Yenisei (yĕnĭsāˈ, Rus. yĕnyĭsyāˈ) [key], chief river of Siberia, c.2,500 mi (4,020 km) long, central Siberian Russia. It is formed at Kyzyl, Tuva Republic, by the junction of the Bolshoi Yenisei and Maly Yenisei rivers, which rise in the E Sayan Mts. along the Russian–Mongolian border. It flows westward, then generally north, past Minusinsk, Krasnoyarsk, Yeniseisk, and Igarka to enter the Kara Sea through a c.250 mi (400 km) long estuary composed of Yenisei Bay and Yenisei Gulf. The Angara, Stony Tunguska, and Lower Tunguska rivers are the Yenisei's chief tributaries. The river is frozen during the winter months. In the spring ice in the upper Yenisei melts before that in the lower river, causing extensive flooding as water backs up behind the frozen portion of the river. The Yenisei's upper course is turbulent, with many rapids, and has a great hydroelectric generating potential; there are giant hydroelectric stations at Krasnoyarsk and at Sayanogorsk. The river's middle course widens and is navigable for steamers. Lumber, grain, and construction materials are transported along the Yenisei. Igarka on the lower river is the region's chief lumber-loading port. There is fishing for sturgeon and salmon in the river's lower reaches.