In recent years, there has been continued interest in the utilization of river discharge as a new energy source. Before this can be achieved, it is essential to examine the stability of the flow rate. In Korea, however, there are no guidelines for calculating the water availability of river water. Therefore, this paper proposes a method of estimating the water availability of river water based on the permissible standard flow rate according to the permission system of river water in Korea, and the amount of domestic river water available is estimated. In addition, the area where sufficient hydrothermal energy can be taken is analyzed considering the heat pump installation conditions of the river water. These results can be utilized as an indicator for evaluating river water hydrothermal energy as an alternative to fossil fuels and the contribution of river water to greenhouse gas reduction.