river flowing through Moscow oblast (province) and part of Smolensk oblast, in western Russia. It is a left-bank tributary of the Oka River in the Volga basin. Rising in the Smolensk-Moscow Upland, the river flows 312 mi (502 km) in a southeasterly direction and drains an area of 6,800 sq mi (17,600 sq km). It is an important source of Moscow's water supply. Major cities along the river include Moscow, Mozhaysk, Zvenigorod, Zhukovsky, Bronnitsy, Voskresensk, and Kolomna. Its principal tributaries are the Kuza, Istra, Pakhra, and Severka.