The most famous place to run the rapids is the Grand Canyon, with steep walls that tower above you as you race down the Colorado River. It's also the most popular spot, with bumper-to-bumper rafts in summer.
You may have less unwanted company on the Colorado upstream in Utah -- which also has good rafting on the Green River. Call the Utah Travel Council (tel. 800/200-1160 or 801/538-1030; www.utah.com) and ask for a copy of Raft Utah. The Snake River south of Yellowstone National Park near Jackson Hole, Wyoming, is also a best bet. The Snake River flows into Idaho, where its wild Hells Canyon offers exciting rides -- as do the Salmon and Middle Fork rivers.
The New River cuts a dramatic, 2,000-foot-deep gorge through the Appalachian Mountains inside New River Gorge National River Recreation Area, near the town of Beckley, West Virginia, making it the most scenic rapids route in the east.
