The Balkan Mountains divide Bulgaria into two nearly equal drainage systems. The larger system drains northward to the Black Sea, mainly by way of the Danube. This system includes the entire Danubian Plain and a stretch of land running 48–80 km inland from the coastline in the south. The Danube gets slightly more than 4% of its total volume from its Bulgarian tributaries. As it flows along the northern border, the Danube averages 1.6 to 2.4 km in width. The river's highest water levels usually occur during the May floods; it is frozen over an average of 40 days per year. The longest river located entirely in Bulgarian territory, the Iskar, with a length of 368 km and a catchment area of 8,640 km2, is the only Bulgarian Danubian tributary that does not rise in the Balkan Mountains. Instead, the Iskar has its origin in the Rila Mountains. It passes through Sofia's eastern suburbs and crosses the Balkan Mountains through a spectacular 65 km–long gorge. Other important tributaries of the Danube include the rivers Lom, Ogosta, Vit, Osam and Yantra. The longest river flowing directly to the Black Sea is the Kamchiya (254 km), while other rivers include the Batova, Provadiya, Aheloy, Ropotamo, Veleka and Rezovo.