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Intermittent streams lose surface flow during part of the year but can provide important habitat for imperiled fishes in residual pools. However, extended intermittency can drive high mortality as pool contraction decreases pool quality, and some pools dry completely. We evaluated the influence of a suite of abiotic habitat characteristics on the over-summer survival of two imperiled salmonid fishes (coho salmon Oncorhynchus kisutch; steelhead trout Oncorhynchus mykiss) at four study sites on two tributaries of Salmon Creek (Sonoma County, California, USA) from 2012 to 2014, during deepening drought conditions. Study sites spanned an intermittency gradient from continuous flow to near-dry conditions, and included alluvial and bedrock stream reaches. We estimated over-summer survival at the pool scale from fish presence-absence data based on paired early-late summer snorkel surveys. We measured pool dimensions and water quality parameters monthly (more frequently during summer dry down) and, in 2013 and 2014, recorded water quality with continuous loggers in selected pools. We performed: (1) logistic regression in a generalized linear modeling framework to identify factors limiting over-summer survival and (2) classification trees using the random forests ensemble learning method to identify abiotic thresholds for sustaining sal-monids. Results suggested that different factors governed mortality of the two species. Coho salmon, which tended to survive in large, deep pools, were limited by minimum dissolved oxygen (DO) concentrations. In contrast, steelhead trout, which tended to survive in pools with large surface area, were sensitive to pool geometry and temperature. Both species persisted for weeks in large pools with low DO levels, including in pools where at least part of the water column reached sublethal or lethal levels. Our results suggest that shallow, lateral hyporheic flow may be important for maintaining DO and temperatures suitable for sustaining salmonids in isolated pools, whereas groundwater discharge originating from deeper flow paths may generate low-DO conditions that inhibit salmonid persistence. Geomorphically complex watersheds with a variety of pool geometries and high rates of lateral hyporheic exchange are those most likely to serve as "sanctuary reaches" for imperiled Pacific salmonid populations in semi-arid regions in the context of a changing climate.