The river environment has the possibility of great change due to the climate change by global warming as a topical issue. It has a close connection to the territory and the native habitat of the living things. The water temperature in a river is one of the important indexes related to the river ecosystem. The global warming has influenced the distribution of the water temperature because the water temperature in the river depends on the air temperature. It is necessary to know depth and the flowmass in the river for the analysis of the water temperature. In this research, a hydrological model and a heat budget model were combined and the water temperature in the rivers of the Natori basin was simulated. The heat budget method was used for the simulation of the water temperature. The degree of the temperature rise in the Hirose River in the simulation on 6th is larger than those on other two dates. This is because there was the relatively little flowmass by few river joins in the whole basin and the water depth in the river was underestimated during the drought by the assumption of constant width of river.