The Manas River runs through the heart of the national park, originating in the Bhutan Himalayas and forming the natural boundary between India and Bhutan in the upper reaches. It is a fast, clear mountain river fed by glacial melt, and its banks attract concentrations of animals throughout the day.
River crossings by elephant during safaris take you to parts of the park otherwise inaccessible by road. During the dry season (November to April) the river level drops and broad sand banks emerge, where you can spot crocodiles basking and a variety of wading birds. Mahseer, one of the hardest-fighting freshwater fish in Asia, inhabit the river — anglers with permits sometimes visit specifically for this.