Introduction to the Sierra Nevada: The peak of the Sierra Nevada is the eastern boundary of the park. The two rivers that flow through the park, the Merced and the Tuolumne, originate from the snow-capped mountaintops. The Merced River flows through Yosemite Valley, while the Tuolumne River carves the massive canyon in the northern half of the park. Although some of the park's spectacular waterfalls flow year-round, they dry out in late summer and early fall.