GEOMORPHIC IMPACT OF CATASTROPHIC FLOODING FROM TROPICAL STORM LEE (SEPTEMBER, 2011) IN GRAVEL-BED STREAMS OF THE APPALACHIAN PLATEAU, NORTH-CENTRAL PENNSYLVANIA

Geomorphic work, whether expressed as sediment transport or landform modification was significant in gravel bed streams emanating from the Appalachian Plateau of north-central Pennsylvania during Tropical Storm Lee flooding in September, 2011. Detailed GIS-based mapping from post-flood helicopter photography and fieldwork documented geomorphic change along more than 200 km of four major tributaries to the Susquehanna River – Lycoming, Loyalsock, Muncy, and Fishing Creeks. Dominant geomorphic responses include: 1) erosion of chutes on the inside of meanders; 2) channel avulsion and reoccupation of prehistoric high flow anabranches disconnected from main channels by anthropogenic activity such as berm construction and highway fill; 3) extraordinary coarse gravel transport (bar growth and migration); 4) episodic downcutting and aggradation; 5) bank erosion; and 6) mass wasting and alluvial fan activation in glaciated headwater tributaries.

While it is likely that geomorphic response to a flood with greater than 100-year recurrence would be significant in steep, gravel bed streams, the extreme magnitude of the response may have been amplified by anthropogenic disturbance. Regional clear cutting in these watersheds between 1850-1920, contributed sediment downstream. Dendrogeomorphic observations suggest that significant filling occurred prior to 1930, creating a low gravelly terrace along these streams. Watershed reforestation since 1930 has yielded relatively clear, aggressive water to downstream channels. 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 now 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. This flood caused extensive damage to homes, farms, and highway infrastructure. Similar responses to floods are likely as these channels evolve through a protracted phase of disequilibrium during their adjustment from constrained single channels to multi-threaded gravel bed systems. Understanding the trajectory of these river systems has enormous implications for management and policy-making in the future.