GEOMORPHIC INVESTIGATION OF THE LOWER WEST BRANCH OF THE SUSQUEHANNA RIVER, NORTH-CENTRAL PENNSYLVANIA
Islands in the present-day channel are relic bars from braided Pleistocene outwash channels, deposited upstream of bedrock outcrops or where the valley suddenly expands. A 0.5-km long x 5-m deep bedrock knickpoint was discovered upstream of the Great Bend near Muncy, PA. The river valley suddenly widens downstream and depth to bedrock increases to over 40 m. The bedrock-alluvium interface over this 30 km2 area is being mapped using numerous well logs and micro-gravity measurements collected along transects spanning the river valley. These features reflect underlying structural and lithologic factors, enhanced by Pleistocene glaciation and subsequent erosion/sedimentation by meltwaters and catastrophic dam-break floods from the 100-m-deep by 100-km-long glacial Lake Lesley located upstream. The data are being used to develop a HEC-RAS hydraulic model and assess the ability of historic flood discharges to transport existing channel bed sediments and hypothesize on the magnitude of the paleofloods that may have shaped the channel and floodplain features of the lower reaches of the West Branch of the Susquehanna River.