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 TID. It then continues on downstream to La Grange Diversion Dam to be diverted into MID and Turlock Irrigation District (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.