Koshi Tappu Wildlife Reserve is recently promoted as safari park within the Sub tropical Terai belt, this is the most smallest (175sq.km) covering wetlands of Sunsari, Saptari and Udaypur district and easternmost wildlife reserve in Nepal. Established in 1976 and nominated as Ramsar site the altitude of this wildlife reserve ranges from 75 to 81 metres consisting of extensive mudflats, reed bed and fresh water marshes in the floodplain of the Sapta koshi River. It is easily accessible from the Mahendra Highway. The vegetation in the forest is defined as deciduous riverine forest, grassland and the marshy vegetation. The coverage of grassland is 68% and only 6% is forest which is dominated by Siso with the patches of Khair. In this wetland 514 species of plants are found some of them are Echornia, Kapok, Sugarcane, Reed, Cattail, Imperata cylindrical, Eelgrass, Hydrilla, Azolla and Lotus. The wildlife reserve also inhibits a wide range of fauna. Here the 31 species of mammal have been recorded which include Asian elephant, Spotted deer, Hog deer, Wild pig and Golden Jackal. The Ganges River Dolphin have been witnessed here.
The last remaining population of approximately 150 Wild Asian Water Buffalo in Nepal commonly called Arna lives here. This place has got different species of birds and is also the ideal destination for migratory birds. During winter 485 different birds species are noted here some of them are Watercock, Indian nightjar, Dusky eagleowl, Black fish eagle, Common golden eye and Gullbilled tern.