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Rancho San Antonio County Park Detention Basin

About This Project

The Rancho San Antonio County Park Detention Basin Project is one of the elements of the Permanente Creek Flood Protection Project.  

After years of planning and design, the Santa Clara Valley Water District began construction on the flood protection improvements along Permanente Creek. Once completed, the Permanente Creek Flood Protection Project will provide natural flood protection for at least 2,200 properties in Mountain View and Los Altos, create recreational opportunities and enhance the environment.

Construction is expected to be completed in 2021.

Project highlights


  • A 15-foot deep depression to collect peak storm flows from Permanente Creek
    Rancho San Antonio Detention Basin location
  • Planting of native trees
  • Removal of non-native trees that compete with native species
  • Replacement of existing maintenance bridge
  • New restroom facilities
  • New enlarged paved parking area with designated equestrian spaces


  • Enhances many acres of wildlife habitat
  • Provides flood protection for thousands of homes and businesses in Mountain View and Los Altos, saving residents thousands of dollars on flood insurance each year
  • Reduces construction impacts to downstream residential and business areas
  • Reduces flow rates, allowing for potential riparian restoration downstream
On Target
Start December 2016 / end Spring 2021
News and UpdatesNews and Updates
Reports and DocumentsReports and Documents
Environmental and Community BenefitsEnvironmental and Community Benefits
History and BackgroundHistory and Background
News & Updates

Work on the flood basin is completed, and a virtual ribbon-cutting event

Valley Water is excited to announce the completion of the Rancho San Antonio detention basin and the reopening of the closed trail. The new detention basin will capture and gradually release stormwater, which was the final element of the Permanente Creek Flood Protection Project.

Valley Water developed a video of the completed Permanente Creek Flood Protection Project with aerial images of all the project elements. Click here to see the video. 

Valley Water, Santa Clara County Parks, and Midpeninsula Regional Open Space District held a virtual ribbon-cutting to celebrate the completion of the Rancho San Antonio detention basin. Below are videos shared during the June 4 event. 

Video 1: Permanente Creek Flood Protection Project with aerial images

Video 2: Ribbon-cutting

Video 3: Event and ribbon-cutting

The completed project provides flood protection to approximately 2,200 properties in Mountain View and Los Altos. Once the Federal Emergency Management Agency (FEMA) updates the Flood Insurance Rate Map, many of these property owners will no longer be required to pay for mandatory flood insurance by their lenders.

We thank our park users and neighbors for their patience while this critical flood protection project was constructed. Rancho San Antonio County Park & Open Space Preserve is one of the many park gems that will benefit us all as a dual-purpose facility, providing flood protection to downstream neighborhoods and recreational space for community members. Among the added project benefits are new ADA restroom facilities, a new replacement parking lot with more paved spaces, new equestrian parking stalls, planting of over 200 native trees and plants, new trail sections, and a new maintenance bridge.

On-going plant care

You may have noticed the newly planted native plants and trees. This young vegetation will require ongoing care from Valley Water staff and contractors for the next three years as they are fully established and can survive independently. 

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Updated June 2021.

For more information: 

Reports & Documents

Final Subsequent Environmental Impact Report (November 2012)

Addendums to the Final Subsequent Environmental Impact Report

Environmental & Community Benefits

What to expect during construction

The Santa Clara Valley Water District, Santa Clara County Parks and MidPeninsula Regional Open Space District are working together to minimize construction impacts as much as practical and in accordance with local ordinances. We appreciate your understanding and cooperation as we embark on this important flood protection project.

