The Alas River cuts through Gunung Leuser from the high country south of Kutacane all the way to the Indian Ocean, and is one of the most spectacular rafting rivers in Southeast Asia. Multi-day trips run between Ketambe and Gelombang, with several grade-three and four rapids, long stretches of forest-lined flat water, and overnight camps on river beaches. Wildlife sightings from the boats are common — hornbills, monkeys, occasionally elephants on the lower banks.
Trips are organised out of Ketambe and Kutacane by a small number of licensed outfitters and run between October and April when water levels are usually high enough. A standard three-day trip covers about 80 km of river and costs in the low millions of rupiah a person including guides, food, equipment and permits. Shorter day trips on calmer sections are an option if you do not have three days to spare.