Summer Use of Side-Channel Thermal Refugia by Salmonids in the North Fork Coeur d'Alene River, Idaho

We observed aggregations of westslope cutthroat trout Oncorhynchus clarkii lewisi, mountain whitefish Prosopium williamsoni, and to a lesser extent, rainbow trout Oncorhynchus mykiss moving into cold side channels in the North Fork Coeur d'Alene River in northern Idaho during the summer of 2007. The use of cold side channels varied throughout the season, as well as by size-class. Such use was much greater for salmonids 300 mm or more in length, and the movement of salmonids into side channel habitats appeared to be triggered by rising water temperatures in the main river and temperature divergences between the main river and the side channels. Larger salmonids were not evenly distributed among the side channels, and channel physical features varied widely. The probability that a side channel would be used as a thermal refugium by salmonids 300 mm or more appeared to be influenced by channel depth, and salmonids of that size used side channels that were both deep (>= 2 m) and cold (<= 20 degrees C). Protection of the remaining hyporheic flow paths and thermal refugia may be important for the conservation of summer habitat for westslope cutthroat trout in the lower North Fork Coeur d'Alene River.