Iskŭr River, longest (after the Danube) river in Bulgaria, formed south of Samokov in the Rila Mountains by its headstreams, the Beli (White) Iskŭr and Cherni (Black) Iskŭr. It cuts a 40-mile (65-km) gorge through the Balkan Mountains to bring the high basin of Sofia (1,800 feet [550 metres]) into communication with the Danube tableland. The river empties into the Danube about 20 miles west of Nikopol after a course of 250 miles (402 km). Along its lower course, southeast of Sofia, is a chain of small hydro stations associated with the Iskŭr Reservoir. The Malŭk Iskŭr and Panega are tributaries.