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Flickr/kymtyr

Where: Arizona

Why: The full 277 miles through the Grand Canyon can take up to two weeks to complete, though shorter excursions are offered beginning at half-day trips. While most rivers run the international scale of one to six, the Colorado’s unique rapids make it one of only three in the world to operate on a one to ten spectrum. Along the way, be sure to marvel at the exposed strata and arresting vistas that make this one of the world’s seven natural wonders. More Info

Runner-up: Big Bend National Park, Texas

For 118 snaking miles, the portion of the Rio Grande flowing through Big Bend National Park provides rafters with an unforgettable journey straddling the U.S.–Mexico border. More Info