Quantifying the effects of stream discharge on summer river temperature

Control of summer river temperature is needed for maintaining water temperature standards to protect aquatic biota and wildlife habitats. Given the fact that instream discharge, among meteorological and hydrological factors, may be the only one that can be practically managed, is it feasible to moderate summer river temperature through reservoir and streamflow regulations? An analysis is conducted to quantify the effects of the magnitude of instream flow on summer river temperature with weather as a reference. Relationships between water temperature and river discharge or flow depth are developed using a simplified model and adopting the concept of equilibrium temperature and bulk surface heat exchange coefficient. The relationships are validated against continuous 5-year field measurements at the central Platte River, Nebraska, USA. It was found that the variation of daily maximum water temperature with now was stronger than that of daily mean. A critical discharge was obtained, which divides dramatic drop and slow variation in river temperature values. The existence of the critical discharge makes it possible to reduce or minimize the occurrence of daily maximum water temperature exceeding a standard at a river reach by increasing discharge to an achievable level. This study advances understanding of impacts of instream flow on summer river temperature and provides information useful in proper planning and design of reservoir operations and streamflow management.