The Vikramshila Gangetic Dolphin Sanctuary covers a 60km stretch of the Ganges between Sultanganj and Kahalgaon, east of Bhagalpur. It is the only protected area in India specifically dedicated to the Gangetic river dolphin, and one of the best places on earth to see this endangered species in the wild.
Dolphin sightings are most reliable between October and March, when water levels drop and the dolphins are more concentrated in the deeper channels. You'll need to hire a boat from the riverbank — most trips depart from Vikramshila Ghat or from Kahalgaon. Dolphins surface to breathe every 30 to 60 seconds, rolling briefly above the water and disappearing again. They are grey-brown, about 1.5 to 2.5 metres long, and move surprisingly fast in the current. Early morning is the best time; bring binoculars.
The sanctuary is managed by the Bihar Forest Department. Entry requires a permit, which can be arranged at the ghat or through guesthouses in Bhagalpur that deal regularly with wildlife visitors.