The First Don Pedro Reservoir
In 1923, Turlock and Modesto Irrigation Districts joined forces to build the first Don Pedro Dam, which had a storage capacity of 289,000 acre feet. The dam held just enough water to accommodate growers’ irrigation needs for a single growing season. To get through consecutive dry years, which happens often in TID territory, the District needed a dam large enough to store enough water for the demands of multiple irrigation seasons.
Original Don Pedro Dam under construction.
Don Pedro Dam releasing water.
Dedication of the original Don Pedro Dam, June 25, 1923.
The New Don Pedro Project
When contemplating a new dam and reservoir project, the Districts were presented with several proposals which they ultimately narrowed to just two different projects. One a “low” concrete arch dam similar to the original Don Pedro but at a height of 485 feet located about a mile and a half downstream of the original dam. The second proposal was for a dam as high as the site would allow. That proposal called for an earth and rockfill dam 580 feet high with a crest length of 1,900 feet requiring 16,400,000 cubic yards of fill material. This new dam would rise to a height of 855 feet above sea level and create a reservoir capable of storing 2,030,000 acre feet of water. The Turlock and Modesto Irrigation Districts ultimately decided to build the larger project and New Don Pedro was born. The dam is the tenth highest in California and creates the sixth largest artificial lake in the state.
Construction began in 1967 and was completed in 1971. The spillway, constructed on the right right abutment of the facility, consists of three radial gates and a 995-foot long uncontrolled spillway. Bonds Flat Road passes below the spillway and connects County Road J59 with recreational facilities at Blue Oaks and Fleming Meadows and eventually connects to Hwy 132.
By constructing the new Don Pedro Dam, power plant and related facilities, the Turlock and Modesto Irrigation Districts firmed up water supplies for their respective districts, increased capacity to generate clean hydroelectric power and provided recreation opportunities and flood control in the Tuolumne River basin.
Although the City and County of San Francisco contributed to the construction of Don Pedro, they own none of the rights to the water stored in it. By partnering with the local irrigation districts in the construction of the new Don Pedro, the City and County of San Francisco was able to double its water supply upstream in the Hetch Hetchy project without having to build new facilities. This arrangement has proven beneficial to all the parties involved and points to the benefits of multiple agencies cooperating to share a resource.
The current Don Pedro Dam and Powerhouse.
Lake Don Pedro, the sixth largest man-made lake in California.
In 1961, an overwhelming majority of voters in the TID and MID service territories along with the City and County of San Francisco, approved bond issues to finance the building of a new dam. Construction was initiated in 1967 and completed in 1971
With a base thickness of nearly 2,800 feet and rising to a crest width of just 40 feet, Don Pedro Dam is truly an amazing structure. Constructed of an impervious clay core protected by earth and rockfill, most of which was locally sourced from dredge tailings in the river channel near La Grange. Using fifteen cubic yard front-end loaders and a fleet of forty 75-cubic yard bottom dump wagons, contractors hauled 16.4 million yards of dam embankment material 8 miles to the dam site placing approximately 60,000 cubic yards of material each day. In addition to access roads into the remote location, a camp to house the construction crews that ranged between 500 and 700 people was built. Two large bridges were also constructed over arms of the reservoir as part of highway relocations from the reservoir area.
Don Pedro Dam
As a public power agency, TID has been delivering power to retail customers since 1923. TID is the majority owner and operator of the reservoir and generation facilities.
Don Pedro’s power plant occupies the entire width of the river channel at the toe of the dam. It’s an outdoor structure, constructed of reinforced concrete and originally contained three 45,500 kilowatt generating units driven by three 70,000 horsepower turbines. In 1989 a fourth unit was added and today; the plant operates three 55,000 kW and one 38,000 kW generators each 85,000 horsepower bringing the total capacity of the plant to 203 megawatts with 139 MW going to TID and 64 MW to MID.
Don Pedro Powerhouse.
When the reservoir is full, Don Pedro has a capacity of 2,030,000 acre feet, covers 12,960 acres of water surface, extends roughly 26 miles upstream and has approximately 160 miles of shoreline. The reservoir provides 1,381,000 acre feet of active storage space for water supplies and 340,000 acre feet of space to regulate flood flows of the river. The minimum pool contains 309,000 acre feet which is 19,000 acre feet more than the original Don Pedro Project held when it was full.
A few years after constructing the original Don Pedro Dam in 1923, the districts realized they needed more capacity to store water. With a maximum storage of 289,000 acre feet, the original reservoir was barely able to accommodate the irrigation needs of a single growing season. After numerous dry winters, the Districts decided to replace the original dam with a much larger one in order to store water necessary to bridge multiple dry years.
The City and County of San Francisco joined with the two irrigation districts in constructing what at the time was known as the New Don Pedro Project. In exchange for its financial contribution to the construction of the Project, San Francisco obtained relief from upstream flood control responsibility on the Tuolumne River and gained greater flexibility in its upstream operations. This arrangement helps to conserve the waters of the Tuolumne and maximize their beneficial use.
Lake Don Pedro, the sixth largest man-made lake in California.
Don Pedro Reservoir provides excellent opportunities for fishing, boating, water sports, camping and other related outdoor recreation activities. The reservoir is easily accessible to people from local areas as well as from the large metropolitan areas of Fresno, Sacramento and the San Francisco Bay Area. Three areas of the reservoir have been developed with funds contributed by the State of California: Fleming Meadows, Blue Oaks and Moccasin Point. Camping facilities, picnic areas, fish cleaning stations, boat ramps, a swimming lagoon, and marina are among some of the improvements offered at various sites.*
*Not all amenities are offered at every recreation area. See entrance
stations or the Don Pedro Recreation Area office for details.
Image credit – Tom King
The Reservoir is licensed by the Federal Energy Regulatory Commission and jointly held by TID and the Modesto Irrigation District (MID). Our original license to operate the power plant at Don Pedro was granted in 1966 and valid for 50 years. The license expired in April 2016 and we are currently operating on a year-to-year renewal basis until a long-term license is granted by FERC.
Tuolumne River Management Plan
Together with Modesto Irrigation District, we’ve developed a comprehensive management plan for the river. The plan describes our proposed operations, improvements and resource protection measures under a new Federal Energy Regulatory Commission (FERC) license for the Don Pedro Project.
Don Pedro Dam and its appurtenant structures are inspected at least:
- Annually by the Federal Energy Regulatory Commission (FERC) and the California Department of Water Resources Division of Safety of Dams (DSOD)
- Quarterly by TID
- As needed by external consultants
In 2017, following the Oroville Spillway Incident, TID performed a focused assessment of the Don Pedro spillway structures. The assessment included a review of the original design and construction documents, detailed inspections and tests, and an analysis of potential failure modes. The assessment was led by qualified external consultants, and involved both FERC and DSOD staff. No safety concerns were identified, and the spillway structures are considered safe for operation.
|Type||Earth & Rockfill|
|Height above foundation||580 feet|
|Crest Length||1,900 feet|
|Crest Width||40 feet|
|Crest Elevation||855 feet above sea level|
|Base Thickness||2,800 feet|
|Volume||16,400,000 cubic yards|
|Dikes||500,000 cubic yards|