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a river in the Mordovian ASSR and Gorky Oblast, RSFSR, a left tributary of the Sura (Volga River basin). The P’iana is 436 km long and drains an area of 8,060 sq km. It is noted for its extreme meandering. Karst caves and sinkholes are frequent along the riverbanks. The river is fed primarily by snow. Its mean flow rate is 25 cu m per sec, with a maximum flow rate of 1,500 cu m per sec and a minimum flow rate of 10–12 cu m per sec. The P’iana freezes in November, and the ice breaks up in April. The lower course is navigable. The city of Sergach is on the P’iana.