A GIS APPROACH TO MAPPING SEDIMENTATION PATTERNS IN THE LETTERKENNY RESERVOIR, FRANKLIN COUNTY PENNSYLVANIA USING KAYAK-DEPLOYED GROUND PENETRATING RADAR

The use of GPR is an environmentally safe, non-invasive way to study subsurface features. GPR is useful because it can be used to study soil thickness, bedrock orientation, sediment grain-size distribution, etc. This study focuses on the use of a Mala X3M GPR to image the bottom of Letterkenny Reservoir to examine sediment distribution patterns and rates. The bedrock-walled reservoir (located in Roxbury, PA) is a protected water supply and is used for recreation and fishing. Due to environmental regulation, deployment of the GPR was not possible with a motor boat, so tandem kayaks were employed to produce radargrams of 21 transverse and longitudinal transects of the lake bottom.

Sedimentation into the lake occurs via a series of minor ephemeral streams and at least one larger inlet stream. Additional sediments enter the lake via mass wasting and runoff from the access road which is eroded during storms. Visual investigation of the shoreline shows J-hooking trees and in some instances bowl-shaped head-scarps suggestive of slump features. Constructed in 1957, the lake has experienced a number of significant flooding events including Hurricane Agnes in 1972.

Trimble GPS units and a data dictionary were used to collect coordinate point data. These were output to ArcMap for production of a bathymetric map of the reservoir. Additional datasets (lake bottom position, bedrock-sediment interface, and pre-lake soil-lake sediment interface) have been identified in radargrams and have been exported into ArcMap for detailed mapping analysis. Interpolation techniques were used to produce a continuous raster surface.

Our analysis revealed, tree stumps, submerged logs, and various bottom types (sand-silt bottom, leaf-litter covered bottom, etc.). The NE end of the lake, as well as the SW end of the lake show bedrock controlled sedimentation. A series of inter-layered convex upward anomalies were identified in a shore parallel transect. These are interpreted as a series of overlapping slump deposits. Other features include an incised sediment bar complex located at the SW end of the lake near the inlet stream. Strong reflectors within this sediment complex are not yet identified, but are inferred to be related to prominent sedimentation events in the reservoir’s history. Future studies will focus on truthing these anomalies.