An Approach for Identifying River Reaches Sensitive to Warming From Daily Solar Radiation

Solar radiation is often the largest energy input for river heat budgets. However, river temperatures are commonly interpreted using paired air and stream temperatures. These methods can be helpful in differentiating watershed characteristics of rivers on a network‐ to regional‐scale, but it is difficult to draw reach‐scaled conclusions about what is influencing high river temperatures. This study determines whether river reaches were more or less sensitive to solar radiation along the Big Hole River in southwestern Montana and discusses the effects of solar radiation on diel temperatures of river inflows (groundwater discharge, tributaries, and canal returns). Areas of groundwater discharge along the river banks were identified with a handheld thermal infrared camera along a 23‐km reach during Summer 2023 and 2024 based on characteristic cooler temperatures. Logged temperatures of the groundwater discharge sites, tributaries, and canals were compared to each other, river temperatures, air temperatures, and solar radiation. Delays in diel fluctuations of upstream and downstream river temperatures were used to develop hysteresis curves to identify river reaches that respond more quickly to daily solar radiation. River reach temperatures that demonstrated a clockwise hysteresis curve on a downstream river temperature versus upstream river temperature plot were tightly associated with measurements of solar radiation. Groundwater discharge temperatures were generally cooler than the river; however, their diel fluctuations increased as stage lowered. At some sites, groundwater discharge flowed overland as runoff and warmed from solar radiation before discharging to the river. Tributaries and canals were either cooler or warmer than the river throughout the day; however, the maximum they could increase the river temperature was 0.1°C. Identifying river reaches sensitive to solar radiation can help focus locations for preservation and/or enhancement of identified groundwater discharges to create thermal refuges for cold‐water species.