Tuolumne River

High in the Sierra Nevada mountains, in Yosemite National Park, is a peak known as Mount Lyell. On Mount Lyell is a great mass of ice known as the Lyell Glacier. Each winter, snow covers everything with a blanket of white.

In the spring, the snow and ice begin to melt and turn into water. This crystal clear water flows downhill into the creeks, streams and lakes below. The streams flow together to form the beautiful Tuolumne River.

The river flows west toward the Pacific Ocean, falling over rocks in splendid waterfalls and rushing down steep canyons. As it flows, other streams flow into it, making it larger.

Near the edge of Yosemite National Park, a dam on the Tuolumne River forms the Hetch Hetchy Reservoir. A reservoir is a large body of water stored behind a dam for use in the future. The people of San Francisco get most of their water throughout the year from this reservoir. The Tuolumne River continues to flow below Hetch Hetchy, downhill through high mountains, beautiful valleys and forests of pine trees until it reaches a place in the foothills 40 miles east of Modesto. There it collects into Don Pedro Reservoir, which is owned and operated by MID and Turlock Irrigation District (TID). It then continues on downstream to La Grange Diversion Dam to be diverted into MID and TID systems or continue down the river.

Below La Grange Diversion Dam, the water that remains in the Tuolumne River continues to flow downhill the way it has for thousands of years. More than 158 miles from where it begins as melted snow, the Tuolumne River empties into the San Joaquin River. Then the water flows more than 100 miles further, through the Delta, all the way to the Pacific Ocean.

Visit the U.S. Forest Service site for more information on the Tuolumne River.

Senior Water Rights

As the second irrigation district established in California, MID – together with TID – holds senior water rights on the Tuolumne River.

All MID water rights are put to beneficial use, either as water storage, irrigation water for agriculture, water for drinking and other urban uses, or environmental water releases that support fish, wildlife, and river ecosystems.


MID has been a faithful steward of the Tuolumne River and Watershed for more than 100 years. Together with our Tuolumne River partners, TID and the San Francisco Public Utilities Commission (SFPUC), we’ve invested millions of dollars and countless hours studying and developing the best available science on the Tuolumne River. We believe that the key to a healthy river system involves stream and habitat restoration, improved river flow schedules and predator suppression to promote the long-time survival of native fish.

The Tuolumne River Partners – MID, TID, SFPUC – have joined dozens of other California water agencies committed to collaborating with the state of California to finalize agreements that will provide water supply reliability to our communities while enhancing river ecosystems. We have proposed a comprehensive alternative originally known as the voluntary agreements, and now referred to as the Agreements to Support Healthy Rivers & Landscapes, for continued management of Chinook salmon and O. mykiss (rainbow trout) within the lower Tuolumne River. The alternative is based on Tuolumne River specific studies and science and answers concerns about the State Water Resources Control Board’s (SWRCB) Bay-Delta Water Quality Control Plan Substitute Environmental Document (SED) and forms the substance of our final license applications for the Don Pedro Project and La Grange Diversion Dam Project.

The Tuolumne River Voluntary Agreement (Agreements to Support Healthy Rivers & Landscapes) provides a foundation for comprehensively managing the Tuolumne River. It provides benefits to fish and their habitats as well as to farms, businesses and communities that depend on the river for water supply.

Learn more about our efforts and current projects underway.

Additional Information

  • Until the Last Drop
    Watch MID’s award-winning documentary and learn more about the rivers that have transformed the San Joaquin Velly, helped create California’s greatest cities and nourished a nation. On the Tuolumne, Stanislaus and Merced rivers we meet the men and women who grow our food; we talk to those studying what salmon really need to survive, and we hear from those who depend on thriving rivers and are working to secure and improve water supplies for the fish and wildlife, farms, businesses and communities that depend on the river.
  • Worth Your Fight
    This is a joint MID and TID customer awareness campaign devoted to informing our region about the potential negative impacts associated with Phase 1 of the SWRCB’s Bay-Delta Water Quality Control Plan SED.
  • Tuolumne River Management Plan
    MID and TID have developed a comprehensive management plan for the Tuolumne River. The plan describes the Districts’ proposed operations, improvements and resource protection measures under a new Federal Energy Regulatory Commission (FERC) license for the Don Pedro Project.
  • Don Pedro Project Relicensing
    MID and TID continue the FERC relicensing process for the Don Pedro Project.
  • San Joaquin Tributaries Authority
    MID is a member of the San Joaquin Tributaries Authority. SJTA’s vision is strengthening the resilience and reliability of the San Joaquin River and its tributaries. Our mission advances collaborative, sustainable water management and flood control while providing dependable water supplies for agriculture, the environment, and the communities we serve.