Predicting the Spatial and Temporal Distribution of Wetted Habitats in Intermittent Streams and its Implications for Threatened Fish Populations

In California, where intermittent streams make up much of the river network, insufficient summer streamflow is often the bottleneck for imperiled species survival, including that of salmon at the southern end of their range. During dry periods, these organisms depend on persistent wetted reaches and isolated pools as their only refuge, from which they may recolonize surrounding habitats upon rewetting. Given the importance of intermittent streams in sustaining populations of imperiled species, an improved understanding of drying dynamics and controls on habitat suitability at the river network scale is required. As the dry season progresses, local variation in geology and geomorphology can become more important than regional climate forcing in influencing flow conditions, yet the hydrologic processes that control the persistence of individual pools remains poorly understood. This study aims to improve understanding of the spatio-temporal controls on stream habitat suitability for endangered Coho salmon (Onchorynchus kisutch) and threatened steelhead trout (O. mykiss) in the intermittent tributaries of the Russian River watershed. Using data from 25 streams, for which at least 3 years of historical summer wetted habitat surveys were available, in combination with antecedent climatic data and landscape-level variables, we developed a statistical model to simulate and predict the degree and distribution of stream drying at the reach scale at multiple sites. Our findings show that important predictor variables include distance along the river from the mouth, late-season precipitation, and canopy cover, indicating that both hydrologic and geomorphic forcing interact variably to influence streamflow permanence. This research helps to advance understanding of the controls on intermittent stream habitats and can be used to inform management and guide conservation action for threatened freshwater species.