Koshi Tappu Wildlife Reserve lies on the alluvial flood plain of Sapta Koshi River in southeastern lowland. Almost all eastern and western embankments of the river define the area.
The reserve was officially gazetted in 1973 giving it a status of the wildlife reserve having an area of 175sq.km. The river Sapta Koshi (largest river of Nepal Kingdom) flows through the reserve that is one of the three main tributes of Ganges.
The mighty river Koshi calm in winter swells up largely in monsoon brining severe flood problems and often changes its course. The depth of the river fluctuates largely from season to season.