Yosemite's beautiful river valleys and canyons were created by the same awesome geological forces. During the earth's last ice age, massive glaciers moved down from the Sierra Nevada's high country, polishing not just the mountain peaks and iconic granite domes, but also softening and widening Yosemite's ancient valleys and canyons.
The Yosemite Valley itself is a natural wonder, carpeted in wildflower meadows beside the bubbling Merced River. Spectacular waterfalls plummet over the valley's sheer walls, guarded by towering granite formations such as El Capitan that prove irresistible to world-class rock climbers.
In the lonely northwest corner of the park, the Hetch Hetchy Valley was once equally impressive. However, a public-works project dammed the Tuolumne River in 1923, flooding the valley that John Muir once described as "a grand landscape garden, one of Nature's rarest and most precious mountain temples." Today, the O'Shaughnessy Dam backs up the Hetch Hetchy Reservoir, where you can glimpse (or even take a day hike to) thundering Wapama Falls.
You'll need to make more of an effort to experience Yosemite's impressive river canyons, located far from paved roads in the park's wilderness areas. One popular backpacking trail in the Sierra Nevada high country leads from Tuolumne Meadows past dramatic waterfalls into the Grand Canyon of the Tuolumne River. Inaccessible to all but expert rock climbers and mountaineers, Tenaya Canyon is incredibly remote and composed of rugged wilderness. But you can get a glimpse of its grandeur from the end of the easy Mirror Lake Trail that starts in Yosemite Valley.