Koshi Tappu Wildlife Reserve lies on the floodplains of the Sapta Koshi River in the south-eastern Terai. The reserve was gazetted 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. Government of Nepal has declared the buffer zone ( 173.5 sq. km ) surrounding the reserve in 2004.
The Sapta Koshi is one of the three main tributaries of the Ganges- Rapid and intense flooding of the reserve, occur during the rainy season. Embankments have been constructed parallel to the river to control the flooding.
The reserve experiences three distinct seasons. Summer (February through May) is intensely hot with minimum precipitation. Shade temperatures can reach 40 c. The monsoon starts in late May/early June and lasts until September bringing heavy frequent rainfalls. The rainfall is greatest during July but high humidity and temperatures are experienced throughout the season. Winter (October through January) is characterized by clear skies and moderate temperature, but can still get quite cold.