Koshi Tappu Wildlife Reserve lies on the flood plains of the Sapta-Koshi in Saptari and Sunsari Districts of Eastern Nepal. The reserve is defined by the eastern and western embankments of the river.
Rapid and complete inundation of the reserve to depths ranging from 10 to 300 cm occurs during the monsoon. The Sapta-Koshi river also changes its course from one season to another.
The vegetation is mainly tall kharpater grassland with a few patches of khair-sissoo scrub forest and deciduous mixed riverine forest. The reserve offers important habitat for a variety of wildlife. The last surviving population (about 100) of wild buffalo or arna are found here. Other mammals are the hog deer, wild boar, spotted deer, and the blue bull.<
The reserve also assists the local economy by providing fishing permits and allowing the collection of edible fruits and ferns in season.
A total of 280 different species of birds have been recorded in the reserve. These include 20 species of ducks, two species of ibises, many storks, egrets, herons and the endangered swamp partridge and Bengal florican. The Koshi Barrage is extremely important as a resting place for migratory birds. Many species recorded here are not seen elsewhere in Nepal. The endangered gharial crocodile and Gangetic dolphin have been recorded in the Koshi River.