BASIN-SCALE INFLUENCE ON THE GEOMORPHIC IMPACTS OF THE TROPICAL STORM LEE FLOOD (SEPTEMBER, 2011) IN NORTH-CENTRAL PENNSYLVANIA

Catastrophic flooding occurred from up to 40 cm of rain over two days in September, 2011 along several major tributaries to the Susquehanna River in north-central Pennsylvania. USGS estimates indicate that flood recurrence interval exceeded 100 years on several gaged streams. Geomorphic response to the flood varied markedly between watersheds. The largest runoff per unit area occurred in Swatara Creek where the discharge nearly doubled the previous record; however, geomorphic response was minor. Swatara Creek heads in the Ridge and Valley Province and enters the Susquehanna near Harrisburg in the Piedmont. Similarly, minor geomorphic change characterized other Ridge and Valley watersheds such as Chillisquaque Creek. In sharp contrast, major geomorphic change, including avulsion, chute formation, massive gravel transport, and bed, bank, and floodplain erosion characterized gravel bed streams emanating from the Appalachian Plateau. These major geomorphic responses occurred in watersheds with less rainfall than those to the south.

Detailed basin morphometric analysis comparing both large-scale watersheds and subwatersheds using ArcGIS watershed modeling show that Plateau watersheds have greater basin relief, higher drainage density, higher ruggedness number, and steeper channel gradient than those to the south. Plateau watersheds also provided an abundant source of flood gravel from two sources: 1) widespread Pleistocene gravelly glacial sediments along steep hillslopes and terraces, and 2) extensive gravelly floodplain sediments inherited from the legacy of regional logging nearly a century earlier. Comparing basin morphometry data with regional studies (i.e. Patton and Baker 1976) shows that Plateau basins are prime candidates for generating flash floods. Major geomorphic change occurred where flashy flows were coincident with an abundant supply of coarse bedload (gravel) from relatively non-cohesive banks, steep channel gradient, and widespread disequilibrium in fluvial systems related to historic land use changes. Minor geomorphic response occurred in streams that experienced flashy flows south of the Plateau, exhibiting lower ruggedness number, lower channel gradient, a lack of available coarse bedload, relatively cohesive banks, and less influence by logging legacy sediments.