a river in Cheliabinsk Oblast, RSFSR, with the upper course in the Bashkir ASSR. The Ui, which partly flows along the border between the Kazakh SSR and Cheliabinsk and Kurgan oblasts, is a left tributary of the Tobol River of the Ob’ River basin. It is 462 km long and drains an area of 34,400 sq km. It rises on the eastern slopes of the southern Urals and flows first across the Trans-Ural Plateau and then, in the lower course, across the Western Siberian Plain. Its basin includes many lakes with no outlets. The river is fed mainly by snow. The mean flow rate 213 km from the mouth is approximately 13 cu m per sec. The Ui freezes in November and opens up in April. It provides water for drinking and irrigation. The city of Troitsk is situated on the river. There are three reservoirs along the Ui, the largest of which is the Troitsk Reservoir, with an area of 10.8 sq km.

a river in Novosibirsk and Omsk Oblasts, RSFSR; a right tributary of the Irtysh River of the Ob’ River basin. The Ui, which rises on the Vasiugan’e Plain, is 387 km long and drains an area of 26,700 sq km. It is fed primarily by snow. The high-water period lasts from April to June. The mean flow rate 48 km from the mouth is approximately 24 cu m per sec. The river freezes in late October or early November and opens up in April or early May.