The Irrawaddy dolphin — a stocky, round-headed freshwater species quite unlike its marine cousins — survives in a small stretch of the Mekong north of Stung Treng near the Lao border. Community-based dolphin watching trips depart from the riverside in town or from the village of Preah Rumkel, about 35 km north. Local boatmen know the dolphins' habits and the trips are well run; sightings are frequent but never guaranteed.
The experience is genuinely special. The dolphins surface in groups, rolling slowly just upstream of the boat, and the setting — wide river, forested banks, Cambodia fading into Laos on the horizon — is one of the finest in the country. The boat trip itself takes you through stretches of flooded forest and past fishing communities that have used this river for centuries. Allow half a day for the full experience.