CATASTROPHIC FLOODING IN NORTH-CENTRAL PENNSYLVANIA: GEOMORPHIC FINDINGS FROM THE SEPTEMBER 2011 EVENT AND THEIR SIGNIFICANCE TO SEDIMENTOLOGY

Tropical Storm Lee produced 25-36 cm of rainfall in north-central Pennsylvania in September, 2011. Loyalsock, Muncy, and Fishing Creeks experienced catastrophic flooding resulting in avulsions, scour of chutes, and deposition/reworking of chute, mid-channel, and point bars. Sampling the largest grain size of gravel bars on each stream reveals downstream fining of resistant sandstone clasts. Differences in downstream trends in grain size are observed between streams, but a strict downstream fining trend does not occur. Inputs from bedrock outcrops, tributaries, and erosion of Pleistocene outwash terraces may explain variations in grain size along streams incised into the Appalachian Plateau. Sedimentological observations of the 2011 flood gravels suggest that these streams experienced hyperconcentrated flow during most of the flood. Surficial clasts on gravel bars are imbricated, but the lack of imbrication and high matrix content of sediments at depth suggests that surface imbrication of the largest clasts took place during the flood peak, armoring the bars. The imbricated clasts on the surface are the largest observed within the bars. Locally, these gravels form knickpoints that will not be disturbed until a flood of similar magnitude occurs. Analysis of bar surface gravels shows variation in grain size and imbrication. While there is little change in the largest clasts on individual point bars with distance downstream along the channel axis, variation is observed transverse to the channel axis across bar surfaces. The largest clasts are often concentrated near the active channel or in localized pockets. Significant fining occurs down the axis of chutes in several locations.

USGS estimates for the flood recurrence interval average 120 years on these streams. Geomorphic observations, however, show that chutes scoured into early Holocene and Pleistocene deposits in several locations. This reworking of long undisturbed sediment suggests that a recurrence of thousands of years is possible. Dendrogeomorphic observations, however, provide evidence of major geomorphic change during the past 100 years. Disequilibrium caused by the recent influx of large gravels into these streams, which appears be related to historic logging, may be a trigger of frequent avulsion and scour during less catastrophic floods.