The Zasavica bog is a marshy lowland in the floodplain of the Sava river. It is a typical elongated oxbow (in Serbian: mrtvaja, dead [water]). The fossilized, parallel bed of the Sava is so elongated that it actually stretches almost to the Drina river. In time, Zasavica river was formed from the subterranean waters from the Drina and from several streams, most notably, Duboka Jovača and Prekopac canal (gravitationally, from the Cer mountain. The final section connecting Zasavica and Sava has been channeled (Bogaz canal). The river slowly meanders into and through the bog, widening from meters to almost . The marsh, consisting of several connected bogs, covers an area of . During normal water levels, the bog is deep on average, but it can get up to deep. The marshland's altitude varies from 76 to 82 meters (250 to 270 ft). The entire biotype includes also the Batar stream and several other canal-tributaries to Zasavica, and consists of an ecological row of water and marsh systems with fragments of flooded meadows and forests. Through the rivers Sava and Danube, the Zasavica biotype belongs to the Black Sea drainage basin.