GEOMORPHIC RESPONSE TO CATASTROPHIC FLOODING IN NORTH-CENTRAL PENNSYLVANIA (TROPICAL STORM LEE - SEPTEMBER 2011): INTERSECTION OF FLUVIAL EQUILIBRIUM AND THE LEGACY OF LOGGING

More than 25 cm of rainfall from Tropical Storm Lee over two days in September, 2011 resulted in catastrophic flooding (USGS estimated recurrence interval > 100 yrs) on several Susquehanna River tributaries emanating from the Appalachian Plateau in north-central Pennsylvania. Helicopter photography and fieldwork was used to prepare a detailed GIS database of geomorphic response to the flood along > 200 km of Loyalsock, Muncy, Lycoming, and Fishing Creeks. Fluvial response to the flood in these gravel bed streams was significant, characterized by: 1) major avulsions and chute development on insides of meanders, 2) erosion of gravel from channel margins and transport downstream in large pulses, 3) headwater landslides and alluvial fan activation, and 4) breaching of anthropogenic berms and reconnection of the main channel to prehistoric floodplain anabranches. Geomorphic work, expressed both as sediment transport and landform change was major.

Streams in this region appear to be in a phase of disequilibrium largely in response to major shifts in sediment delivery from their watersheds caused by historic land use change, chiefly logging. Widespread clear cutting (1850–1920) contributed large volumes of sediment to downstream reaches. Dendrogeomorphic observations indicate that this logging legacy fill stabilized by the 1930’s, creating a significant low terrace inset into Pleistocene outwash and glacial sediments. Most of these streams were disconnected from high-flow anabranches by berms related to logging, agriculture, and highway construction. Major floods during the past 40 years (1972, 1996, 2004, 2011) appear to have initiated widespread disequilibrium and erosion of the logging legacy gravel. Sediments from this remobilized fill appear to be moving through these watersheds as major gravel pulses. Simultaneous aggradation of gravel bars above bankfull during floods is promoting bank erosion and channel avulsion. Consistent with the results of pioneering studies by Schumm, the increased coarse gravel load available to channels is causing rivers to adjust their morphology and return to multi-threaded systems where flood flows occupy numerous channels across wide floodplains. Understanding the trajectory of streams in this region is critical in making wise management and land use policy decisions.