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a river in Kalinin Oblast, RSFSR; a left tributary of the Volga. The Tvertsa is 188 km long and drains an area of 6,510 sq km. Its source is generally accepted to be the mouth of the Starotveretskii Canal (2.9 km), which connects the Tvertsa with the Vyshnii Volochek Reservoir (1703–09). From the reservoir, the Tvertsa draws between 75 and 80 percent of the runoff of the Tsna River. The Tvertsa is fed primarily by snow. The mean flow rate 40 km from the mouth is 60 cu m per sec. The river freezes in November, December, or early January and opens up in later March or April. It flows through a system of locks and is part of the Vyshnii Volok Water System. Located on the Tvertsa are the cities of Torzhok and Kalinin—the latter at the river’s mouth.