The Iskar (, ; ) is a right tributary of the Danube. With a length of 368 km it is the longest river that runs entirely within Bulgaria. Originating as three forks in Balkan's highest mountain range Rila, it flows in northern direction until its confluence with the Danube River. As it flows northwards it fuels the largest artificial lake in the country, the Iskar Reservoir, forms the divide between the Vitosha and Plana Mountains in the west and the Sredna Gora mountain range in the east before entering the Sofia Valley, which contains the nation's capital Sofia. From there the Iskar runs through the Balkan Mountains forming the spectacular 84 km long Iskar Gorge and as it crosses the mountains its water course turns in north-eastern direction at Lakatnik. North of the Balkan Mountains the river crosses the Danubian Plain and finally flows into the Danube between the villages of Baykal and Gigen. Geologically, Iskar is the oldest river in the Balkan Peninsula.
Its watershed drains 8,617 km2 in the provinces of Sofia, Sofia City, Vratsa, Lovech and Pleven. The Iskar flows through nine towns and numerous villages. The Iskar river basin is home to more than 50 species of fish, including Cottus haemusi that is endemic to the upper Iskar and Vit drainages.
The Iskar has a significant economic importance, providing water for the needs of the capital and a number of manufacturing plants. It waters are also utilised for irrigation and electrical energy generation, while the scenic gorges and rock formations throughout its course provide excellent conditions for recreation. Although the Iskar is not navigable its valley is very important for transportation, especially along the Iskar Gorge, which is one of the only three passes in the Balkan Mountains crossed by a railway.
Overview
The Iskar is 368 km long, which makes it the longest river that runs entirely within the territory of Bulgaria. The Danube and the Maritsa rivers are both longer but they also flow through other countries. The Iskar is a tributary to the Danube and forms part of the Black Sea river basin. It is the only river that takes its source in southern Bulgaria and flows north through the Balkan Mountains that form the water divide between the basins of the Black Sea to the north and the Aegean Sea to the south. In terms of geology, the Iskar is the oldest river in the Balkans and also the only one to have preserved its original direction despite the significant geological changes in later stages. The Iskar basin spans a territory of 8,646 km2 in six Bulgarian provinces, Sofia, Sofia City, Vratsa, Lovech and Pleven, with a total population of about 1,600,000 people. The most significant right tributaries from south to north are the Lesnovska River (65 km), Malki Iskar (85 km) and Zlatna Panega (50 km); the largest left tributaries are the Palakaria (39 km), Vitoshka Bistritsa (12 km) and Perlovska (31 km). Its average discharge at the mouth is .
Course
Upper course
The Iskar headwaters consist of three forks in the northern slopes of Rila, Bulgaria's highest mountain range. The 23 km long Cherni (black) Iskar is considered the main stem of the river. It takes its source from the Chamovsko Lake (2,500 m) to the north-east of the Damga Peak (2,669 m) and from there the river flows in north-eastern direction through the Govedartsi Valley, fuelled by the waters of the Chanakgyolski, Malyovishki and Urdini Lakes. Cherni Iskar's discharge at Goverdartsi is 1.60 m3/s; its basin covers 237 km2. Beli (white) Iskar is the longest of the three with a length of 28 km and its springs are at the northern slopes of the Redzhepitsa Peak (2,677 m). It forms a deep and steep valley and has a basin of 91 km2. One of its tributaries, the Bistritsa, takes its beginning from the Musala Lakes under the slopes of Musala (2,925 m), the highest summit of the Balkan Peninsula. In its upper course at an altitude of 1,900 m is located the Beli Iskar Reservoir, constructed between 1939 and 1945, that powers a small 16 MW hydro power station. Levi (left) Iskar is the smallest of the three and has its origins at the northern slopes of the Marinkyovistsa Peak (2,636 m). It flows through a deep valley for 18 km until its confluence with Cherni Iskar downstream of the village Mala Tsarkva and has a watershed of 56 km2.