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a river in Moscow and Kaluga oblasts, RSFSR, a left tributary of the Oka. It is 282 km long and drains an area of 4,620 sq km. The river originates in the Moscow Upland and is fed primarily by snow. High water is in April and May. The mean flow rate is approximately 25 cu m per sec, with a maximum of 800 cu m per sec and a minimum of 5–6 cu m per sec. The Protva freezes in early December, and the ice breaks up in early April. Its main tributary is the Luzha. The cities of Vereia, Borovsk, and Obninsk are situated on the Protva.