The river's source is located in the Ansel Adams Wilderness, in the south-central Sierra Nevada at the confluence of three major affluents: the Middle Fork, which rises from Thousand Island Lake at almost above sea level, the North Fork, which starts southeast of Mount Lyell, and the South Fork, which begins at Martha Lake in Kings Canyon National Park and flows through Florence Lake, joins a short distance downstream. The Middle Fork is considered the largest of the 3 forks. From the mountainous alpine headwaters, the San Joaquin flows generally south into the foothills of the Sierra, passing through four hydroelectric dams. It eventually emerges from the foothills at what was once the town of Millerton, the location of Friant Dam since 1942, which forms Millerton Lake.