GEOLOGIC AND HYDROLOGIC CONTROLS ON SOUTHERN STEELHEAD TROUT HABITAT IN THE SANTA MONICA MOUNTAINS

A common challenge in river restoration is the assessment of anadromous fish habitat over broad spatial scales, such as small coastal mountain ranges. In the semi-arid region of southern California, this frequently requires estimation of both the storm flow produced by a given watershed as well as the baseflow provided during the summer low-flow months. However, prediction of the watershed response to a given rain storm is often difficult due to the lack of gauged basins in many mountain streams. The primary goal of the present study was to identify which basins in the Santa Monica Mountains (SMM) were most capable of supporting steelhead trout populations, in terms of adequate runoff and baseflow. This was achieved through 1) an analysis of the relation between baseflow and geology and 2) modeled predictions of rainfall-runoff relations between 15 small, coastal watersheds.

In the SMM, the geology was found to be an important factor in enhancing the presence of summer low flow habitat, which is perhaps the major limiting factor to steelhead survival in southern California streams. Where aquifers are present and groundwater is forced to the surface due to the existence of steep faults, seeps and springs are more common. The presence of rocks with low hydraulic conductivity and an absence of faulting appear to lead to little or no baseflow. On a regional scale, the eastern portion of the SMM likely offers a higher potential for summer low flow due to the more favorable geology. Modeling results indicate that both peak and volumetric disharge were both tightly coupled with the drainage area, with larger basins in general producing higher flows that were sustained over longer time periods. Both a strong precipitation gradient and favorable geology contribute to creating better habitat in the eastern portion of the SMM. The results of this analysis highlight the importance of considering both hydrologic and geologic factors in stream restoration studies, especially when assessing fish habitat over a regional scale. Such an approach can provide the framework for a decision-making process that can focus restoration efforts towards those watersheds that are most likely to sustain trout populations.