The vegetation here is mainly of wet grassland, scrub and deciduous forests. The reserve is unique for its wild water buffaloes. Other mammals are the hog deer, wild boar, spotted deer and blue bull. The species of birds total 350 here, including 20 of ducks, many storks, egrets, herons, and the endangered swamp partridges and Bengal floricans. The sanctuary is a resting place for migrating birds not seen anywhere in Nepal. There have also been recorded sightings of the endangered gharial, marsh crocodile and Gangetic dolphin in the Koshi River.