Mechanics of the energy balance in large lowland rivers, and why the bed matters

Along many rivers, dams trap sediment and water released from the dams is clear. Downstream of the dam, temperature variability along the river is controlled by climate that warms or cools the water, the flow magnitude, and the spectral properties of water and the river's bed. Using field observations, a synoptic numerical model without calibration couples a full‐spectrum radiation balance with turbulent heat fluxes, bed conduction, and a hydraulic model that estimates depth and velocity. We show that variations in the river's temperature are sensitive to the albedo of the sediment on the bed, especially at shallow depths and smaller discharges. However, about half the solar radiation lies in a spectral range where water is highly absorptive; in these wavelengths, absorption is independent of depth. In spring and summer with many hours of sunlight, releases of cold water will have limited influence on temperatures beyond tens of kilometers downstream of a dam. Key Points A full‐spectrum fluvial energy balance model shows river temperature depends on depth, the spectral distribution of sunlight, and bed albedo Sand to gravel variations in bed sediment albedo can alter the energy balance and river temperature by up to 2°C Plausible releases of cold water from a dam cool the river for no more than tens of kilometers downstream