Cooperative Fish and Wildlife Research Units Program: Montana Fishery
Education, Research and Technical Assistance for Managing Our Natural Resources

Ritter, T. D., A. V. Zale, G. Grisak, and M. J. Lance. 2020. Groundwater upwelling regulates thermal hydrodynamics and salmonid movements during high temperature events at a montane tributary confluence. Transactions of the American Fisheries Society


The Smith River is a popular recreational sport fishery in western Montana, but salmonid abundances there are thought to be limited by riparian land-use alterations, irrigation water withdrawals, and high summer water temperatures. We used integrated networks of temperature loggers, PIT-tag antenna stations, and in-situ temperature mapping to investigate thermal hydrodynamics and associated movements of PIT-tagged salmonids at the confluence of Tenderfoot Creek, a major, unaltered coldwater tributary of the Smith River. Contrary to expectations, Tenderfoot Creek itself was not used as a thermal refuge by salmonids during periods of high main-stem water temperatures; rather, its cool outflow into the Smith River was used instead. Mean outflow water temperatures were 2.9 °C lower than those of the Smith River during summer and ranged from 0.5 °C to 6.1 °C less. Moreover, measured and estimated temperatures in the outflow were cooler (by up to 2.8 °C) than in Tenderfoot Creek itself at times as a result of groundwater upwelling at the confluence. Detections of PIT-tagged fish in the thermal plume increased, especially at night, when daily mean water temperatures exceeded 20°C in the main stem Smith River; 4 times as many PIT-tagged fish were detected in the plume (N = 52) than along the opposite bank (N = 12), which ostensibly afforded better cover. Coldwater tributary confluences may provide superior thermal refuges for salmonids—cooler and more secure than the tributaries themselves—when water temperatures in river main stems are stressful.