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 doi.org/10.1002/tafs.10259
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.