LEGACY OF MID-PLEISTOCENE JöKULHLAUPS ON THE FLUVIAL GEOMORPHOLOGY AND ECOLOGY OF THE WEST BRANCH SUSQUEHANNA RIVER AND ITS TRIBUTARIES

Mid-Pleistocene glacial outburst floods (jökulhlaups) have significantly influenced the modern fluvial geomorphology and ecology in the Susquehanna watershed. Studies of valley fill deposits (Peltier 1949), a massive mid-Pleistocene (800-900 ka) ice-dammed lake (glacial Lake Lesley) on the West Branch (Ramage et. al 1998) and smaller ice-dammed lakes in tributaries (Kochel et. al. 2009), used soil chronology, seismic refraction, electrical resistivity, micro-gravity, and geomorphic mapping to document jökulhlaups. Lidar, side-scan sonar, and numerical models were used to map river and valley morphology, estimating Lake Lesley jökulhlaups as great as 178,000 m3/s (Newlin and Hayes, 2015); two orders of magnitude > 100-yr floods (8,500 m3/s). These paleofloods reworked the glacial debris in the valley, depositing 10 to 40m of coarse, glaciofluvial sediments.

Mid-Pleistocene ice sheets also moved up into tributaries, dammed headwater streams, and formed glacial lakes. As ice sheets retreated, the ice dams failed and subsequent jökulhlaups formed low-gradient (1-2^o) surfaces composed of matrix-supported cobble-boulder deposits, extending 5-8 km from the ridges. Because the modern stream hydrology is incapable of mobilizing these deposits, some streams have shifted to the margins of the jökulhlaup surfaces and cut channels in the weathered shale bedrock; inverting topography as jökulhlaup surfaces became local drainage divides. They can’t be farmed and remain as prominent forested corridors that provide critical habitat and migration routes for wildlife.

The fingerprints of glacial Lake Lesley jökulhlaups are clearly visible in the valley and channel morphology of the Susquehanna. Bathymetric mapping reveals both smaller-scale modern bedforms and extremely large pools and riffles with a size/spacing reflecting Lake Lesley jökulhlaups, when flows were as deep as 40m. These legacy pools now provide refuge for native fish, amphibians, mussels, and reptiles during low water periods. Extensive backwater flooding occurred upstream of valley constrictions and clusters of islands formed at expansive reaches and at low-gradient reaches upstream of bedrock knickpoints. Braided paleochannels, which are clearly visible on lidar images, now form bog and palustrine wetlands throughout the valley.