Environmental flow needs assessment for salmonids in the Coldwater River

Climate change is driving drought impacts on salmon in the Coldwater River, a major tributary of the Nicola River. Determining specific flow optima for salmon is therefore important for water managers seeking to balance anthropogenic and ecosystem water needs. We conducted a habitat suitability-based environmental flow needs (EFN) assessment in the Coldwater River between July and September, 2024. We also analyzed over 50 years of streamflow data to frame environmental flows in the context of accelerating climate change. Our study produced curves that modelled the relationship between flows and Weighted Useable Width (WUW)–a weighted metric for fish habitat–for spawning Chinook and coho salmon, spawning steelhead, and aquatic macroinvertebrate production. WUW for spawning coho was maximized at a discharge of 4.88 m3/s, 5.49 m3/s for spawning Chinook, and 6.58 m3/s for spawning steelhead. WUW for aquatic macroinvertebrate production was maximized at 5.61 m3/s. These flow values have not been attained in the Coldwater River in the summer for some time and are unlikely to be reached without a comprehensive overhaul of water management and land use practices in the basin. Our analysis of historical streamflow revealed clear hydrological shifts in the watershed indicative of climate change. Median monthly streamflow during the summer (July-September), and median freshet flows (May-June) have declined dramatically since 1960. In this new era of water scarcity, decision-makers must adapt with proactive, watershed-scale policies to protect streamflows during the summer, or risk facilitating the extirpation of irreplaceable wild salmon populations.