A Thermally Stratified Reservoir Module for Large-Scale Distributed Stream Temperature Models With Application in the Tennessee River Basin

River temperatures affect water quality, power plant cooling, and freshwater ecosystems. Stream temperature models that treat river reaches and reservoirs as well-mixed segments do not capture thermal stratification in reservoirs. To account for the effects of reservoir stratification on downstream water temperatures, we developed a two-layer stratified reservoir module, which was integrated into the River Basin Model (RBM) to simulate river temperature across a river network with multiple large thermally stratified reservoirs. To evaluate the performance of this model configuration compared to RBM without thermally stratified reservoirs, we simulated river temperature in the Tennessee River Basin in the southeastern United States. We simulated land surface hydrologic fluxes using the Variable Infiltration Capacity (VIC) model and routed runoff using the river routing model RVIC. The two-layer model configuration reduced the bias in simulated summer river temperature from 6.7 to −1.2 °C downstream of a reservoir with a residence time of 92 days and from 3.0 to −0.7 °C downstream of a reservoir with a residence time of 8 days. Improvement in fall and winter, when reservoirs tend to be well mixed, is minimal. RBM with the two-layer module also captured the observed longitudinal river temperature variation downstream of a reservoir, with cool temperatures immediately downstream of the reservoir and gradual warming of the river as it flows downstream. Incorporating a simple stratified reservoir module into RBM improves model performance and increases the ability to apply the river temperature model to large basins with multiple large reservoirs.