TOPOGRAPHIC AND COLLUVIAL SIGNATURES OF LITHOLOGY, BASE LEVEL, AND CLIMATE IN THE SHAVERS CREEK WATERSHED, PENNSYLVANIA

Integrative models of critical zone processes aim to provide a framework for interpreting the climatic and tectonic histories of landscapes over geologic time, and for predicting landscape and ecosystem response to short-term changes in climate and land use. At the Susquehanna Shale Hills Critical Zone Observatory in central Pennsylvania, the overprinting of structural, lithologic, climatic, and base level controls on landscape form present both challenges and opportunities for testing such critical zone models. Here we present preliminary geomorphic analyses of the Shavers Creek watershed (164 km²), focusing on a comparison of the recently instrumented Garner Run sub-catchment (1 km²; predominately underlain by sandstone bedrock) and the Shale Hills sub-catchment (0.1 km²; underlain by shale bedrock). Using a combination of lidar topographic analysis and detailed field mapping, we note three main observations that merit further research. First, hillslopes at Garner Run are less steep than at Shale Hills, in contrast to expectations of stronger rock supporting steeper topography. We hypothesize that this is due to the isolation of the Garner Run sub-catchment above a prominent stream knickpoint, and that local baselevel fall may be up to three to four times lower than in Shale Hills. Second, there is a strong imprint of Pleistocene periglacial processes at the Garner Run sub-catchment, both in the topographic expression of solifluction lobes, and in the extensive coarse grained colluvium and boulder fields that cover hillslopes. On these hillslopes, we find aspect and slope-dependent variations in the surface texture and grain size of colluvial material that likely impart a strong control on patterns of infiltration, runoff, and vegetation. Third, throughout Shavers Creek there is a strong lithologic control on drainage density and hillslope length; hillslopes underlain by sandstone are five to ten times as long as shale hillslopes, potentially as a result of the changes in hillslope hydrology described above.