GEOLOGIC CONTROLS ON SURFACE-WATER HYDROLOGY ON SANTA ROSA ISLAND, CHANNEL ISLANDS NATIONAL PARK, CA

In contrast to positive relationships between stream discharge and drainage area in humid climates, surface water on Santa Rosa Island (SRI), CA, with a Mediterranean climate, rugged topography, and complex underlying geology is spatially discontinuous. To determine potential causal mechanisms for the map patterns observed on SRI within the Channel Islands National Park, we ground-surveyed surface water characteristics at the driest time of year, after a prolonged drought (September 2014), to establish base-flow data for comparison with both bedrock and Quaternary geologic maps. Using GPS to locate features, 335 km of channel network, >2^nd order, were surveyed, identifying over 1100 surface water features including the starting and stopping points of streamflow, springs, and pools. Comparing locations of these hydrologic features with the underlying mapped geology revealed numerous associations with structure, material type, and topography: i) faults appear to act as both conduits and barriers to surface flow, ii) stream profiles aligned with dip-slope exposures of impermeable rock correlate with flow, iii) volcanic rocks and less permeable sandstones correlate with more surface water presence than fine-grained, highly fractured shale, and iv) landscapes at higher elevations that are exposed to fog during the summer months tend to be wetter. Valley floors composed of fine-grained shale are generally highly fractured, presumably inducing secondary permeability, with little correspondence with surface water. In these mostly dry, lower gradient valley floors, isolated seeps and pools were, though, located close to both active Quaternary and older faults. Somewhat laterally continuous fault-core clay gouge, and fault juxtaposition of rock types with strongly contrasting permeabilities, likely act as effective flow barriers forcing water to the surface. Lastly, valleys with a greater degree of incision and relative relief tend to reveal greater flow, possibly due to intersecting the ambient ground water table. Spatial correlations between the independent geologic and hydrologic mapping provide the opportunity to evaluate potential causal mechanisms for surface water presence on the water-limited Channel Islands and assist restoration efforts.