The role of large, glaciated tributaries in cooling an important Pacific salmon migration corridor: a study of the Babine River

Climate change and its associated symptoms, such as warming air temperatures, glacier retreat, reduced snowpack, and increased variability in precipitation, are warming rivers and lakes. Such warming water temperatures threaten cold-water fishes in some regions. For instance, warm water temperatures can kill or harm anadromous Pacific salmon as they migrate upstream to spawning grounds. In this study, we assessed how tributaries, and their relative watershed properties, shape temporal and spatial dynamics of temperatures during the summer months in a salmon migratory river. We focused on the mainstem of the Babine River of British Columbia, an important migratory corridor for steelhead and the five eastern Pacific salmon species, but particularly for sockeye salmon that spawn in stream reaches above the Babine Lake, at the river’s headwaters. We discovered that large glacier- and snowpack-fed tributaries cooled the Babine River by approximately 2 °C over its 96 km length. Different tributaries played different temperature functions. Cooler and more glacierized rivers were associated with a bigger change in temperature between upstream and downstream sites. Understanding how water temperatures change during adult salmon migration, especially within riverscapes on the edge of potentially harmful temperatures, can help inform proactive management options in a warming world.