentation
DE: 1821 Floods
DE: 1860 Runoff and streamflow
DE: 4221 Dendrochronology
SC: Biogeosciences [B]
MN: 2003 Fall Meeting