Why: 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 900 MW hydropower 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. "As per the Environment Impact Assessment report of the project, 56 families will have to be relocated and a total of 239 families will be affected by the project," says Samir Mehta, South Asia Program Director at the International Rivers organization. "The project is designed as a peaking run-of-river project. There will be a daily drought and flood in the river, and water will flow for only four peaking hours, which are in the evening."

What to do: 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.

More info: welcomenepal.com