The Tuolumne River, which drains a 1,960 square-mile watershed on the western slope of the Sierra Nevada range, is the largest of three major tributaries to the San Joaquin River. The Tuolumne originates in Yosemite National Park and flows southwest through Yosemite, Stanislaus National Forest, and private lands to its confluence with the San Joaquin River, approximately 10 miles west of Modesto.
At higher elevations, the watershed is composed primarily of granitic bedrock that was scoured by glaciers during glacial periods down to the O’Shaughnessy Dam location, resulting in mountainous terrain, patchy forests, and a variety of steep canyons and mountain meadows. The middle portion of the watershed from New Don Pedro Reservoir to above Hetch Hetchy Reservoir is characterized by deep canyons and forested terrain. Near the town of La Grange, the river exits the Sierra Nevada foothills and flows through a gently sloping alluvial valley that is incised into Pleistocene alluvial fans.
We operate the Hetch Hetchy Project, located within Yosemite National Park and the Stanislaus National Forest in the Upper Tuolumne River watershed. The Hetch Hetchy Project supplies water and power to the City of San Francisco and surrounding Bay Area communities, and regulates stream flow in the Upper Tuolumne River, Cherry Creek, and Eleanor Creek.
We are responsible for environmental stewardship issues related to the operation of Hetch Hetchy Project facilities in the Upper Tuolumne River watershed. Through implementation of our Environmental Stewardship Policy and collaborative efforts, including the Upper Tuolumne River Ecosystem Project, we and project partners are building a scientific understanding of the river with the goal of improving management of the overall river ecosystem.