Why to go now: Originating on the Tibetan plateau and cutting through the Himalayas to India, the 315-mile Karnali is Nepal's longest, largest, and wildest river. For serious floaters, it's also one of the planet's premier rivers for multiday rafting trips, featuring clear water, consistent flow, and big Class V rapids. Unfortunately, the abundant water and fast flow make the river a prime prospect for hydropower development. The Nepali government is currently seeking investors and financiers for a large dam project on the Upper Karnali. If built, the dam would generate electricity for export to China and India at the expense of world-class whitewater.

How to get there: Ultimate Descents specializes in running rivers in Nepal and the surrounding area, including a 10-day trip on the Karnali. Founder David Allardice, a New Zealander, has spent 20 years exploring and pioneering rivers in the Himalayas and is the co-author of the original 'White Water Nepal' guidebook.