Koshi Tappu Wildlife Reserve is situated on the flood plains of the Sapta-Koshi River in Eastern Nepal.
The reserve is a wetland of international significance. It is home to more than 280 bird species including 20 duck species, 2 species of ibises, many storks, egrets and herons. The endangered swamp partridge and Bengal florican are found here. The Koshi Barge is an extremely important resting-place for migratory birds.

The last surviving population of wild buffalo or arna is found here. The reserve is also home to many types of mammals such as hog deer, spotted deer, wild boar and blue bull. The endangered Gharial crocodile and Gangetic dolphin have been recorded in the Koshi River.

The vegetation mainly includes tall khar-pater grasslands with a few patches of khair-sissoo scrub forest and deciduous mixed riverine forest.

During the monsoon, the reserve is flooded with depths ranging from 10 to 300 cm. From season to season the Sapta-Koshi River changes its course.