Koshi Tappu Wildlife Reserve is situated on the flood plains of the Sapta-Koshi River in Eastern Nepal. The reserve was gazette in 1976 to preserve habitat for the only remaining population of Wild buffalo, Arna (Bubalus arnee). The 176 sq. km. reserve is Nepal’s smallest wildlife reserve. The eastern and western embankments of the Sapta Koshi River define the area. In 1987, Koshi Tappu was declared a Ramsar site, a wetland of international significance. It is home to more than 441 bird species including 20 duck species, 2 species of ibises, white tailed stonechat, striated marsh warbler, 30 shore birds, 114 water birds, and the endangered swamp partridge and Bengal florican. The Koshi Barrage is an extremely important resting place for many migratory birds, containing 87 winter and trans-Himalayan migratory species.
The Koshi River is home to 80 species of fish. The endangered Gharial crocodile and Gangetic dolphin have been recorded in the river as well. 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 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.