The Teshio River flows northward from its source at Mt. Teshio in the Kitami Mountains, joining the Kenbuchi River, the Nayoro River, and other tributaries to flow into the Sea of Japan at Teshio Town. In the upstream area, Lake Iwaonai of the Iwaonai Dam, which is responsible for flood control and supply of water for domestic use, is filled with rich water, and unspoiled nature remains in abundance.
In the middle reaches, it flows through the countryside and joins a branch river. A monument to Takeshiro Matsuura, the godfather of Hokkaido, is also scattered around the river.
In the lower reaches, the river becomes wider and the flow slows down. In winter, the surface of the river can be seen freezing over.