Abstract

Understanding how soil thickness and bedrock weathering vary across ridge and valley topography is needed to constrain the flowpaths of water and sediment within a landscape. Here, we investigate how soil and weathered bedrock properties vary across a ridge-valley system in the Northern California Coast Ranges where topography varies with slope aspect such that north facing slopes, which are more densely vegetated, are steeper. In this study, we use seismic refraction surveys to extend observations made in boreholes and soil pits to the hillslope scale and identify that while soils are thicker on north facing slopes, the thickness of weathered bedrock does not vary with slope aspect. We estimate the porosity of the weathered bedrock and find that it is several times the annual rainfall, indicating that water storage is not limited by the available pore space, but rather the amount of precipitation delivered. Bedding-parallel and bedding-perpendicular seismic refraction surveys reveal weathering profiles that are thickest upslope and taper downslope to channels. We do not find a clear linear scaling relationship between depth to bedrock and hillslope length, which may be due to local variation in incision rate or bedrock hydraulic conductivity. Together, these findings, which suggest that the aspect-independent weathering profile structure is a legacy of past climate and vegetation conditions and that weathering varies strongly with hillslope position, have implications for hydrologic processes across this landscape.