The Park is listed as a World Heritage Site and is the most popular safari destination in Nepal. It covers a unique ecosystem, containing hills, ox-bow lakes and flood plains. Dominated by sal forests, the park has 50 species of grasses. The 43 species of animals include the endangered one-horned rhino, Royal Bengal tiger, gharial, crocodile, gaur, wild elephant, four horned antelope, stripped hyena, pangolin, Gangetic dolphin, monitor lizard, cobra, krait and rat snake.

The park is the largest and most undisturbed wilderness area in the Terai. It is thickly forested by sal trees, and is carpeted with grass, savannah and river line forest. It is regenerated habitat for endangered species like the rhinoceros, wild elephant, tiger, swamp deer, black buck, gharial, crocodile and Gangetic dolphin. Endangered birds include the Bengal florican, lesser florican, silver-eared mesia and sarus crane.

The vegetation here is mainly of wetlant grassland, scrub and deciduous forests. The reserve is unique for its wild arna buffaloes. Other mammals are the hod 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 reserve is resting place for migrating birds not seen anywhere in Nepal. The endangered gharial, marsh crocodile and Gangetic dolphin are also recorded in the Koshi River.