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.