Oka
an important navigable river of central Russia, the principle affluent of the Volga from the S., rises in the government of Orel, and flows in a generally N. E. direction, and joins the Volga at Nijni-Novgorod, after a course of 706 miles. Its basin comprises the richest and most fertile region of Russia.
The principal towns on its banks are Orel, Bielev, Kaluga, Riazan, and Muron; the most important affluents are the rivers Moscow, Kliasma, and Tzna.
During spring the Oka is navigable from Orel to the Volga; but in summer the navigation is obstructed by sandbanks.