By Catherine Corbett, Lower Columbia Estuary Partnership
The importance of cold water refuges to Pacific salmon and steelhead migrating through the Columbia River Basin recently has been well documented. Summertime water temperatures in the mainstem Columbia River have steadily increased over the last few decades, as has the length of these warm periods. Annual peak temperatures routinely exceed 21°C in most years and have been as high as 24°C. The warmest period typically occurs in July to early September, coincident with late-migrating summer Chinook and sockeye salmon and substantial portions of the fall Chinook salmon and summer steelhead runs. Water temperatures in the 19-22°C range are a significant concern because these temperatures can cause behavioral changes and a variety of sub-lethal effects on physiology, disease susceptibility, reproduction, and survival.
Since 2015 the Estuary Partnership has been filling data gaps of cold water refuge locations in the lower Columbia River by assessing the extent, magnitude, and temperatures of cold water plumes at tributary confluences and within the mainstem. Many adult salmonids temporarily use such thermal refuges when mainstem Columbia water temperatures are high. The University of Idaho with the USACE and NOAA National Marine Fisheries Service identified a series of cold water refuges located along the migration corridor at tributary confluences with the mainstem river between Bonneville and John Day dams, where cold-water tributaries draining the Cascade Range enter reservoirs. Those most used by adult salmon (e.g., up to 70% of adult steelhead) among these include Herman Creek and the Little White Salmon, White Salmon, and Deschutes Rivers. These plumes are often 2-7 °C cooler than the mainstem during summer months, and are adequately sized and protected from mainstem currents by structures or topography, so as to be detectable by migrating salmonids. This research did not extend below Bonneville Dam, so we addressed this spatial data gap in 2015 and 2016. Results showed that some tributary confluences, mostly in the lower Gorge area, had summer water temperatures at least 2°C lower than the mainstem. However, plume (i.e., cold water refuge) formation at mainstem confluences was limited or non-existent due to the very small size of these streams relative to the mainstem river. Cold water exiting the tributaries immediately sinks or is diluted by mainstem flows. To address this problem, we are assessing the feasibility of techniques that enhance topography at lower Columbia tributary confluences in order to facilitate the formation of protected pockets of cold water. We are applying a hydrodynamic model at three “pilot” tributary confluences in the lower Columbia Gorge to assess potential enhancement techniques that could increase the extent and retention of cold tributary water at these locations. Potential techniques include placement of diversion structures such as large wood to deflect mainstem river flows and encourage eddy formation, as well as modification of existing topography to create larger, isolated pockets of cold water.
The information is carefully vetted with regional experts, at forums including the Estuary Partnership Science Work Group, the Columbia River Estuary Conference, and others, and results are widely disseminated so others can use the results in habitat restoration design or species recovery planning. If interested in learning more, please contact the Estuary Partnership.
