THE “GRAVEL FLOOD”: COARSE-GRAINED SEDIMENT TRANSPORT IN GRAVEL BED STREAMS DURING TROPICAL STORM LEE IN NORTH-CENTRAL PENNSYLVANIA (SEPTEMBER, 2011)

Calls for “stream cleaning”, the local term for dredging of gravel from stream channels, in north-central Pennsylvania streams have been overwhelming since catastrophic flooding by Tropical Storm Lee in September, 2011. Streams emanating from the Appalachian Plateau transported extraordinary volumes of coarse gravel, while also experiencing major bank erosion, avulsion, and chute formation during the flood. Detailed GIS-based mapping from helicopter reconnaissance photography and fieldwork in four major tributaries to the Susquehanna River document major pulses of gravel that moved downstream during the flood. New gravel bars typically range from 2-3 m thick and have areas over 300,000 m². Clasts on these bars commonly have long axes over 1.5 m and are mostly in the boulder and cobble gravel range. Estimated gravel transport by this flood along > 200 km of the streams studied is estimated at ~ 5,000,000 m³.

The occurrence of gravel pulses in these Plateau streams are similar to those discussed by Jacobson and Gran (1999) in the Ozark Plateau of Missouri. Comparison of pre-flood and post-flood GIS mapping documented significant overall increases in gravel bar volume after the flood. In most cases, mapping showed clear downstream migration of gravel pulses. Most of the gravel appears to have been sourced from low terraces composed of gravelly sediments deposited in response to widespread logging of these watersheds nearly a century ago. However, contribution of gravel from tributaries is also suggested at some locations. Headwater tributary sources of gravel appear to be from logging legacy sediments as well as mass wasting of unstable glacial deposits. These rivers are experiencing a phase of disequilibrium in response to increased availability of gravel and are currently adjusting their morphology to return to multi-threaded systems where flood flows occupy numerous channels across wide floodplains. This morphology appears to have been common in the region prior to anthropogenic disturbance by logging, agriculture, and highway construction over a century ago. Understanding the trajectory of stream morphology in this region is critical in making wise management and land use policy-making decisions.