river, c.425 mi (680 km) long, rising in many branches in the Sawtooth and the Salmon River mts., central Idaho. It flows northeast and is joined, at Salmon, by the Lemhi River, after which it flows west and is joined by the Middle Fork and the South Fork, then goes north to join the Snake River. The river's canyon, c.1 mi (1.6 km) deep and 10 mi (16.1 km) wide in some places, threads through a wilderness preserve. In 1935 a party sponsored by the National Geographic Society explored the canyon. Though the swift waters and rapids are navigable downstream, it is impossible to return by the water route, thus giving the Salmon the name River of No Return. Salmon travel up the river to spawn.