Migration and spawning phenology of Pacific Salmon is linked to the hydroregime, and thought to be triggered by increases in river discharge and decreases in water temperature. However, little data exists that describes direct fine-scale linkages between the hydroregime and spawning in Coho Salmon. This study evaluated the spatial and interannual variability in the relationships among streamflow, water temperature, and the spawning timing of coastal Coho Salmon returning to three tributaries of the Smith River. Generalized linear mixed models indicated that water temperature and several discharge metrics, including annual cumulative discharge, cumulative discharge since the last site visit, and daily discharge were significant both at the regional scale and at the individual sub-basin scale. However, the relative importance of the discharge and water temperature parameters differed, depending on the year and the sub-basin. Furthermore, a majority of spawning tracked closely with the 25th percentile of cumulative annual discharge. These relationships between hydroregime and Coho salmon spawning timing will likely be sensitive, at an individual sub-basin scale, to the effects of climate change. It will be important to continue monitoring and studying these relationships, to support management of these endangered populations for resilience and sustainability in the long-term.