The source of the Sendai River is in the vicinity of Okinosen () and Mount Nagi (), deep in the Chūgoku Mountains on the border of Tottori and Okayama prefectures. Several tributaries of the Sendai similarly emerge from the Chūgoku Mountains in this area, and form a fan-shaped area before flowing into the main course of the Sendai River. The region of gorges and ravines is commonly known as by residents of the area. The upper reaches of the Sendai and its tributaries is protected as part of Hyōnosen-Ushiroyama-Nagisan Quasi-National Park (), established in 1969.