IMPRINTS OF PLEISTOCENE GLACIATION ON STREAM MORPHOLOGY IN NORTHWESTERN PENNSYLVANIA

Northwestern Pennsylvania provides an opportunity to study the legacy of Pleistocene glaciation on stream morphology. The region was extensively glaciated at the LGM and the drainage divide between the Allegany and St. Lawrence Rivers occurs along an escarpment between the Appalachian Plateau and the Great Lakes Plain, ~20 km south of Lake Erie. The divide creates different base levels for streams flowing north into Lake Erie and streams flowing south into the Allegany River. In this preliminary study, two similarly sized streams (basin areas ~ 250 km²) were analyzed: the north-flowing Elk Creek and the adjacent south-flowing Cussewago Creek. In this setting, base level is a primary driver of stream morphologic differences because the basins have similar climate, vegetation, and bedrock. Slope, sinuosity, and terraces were extracted from topographic maps, aerial photographs, and ~1 m DEMs derived from airborne LiDAR. Base level on Cussewago Creek is set by the Allegheny River, which has been relatively stable since the LGM. The average channel slope and sinuosity on Cussewago Creek is 0.0035 and 1.2720, respectively. On Elk Creek, the average channel slope is steeper and less sinuous than Cussewago Creek, 0.0049 and 1.116, respectively. The base level of Elk Creek, Lake Erie, has fluctuated dramatically since deglaciation because of multiple glacial and post glacial lake stages caused by retreating ice and isostatic rebound. Net base level lowering on Elk Creek has created stream disequilibrium and subsequent channel incision into glaciolacustrine sediment and sedimentary bedrock. Incision has produced a convex stream profile that is significantly steeper (p < 0.01) than Cussewago Creek's concave profile. The steep channels result in greater unit stream power and more erosion, which is consistent with the observation of many bedrock reaches. Elk Creek has incised 36 m since the LGM and five terraces are preserved. The two highest terraces, 33.0 and 30.5 m above the current channel, are associated with Glacial Lake Warren stages that occurred ~15.5 ka. The lowest terrace, 3 m above the current channel, correlates with the Lake Erie high stage, ~5.5 ka. Although most incision occurred in the early and middle Holocene, steep bedrock channels and a convex stream profile suggest that Elk Creek has not yet achieved equilibrium.