Sava River, a river in the Balkan Peninsula. It is a principal tributary of the Danube. From its source in the Julian Alps in Slovenia, the Sava flows southeastward for about 585 miles (940 km); through northern Croatia, along part of Bosnia-Herzegovina's northern border, and into Serbia, where it joins the Danube at Belgrade. The Sava is navigable for about two-thirds of its course. Its main tributaries are the Kupa, Una, Vrbas, Bosna, and Drina rivers.