Koshi Tappu Wildlife Reserve, with an area of 175 square kilometers, is situated in the embankments of the Sapta Koshi river system. The vegetation consists mainly of grassland, scrub and deciduous forests. This reserve is unique for its 100 or so surviving wild water buffaloes. Other animals are the hog deer, wild boar, spotted, and the Gaur.
The species of birds total 280 here and includes 20 of ducks, two of ibises, many storks, egrets, herons and the endangered swamp partridges and Bengal floricans. The region is a resting place for migrating birds not seen anywhere in Nepal. Endangered Gharial crocodiles and gigantic dolphins have also been sighted in the Koshi River.