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a river in Bulgaria; a right tributary of the Danube. It is 314 km long and drains an area of 2,874 sq km. It originates in the Stara Planina (Balkan Mountains) and flows mainly through a deep valley. It is fed by rain and snow. High water occurs from March to June; low water occurs in summer and autumn. The river is not navigable. It is used for irrigation. The city of Lovech is situated on the Osum.