  • Regular construction work hours are 8 a.m. to 5 p.m. Work is scheduled Monday through Friday, excluding legal holidays.
  • The South Meadow parking lot will be open throughout construction. The new, expanded parking lot and restroom will be built first, slightly north of the existing parking lot. Upon completion of the new facilities, the existing parking lot will be closed.
  • Various parts of the South Meadow and Hammond-Snyder Loop Trails will be closed during construction. Trail accessibility signage will be installed at least two weeks in advance of any closures.
  • A large amount of dirt will be removed from the site. The contractor will construct a 15-foot wide road behind the Gate of Heaven Cemetery for the haul trucks. The trucks will drive on the Hammond-Snyder trail to Stevens Creek Boulevard, to Foothill Boulevard, then onto I-280 (to avoid Cristo Rey Drive).
  • A visual screening fence will be installed around the construction areas.
  • Hundreds of native trees and removal of non-native trees that compete with native species along the creek.
  • Safety of the community and our employees is a priority. Barricades, railings, lights, fences and other warning devices will be used for public safety and convenience.
    Existing view of Rancho San Antonio County Park

What is a flood detention area?

Flood protection methods can include berms around buildings, widening channels, raising floodwalls, elevating structures and roadways, and/or constructing a bypass channel. In areas where development limits widening the creek or raising floodwalls, flood storage basins are used to temporarily divert and store floodwaters until a major storm passes.

The Rancho San Antonio detention areas will be approximately 12 acres in size and 15 feet deep with mild side slopes, contoured to the surrounding area and replaced with native trees and grass. Flood flows would inundate the site very rarely and quickly drain away. A 25-year flood, which has a 4 percent chance of occurring in any given year, would result in about one foot of water in the detention area that would drain away in hours. A 100-year storm, which has a 1 percent chance of occurring in any given year, would fill the area and drain in one to four days.

Use Access Valley Water to submit questions, complaints or compliments directly from your computer or smart phone to a water district staff person who can help you.

History & Background

Flooding History and Project Background 

Permanente Creek has a history of flooding, having experienced major flooding in 1862, 1911, 1940, 1950, 1952, 1955, 1958, 1963, 1968, 1995 and 1998. Flooding can result in millions of dollars in damage to homes, businesses and schools. In addition, disruption to businesses and transportation networks can result in significant loss of productivity and revenue. One of the project’s goals is to avoid utility and transportation shutdowns and prevent potential damages that could exceed $48 million (1999 value).

Each winter, thousands of households, schools and businesses in Mountain View and Los Altos are susceptible to flooding from Permanente Creek during a major storm. The Santa Clara Valley Water District has initiated planning of a flood-protection project along 10.6 miles of Permanente Creek, from San Francisco Bay’s southwest shoreline through Mountain View to Foothill Expressway in Los Altos.

The Permanente Creek Flood Protection Project offers a tremendous opportunity for a multi-purpose project to improve flood protection, create recreational opportunities and enhance the environment. The District worked with the cities and the community to design the most suitable alternative. 

About the Safe, Clean Water and Natural Flood Protection Program

In November 2012 the voters of Santa Clara County overwhelmingly approved Measure B, the Safe, Clean Water and Natural Flood Protection Program, as a countywide special parcel tax for 15 years with a sunset date of June 30, 2028. This Program replaced the Clean, Safe Creeks and Natural Flood Protection Plan, which voters approved in November 2000.

The Safe, Clean Water Program was developed with input from more than 16,000 residents and stakeholders and was created to match the community’s needs and values. The voters of Santa Clara County identified five priorities:

Priority A: Ensure a Safe, Reliable Water Supply

Priority B: Reduce Toxins, Hazards and Contaminants in our Waterways

Priority C: Protect our Water Supply from Earthquakes and Natural Disasters

Priority D: Restore Wildlife Habitat and Provide Open Space

Priority E: Provide Flood Protection to Homes, Businesses, Schools and Highways

Other: Six projects from the Clean, Safe, Creeks Plan have been carried forward into the Safe, Clean Water Program.  

Each year, the District prepares a report providing a progress update for each of these Program priorities, along with fiscal year accomplishments.

To ensure transparency and accountability to the voters, the ballot measure also created an Independent Monitoring Committee, appointed by the Santa Clara Valley Water District Board of Directors. The Independent Monitoring Committee annually reviews the Program’s progress to ensure the outcomes are achieved in a cost-efficient manner and reports its findings to the Board.

In addition, the Program requires three independent audits, the first of which was conducted in FY 2017.

View the Safe, Clean Water Program’s annual reports, annual IMC audit reports, and independent audits, including a staff response, on the District website.