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San Luis Reservoir Low Point Improvement

The Santa Clara Valley Water District has partnered with the U.S. Department of the Interior, Bureau of Reclamation (Reclamation), Mid-Pacific Region and the San Luis & Delta-Mendota Water Authority (Authority) to complete the San Luis Low Point Improvement Project (SLLPIP). The purpose of the SLLPIP is to identify a feasible alternative that will address the uncertainty of Central Valley Project delivery schedules and the water supply reliability problems associated with the San Luis Reservoir "low-point".

Project objective:

The objective of the SLLPIP is to optimize the water supply benefits of San Luis Reservoir while reducing additional risks to water users by:

  1. Avoiding supply interruptions when water is needed by increasing the certainty of meeting the requested delivery schedule throughout the year to south-of-Delta contractors dependent on San Luis Reservoir.
  2. Increasing the reliability and quantity of yearly allocations to south-of-Delta contractors dependent on San Luis Reservoir
  3. Announcing higher allocations earlier in the season to south-of-Delta contractors dependent on San Luis Reservoir without sacrificing accuracy of the allocation forecasts.

The San Luis Low Point Improvement Project may provide opportunities for ecosystem restoration.

The need to solve the San Luis Reservoir "low-point” issue was identified by CALFED, a consortium of state and federal agencies committed to restoring the Bay-Delta environment and improving the management of California’s water resources. CALFED recommended that the district be the lead agency for this project; in 2001 the district was awarded a California Department of Water Resources (DWR) Proposition 13 grant to conduct a feasibility analysis and formulate a solution to the low-point problem. In 2005, the project lead transitioned to Reclamation, where the district began its partnership with Reclamation and the Authority.

Pacheco Intake(Photo at right: In the summer of 2008, San Luis Reservoir’s low levels left this Upper Pacheco Intake structure exposed, affecting water quality in Santa Clara County.)

Jointly owned and operated by Reclamation and DWR, San Luis Reservoir is one of California's largest reservoirs and a critical component of the state's water supply system. In fact, with a capacity of more than 2 million acre-feet (AF), it is the largest off-stream storage facility in the world.

Each year, water from California's Sacramento-San Joaquin Delta is delivered to San Luis Reservoir via the California Aqueduct and Delta-Mendota Canal for temporary storage during the winter or rainy season. During the summer or dry season, this water is released for use by State Water Project (SWP) and federal Central Valley Project (CVP) contractors located south of the Sacramento-San Joaquin Delta, including the district and San Benito County Water District (SBCWD). Water is delivered to these users through CVP San Felipe Division, located on the west side of the reservoir. Additional "south-of-Delta" water contractors receive water via the William R. Gianelli Pumping-Generating Plant, located on the east side of the reservoir.

A significant proportion of the water supply conveyed to Santa Clara County and San Benito County is at risk when water levels in San Luis Reservoir reach very low levels during late summer and early fall months. The high temperatures combined with declining water levels foster growth of an algae layer, as much as 35 feet thick, on the reservoir’s surface.

As the water levels lower, algae is captured by the Upper Pacheco Intake that delivers water to the San Felipe Division. The water quality within the algal blooms is not suitable for agricultural water users with drip irrigation systems in San Benito County and district cannot treat the water with existing facilities for municipal and industrial water users. The presence of algae can interrupt water deliveries during the peak demand season affecting the district’s ability to provide reliable supply of healthy, clean drinking water.

The San Luis Reservoir Low-Point Improvement Project was established to study ways that allow the San Luis Reservoir to be fully utilized without interrupting water deliveries or impacting water quality to water contractors.

Low-point issue San Luis Reservoir - algea growth
During the summer, as San Luis Reservoir is drawn down, a thick layer of algae grows on the reservoir’s surface. When the reservoir elevation drops to approximately 300,000 acre-feet (the beginning of the low-point issue), algae begins to enter the San Felipe Intake valve degrading water quality and making it more difficult to treat. This limits the flexibility of federal and state contractors to fully utilize reservoir storage. The low-point problem poses a threat to about half of the imported CVP supplies for the San Felipe Division agencies.

Contact information:
For more information about this project, please contact: Kellye Kennedy, Senior Project Manager, Santa Clara Valley Water District, at (408) 265-2600, or e-mail comments and feedback to