CHANNEL ADJUSTMENTS TO HISTORICAL LAND USE CHANGES IN THE GRAYS RUN WATERSHED, LYCOMING COUNTY, PENNSYLVANIA

During the latter part of the 19th and early 20th century, the forests in Central Pennsylvania and most other areas in the Northeastern United Sates were drastically reduced in area due to the influx of logging and agricultural practices. Since the 1920's these forests have been allowed to grow to nearly the size at which they had once thrived. These anthropogenic changes have significantly altered sediment delivery to stream channels. Since the dynamic equilibrium of streams depends upon water discharge and sediment loading, these changes in land-use have significantly altered the geometry and behavior of channels in the region. This study focuses on Grays Run, a medium-sized stream within the Lycoming Creek watershed of the Appalachian Plateau. Lycoming Creek is a tributary to the West Branch of the Susquehanna River.

Grays Run is currently experiencing major channel avulsions and shifts from a single channel to braided stream in its lower reaches. The present disequilibrium conditions in Grays Run with respect to channel dynamics and sediment movement seems to have directly resulted from changes in land use due to the influx of historical logging in the area, followed by reforestation. During deforestation from the 1870's to the 1920's the original floodplain (Qa1) accumulated with a geomorphic surface (Qa2-the “logging surface”) with the abrupt increase in sediment loading from logging practices. Following reforestation, Grays Run began to incise into this surface (Qa2) and widen its channel. Gravel point bars and mid-channel bars (Qa3) accumulated from the eroded Qa2 sediment and are episodically moving downstream. Simultaneous erosional and aggradational processes are occurring along Grays Run, often at the same locations. The channel bars (Qa3) seem to be moving primarily in accordance with large floods, the latest being in January 1996 and September 2004. A distinct change in channel pattern (from a single channel system to a wider and shallower braided system) is occurring at the zones of channel avulsion due to tree jams and bar aggradation. The implications of this study are critical with respect to regional applications to infrastructure and stream restoration. Studies such as this imply that reach-based approaches may not be appropriate for such complex systems.