Sava (säˈvä) [key], Hung. Száva, c.580 mi (930 km) long, rising in two headstreams in the Julian Alps, Slovenia, and flowing generally SE past Ljubljana and Zagreb, Croatia, then forming part of the border between Croatia and Bosnia and Herzegovina, and through Serbia to the Danube River at Belgrade. The Drina is its chief tributary. The Sava is navigable to Sisak; its valley is an important transportation corridor. The Sava basin is a fertile agricultural region.