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a river in Bulgaria, a right tributary of the Danube. Length, 368 km; basin area, 8,600 sq km.
The river rises in the Rila Mountains. In its upper reaches it flows through the Samokov and Sofia basins, then intersects the Stara Planina (Balkan Mountains) via the Isker Gorge and in its lower course traverses the Walachian Plain. High water is in spring, low water in summer. The average rate of flow at the mouth is 54 cu m per sec, and the maximum, in years of abundant flow, is 800 cu m per sec. The basin of the Isker contains reservoirs and hydroelectric power plants. Situated on the Isker are the cities of Samokov, Mezdra, and Cherven-Bryag; the river also flows through the eastern outskirts of the Bulgarian capital, Sofia.