Geomorphic Investigation of the Lower West Branch of the Susquehanna River, North-Central Pennsylvania

Elaine Langone Center, Terrace Room

The channel morphology, island and floodplain features, and aquatic habitat along a 54-km stretch of the lower West Branch of the Susquehanna River were mapped to characterize the fluvial landforms and assess its geomorphic history. Channel features on the horizontal scale of three meters or greater were delineated using a high-resolution depth finder and side-scan sonar imager which provided over 300,000 depth/location readings. The data were processed and combined with terrestrial LiDAR coverages to produce sonar image maps (SIMs) and digital bathymetry maps (DBMs) of the channel and adjacent floodplain features. These maps reveal two scales of river bedforms: (1) large, elongate pools, riffles, and side channels interpreted to have been formed by catastrophic Pleistocene flood erosion and sedimentation; and (2) superimposed, smaller-scale and finer-grained gravel bars formed by historic-floods. The older bedforms are typically coarser and more armored, weathered and rounded than the modern flood sediments. Underwater digital photographs and grain size measurements are being combined with side-scan sonar images to characterize textural variations in the river bed sediments. 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.