It was ancient glaciers that finally carved and polished Yosemite's mighty valleys, but these natural wonders were originally cut by rivers. In late spring and early summer, when melting snow from Sierra Nevada peaks thunders down into the valleys below, it's easy to imagine how millennia of erosion has shaped the landscape of the park today.
Flowing through the Yosemite Valley, the Merced River may be the most iconic river in Yosemite National Park. Protected as a National Wild and Scenic River by Congress in 1987, the Merced River is naturally free-flowing throughout the park, continuing unfettered through whitewater rapids west of El Portal all the way to Lake McClure outside the national park boundaries near Mariposa. The Merced River changes its moods with the seasons: it can be a frozen ice queen in winter, a fearsome flooding monster in spring, a playful friend in summer, or a calm whisper of water in fall.
The most popular activities on the Merced River are swimming or floating downstream in a rubber raft or kayak during the early days of summer, usually in June and July. At fishing holes alongside the river, you can hook trout. Outside the park boundaries, whitewater rafting trips launch from April through July. If you'd rather just sit beside the river and take a moment to contemplate the natural beauty of Yosemite, the entire valley is filled with meadows perfect for having a picnic beside the flowing waters. Favorite viewpoints for photographing the Merced River are from Sentinel Bridge; at the Happy Isles, where the river tumbles musically over polished granite rocks; or from Valley View, a roadside pull-off that drivers will pass as they leave the valley.
Like the Merced River, the colossal Tuolumne River has its headwaters high in the Sierra Nevada and flows down through the western side of the Sierra Nevada range into California's central valley. If you want to see for yourself where the river starts, you'll need to ascend Mount Dana (elevation 13,053 feet) or Mount Lyell (elevation 13,114 feet) -- the latter is the tallest peak in Yosemite National Park. The most popular way to get up close to the river, however, is by hiking to Waterwheel Falls, where whitewater explodes off the faces of river rocks, and into the Grand Canyon of the Tuolumne, beyond which the river tumbles down into artificial Hetch Hetchy Reservoir.