 The Merced River ( pronounced <unk> ) , in the central part of the U.S. state of California , is a 145 @-@ mile ( 233 km ) -long tributary of the San Joaquin River flowing from the Sierra Nevada into the San Joaquin 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 . 
