Morphologic Investigation of the West Branch of the Susquehanna River in North-central Pennsylvania

Channel pattern and habitat features in a 32-km stretch of the West Branch of the Susquehanna River in north-central Pennsylvania were mapped to characterize existing features and to assess the geomorphic history of the river. River bed features on the horizontal scale of five meters or greater were mapped using a high resolution 455 kHz Lowrance StructureScan side-scan sonar imager and 83kHz HDS-10 Gen2 depth finder, as well as a Sontek River Surveyor ADCP and DGPS unit to collect more than 100,000 depth/location readings. The river was longitudinally traversed in a zig-zag fashion with an average cross-sectional spacing of 40 m. The data readings were processed using ArcGIS to produce both sonar image maps (SIMs) and digital bathymetry maps (DBMs) by contouring and gridding the sonar readings and combining them with one-meter terrestrial LiDAR data of the adjacent floodplain areas. The DBMs revealed a variety of bedforms in the channel that are interpreted as larger glacial outwash bedforms with smaller-scale modern flood bedforms superimposed upon them. Bedrock exposed on the channel bed were mapped and surveyed with respect to mid-channel islands. At various locations along the transect, pebble counts were made to characterize the size and nature of the riverbed sediments. Upstream of the Great Bend at Muncy, PA, the suspected location of a Pleistocene glacial ice dam, a five-meter deep channel is incised into the bedrock. The channel morphology data were used with hydraulic modeling to analyze the ability of a range of discharges to move the channel bed sediment and to hypothesize glacial dam-break events that may have shaped the current features of the West Branch of the Susquehanna River in north-central Pennsylvania.