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|Main source||Mačva, west of Bogatić, Serbia|
|River mouth||Sava river, at Sremska Mitrovica, Vojvodina, Serbia|
|Basin size||120 km2 (46 sq mi)|
|Length||34 km (21 mi)|
The Zasavica is a 33.1 km-long right tributary to the Sava river which entirely flows through the region of Mačva. It originates from the several streams out of the swamps north of the village of Salaš Crnobarski, in the floodplain of the lower course of the Drina river. The river flows in the north-east direction, for 10 km parallel to the flow of the Sava and next to the villages of Glogovac, Sovljak, Crna Bara, Banovo Polje and Radenković, where the river crosses the administrative border of Central Serbia and the province of Vojvodina, where it flow near the settlements of Ravnje, Zasavica I, Zasavica II, Noćaj, and Mačvanska Mitrovica. At village Banovo Polje two major headstreams, Jovača and Prekopac, meet and from that point the river is called Zasavica.
Near the village of Zasavica, the river enters the marshy area of the Zasavica bog where the 50–60 metres wide stream spreads to almost 300 meters and gets 2 metres deep, and meanders through the middle of it until it flows into the Sava at Mačvanska Mitrovica, right across the town of Sremska Mitrovica on the Sava. The final section is channeled (Bogaz canal) and the river often floods the surrounding area.
The name of the river could be translated as the “behind Sava”. It actually flows through the typical elongated "mrtvaja" (oxbow), the old (fossil) bed of both the Sava and later, the Drina rivers. Because of the river’s meandering course and the low terrain, a bog was created, while the river itself changes the course depending on how much atmospheric waters it gets during the year.