The Sava is a river in Southeast Europe, a right side tributary of the Danube. It flows through Slovenia, Croatia, along the northern border of Bosnia and Herzegovina, through Serbia, discharging into the Danube in Belgrade. Its central part is a natural border of Bosnia-Herzegovina and Croatia. The Sava forms the northern border of the Balkan Peninsula, and southern edge of the Pannonian Plain.The Sava is 990 kilometres long, including 45-kilometre Sava Dolinka headwater rising in Zelenci, Slovenia. It is the greatest tributary of the Danube by volume of water, and second-largest after Tisza in terms of catchment area (97,713 square kilometres) and length. It drains a significant portion of the Dinaric Alps region, through the major tributaries of Drina, Bosna, Kupa, Una, Vrbas, Lonja, Kolubara, Bosut and Krka. The Sava is one of the longest rivers in Europe and among a handful of European rivers of that length that do not drain directly into a sea.The population in the Sava River basin is estimated at 8,176,000, and it connects three state capitals—Ljubljana, Zagreb and Belgrade. The Sava is navigable for larger vessels from the confluence of Kupa River in Sisak, Croatia, approximately two-thirds of its length.