Middle Fork of the Salmon River, Idaho River Odysseys West's six-day, Class IV raft trip on central Idaho's celebrated river will see the biggest boost from high flows. Most years, low water in late summer makes the upper 23 miles—the prettiest part of America's most pristine mountain stream—unnavigable. This year, guests will likely experience the entire 100-mile float, including all of the hot springs, through August ($2,000; rowadventures.com).

Tuolumne River, California OARS's three-day raft outing down California's 18-mile, Class III–IV Tuolumne starts at the northern border of Yosemite National Park and descends pounding rapids through a granite gorge. The Tuolumne flows mostly over bedrock, creating an effect similar to that of a theme-park inner-tube ride with much wilder water—this year especially (from $570; oars.com).