High Quality Content by WIKIPEDIA articles! The Merced River (pronounced mer-SED), in the central part of the U.S. state of California, is a 145-mile (233 km)-longtributary of the San Joaquin River flowing from the Sierra Nevada into the Central Valley. It is most well known for its swift and steep course through the southern part of Yosemite National Park, and the world-famous Yosemite Valley. The river`s character changes dramatically once it reaches the foothills and the lowlands, becoming a slow-moving waterway meandering through irrigated fields.