Oka əkä´ [key], river, c.925 mi (1,490 km) long, rising S of Orel, central European Russia. It flows N past Orel and Kaluga, E past Serpukhov, Kolomna, and Ryazan, and then NE past Murom to join the Volga River at Nizhny Novgorod. It is navigable by large vessels below Kolomna, c.550 mi (890 km) upstream, and traverses densely populated agricultural and industrial areas. Among its tributaries are the Moskva, the Klyazma, and the Moksha.
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