EXPLORING HYDROLOGIC PROCESSES AND THE WEATHERING FRONT UNDER A STEEP, RAPIDLY ERODING HILLSLOPE: RESULTS OF DRILLING AND GEOPHYSICAL INVESTIGATIONS

Observations from roadcuts and a small number of field studies suggest that near-surface bedrock underlying hillslopes is commonly highly fractured, with fracture openings and density greatest near the surface. The development of this weathered front into the underlying fresh bedrock creates subsurface flow paths and a seasonal water storage zone that may strongly influence runoff generation, slope stability, and water availability to plants. At an intensively monitored hillslope in the Angelo Coast Range Reserve along the Eel River in Northern California we are exploring the use of geophysical tools to reveal how this subsurface domain is structured, evolves and influences hydrologic and geomorphic processes. The 4000 m² catchment has a mean slope of 30 degrees and is underlain by nearly vertically bedded Franciscan Coastal Belt argillite. A thin colluvial soil mantles the weathered bedrock. Average rainfall is about 2 m with nearly all of it occurring between November and April. Elder Creek, into which the hillslope drains, is eroding at about 0. 2 mm/year which is comparable to the estimated uplift. In 2007, 7 deep wells up to 30 m deep were drilled across the hillslope and water levels have been continuously monitored since then. Hundreds of devices for monitoring soil moisture, sap flow, and climate attributes have been installed throughout the catchment. Drilling using standard blow count testing revealed an abrupt transition to very resistant, unoxidized fresh bedrock at depth. This front varied systematically from 4 m beneath the surface at the toe to 16 m depth 100 m upslope near the divide. All rain that is not returned to the atmosphere through evaporation or transpiration passes through the soil into the underlying fractured bedrock and then travels laterally to the bedrock floored Elder Creek at the hillslope base. The water table varies significantly both seasonally and in response to individual storms. We have conducted 5 time-lapse electrical resistivity tomography surveys along two 100 m long profiles along and across the hillslope. In addition, we have monitored moisture content dynamics in the 7 wells using a neutron probe. Taken together, these data reveal a heterogeneous but spatially structured pattern of moisture dynamics that we propose reflects the underlying pattern of bedrock fracturing.