We have developed a river-temperature model at the 8-digit Hydrologic Unit of Calculation (HUC) scale over the United States (~2,100 basins) to assess the vulnerability of thermoelectric power plants under a changing hydro-climate. The model updates river temperature on an hourly time-step and also provides mixing-zone extent (of the thermal plume) and cooling-violation frequency. We first present and evaluate this physically based semi-Lagrangian river temperature model. We apply meteorological data and thermal pollution from river-based power plants to historical river flow data in order to obtain estimates for river temperature and flow. The model is run with a 1 hour time step and tested initially on a 190 km river reach. The model is then further validated for 17 river stations throughout the United States. The model performs well with an R-squared of 0.94 for the 190 km river reach and a median R-squared of 0.846 among the 17 U.S. river stations. To assess the impacts of future U.S. thermoelectric power production, a set of future climate scenarios are applied to the HUC-8 networked configuration to identify regions of high vulnerability to river temperature violations. Generally speaking, greater vulnerabilities to U.S. thermoelectric power plants to climate change are widespread over southeastern U.S. as well as the Ohio and Delaware River Valleys. More localized and intense hot spots are found over the southwestern U.S. As presented the model is intended for use in integrated assessment models to represent the interdependencies between the managed water and energy systems.