Novel Dimensionless Index For Rapid Assessment of Thermal Refugia Characterizes Off-Channel Habitat On Gravel Bed River

This study presents a novel dimensionless index for rapid assessment of thermal refugia on the upper Willamette River, Oregon. In the Willamette River, main channel temperatures can be too warm for cold water fishes, causing fish to concentrate in secondary channel features that provide thermal refugia. Temperatures vary among and within features, and improved understanding of physical processes controlling thermal regimes is needed. In turn, this study developed the Hyporheic Insolation (HIN) index, which can predict and thereby classify thermal refugia based on minimal field data. We took continuous water temperature measurements at one side channel, eight alcoves, and six beaver ponds provided data to ground truth HIN index predictions. HIN index calculations require stratification in the water column, which we calculated using the dimensionless Richardson number. At our 13 stratified sites, calculation of the novel HIN index characterized the ratio of cooling flux from hyporheic discharge to heat flux from incoming solar radiation. As the HIN index increased, measured temperatures at sites decreased, showing a bin-averaged logarithmic fit (R2=0.97). Calculation of the Richardson number and the HIN index indicate that secondary channel features characterized by stratification and cool hyporheic discharge can provide thermal refugia. The new HIN index provides a tool to practitioners that is fast, simple, and accessible, and guides restoration in a more quantifiable direction.