Ross Valley Flood Protection & Watershed Program

What is the Ross Valley Watershed Flood Reduction Program? 

The Ross Valley Watershed Flood Risk Reduction Program (Program) is a regional effort led by the Marin County Flood Control and Water Conservation District (Flood Control District) with an overall objective to substantially reduce the frequency and severity of flooding throughout the Ross Valley Watershed, in an economically viable manner while prioritizing public safety and minimizing environmental impacts.

The Flood Control District intends to meet this objective through the following actions: 

  • Increasing creek and floodplain capacity to convey floodwaters
  • Enlarging some channels through the removal or modification of existing obstructions to flow
  • Reducing peak discharge and attenuating the flows by increasing floodplain detention storage
  • Flood preparedness including regular creek maintenance, and structure modification
  • Community flood education including flood preparedness real-time rain and stream monitoring, flood response checklists and information

This map shows the major Program flood reduction measures (measures) that work cooperatively to reduce peak out-of-bank flows and achieve protection from a 100-year flood event (1% chance of occurring or being exceeded in any one year).  Measures include detention basins, located in the upper reaches of the watershed to detain peak flows during flood events; bridge replacements in Fairfax, San Anselmo and Ross to remove impediments to flows and reduce localized flooding; dredging of channels in the lower watershed; and creek improvements watershed-wide to increase capacity and handle flood flows as they move through the watershed.  Just a few examples of the approximately 180 creek improvements to be considered include clearing debris, removing invasive vegetation, creek bank stabilization and habitat enhancement.  While numerous potential detention basin sites have been identified, in the event one or more are deemed infeasible, only a subset will be constructed throughout the watershed to meet the peak flow reduction goal.  Additional measures including policies to encourage low impact development, flood preparedness and educational activities will be led by local municipalities and occur in parallel throughout the lifespan of the Program.  

As shown in the figure below, the projects will be implemented in two phases.  Phase 1 (2017-2027, the period of the current flood fee) will target a goal of 25-year flood protection (4% chance of occurring in any one year) currently undergoing community participatory planning and scoping for the Program environmental review (began January 2017).  Phase 2 (2028- 2050, depending on securing funding sources such as grants or a renewal of the flood fee) will add additional measures to achieve a target goal of 100-year flood protection.  


Who are the Program Partners?

The Program is led by the Flood Control District in partnership with the County of Marin, Towns of Fairfax, San Anselmo, Ross, and City of Larkspur along with the unincorporated areas of Greenbrae, Kentfield, Sleepy Hollow and Oak Manor.  Additional Program partners include environmental, business and community organizations, the U.S. Army Corps of Engineers, Marin Municipal Water District, Marin County Parks and Open Space District, Ross Valley Fire Department and Ross Valley School District.

Why is the Program needed? 

The Program is needed to help reduce the risk of flooding in Ross Valley which not only threatens the lives and homes of those living in the floodplain, but also impacts the broader community, including local businesses, schools and emergency services. Below are several key facts about flooding in Ross Valley:

  • The December 2005 flood resulted in over $95 million in damages to homes, businesses and public agencies
  • During a flood, police, fire and emergency services cannot function properly; it is estimated that severe storms and flooding can increase emergency response times by 2-10 times
  • An estimated 2,904 land parcels could be inundated during a 100-year flood event (a flood that has a 1% chance of occurring or being exceeded in any one year)
  • Implementing all of the Program elements in the Capital Improvement Plan (Stetson, 2011) would reduce the risk of flooding in Ross Valley and provide up to a 100-year level of flood protection

The Program’s overall goal is to increase the existing 6-year level of protection to a 100-year level.  The figure below shows the historical peak discharge for Corte Madera Creek, recorded at the Flood Control District stream flow gage in Ross.  As shown, large flooding events occur on a semi-regular basis, causing extreme flooding in Ross Valley. The orange horizontal bar indicates the current creek capacity, a 6-year flood event (a flood that has about a 20% chance of occurring in any one year). Implementing Phase 1 measures would increase the capacity to a 25-year level of flood protection (shown by the green line). Phase 2 would raise the flood protection level from 25-year to a 100-year level (shown by the purple line) to address the most severe flooding events such as those that occurred in 1982 and 2005. Phase 2 is dependent upon securing additional funding such as grants and renewal of the flood fee. 


View the video below to hear from Ross Valley residents on how flooding affects them.

How is it Funded? 

In response to local flooding in 2005, Ross Valley residents passed the Ross Valley Storm Drainage Fee (Fee), which helps fund the Program and leverage additional state and federal grants to design and implement flood reduction measures. The Fee will raise over $40 million during a 20-year period (2007-2027) to implement solutions for helping reduce flood risk within the most flood impacted areas of the Watershed. To date, close to $29 million dollars in grant funding has been awarded to the Program and additional funding opportunities are continually being pursued. 

Program History

The Program was initiated following the devastating flooding that occurred in Ross Valley during December 2005, which caused nearly $95 million in damages in the communities of Fairfax, San Anselmo, Ross, Kentfield and Larkspur. The Program utilizes a community-based participatory planning and design process that incorporates input from Ross Valley residents and stakeholders to help shape and implement solutions which significantly reduce local flood risk.

Since the Fee’s passage, the Flood Control District has utilized these funds to conduct several studies addressing flood reduction in Ross Valley including the Capital Improvement Plan Study for Flood Damage Reduction and Creek Management for Flood Zone 9/Ross Valley (CIP) (Stetson, 2011) and the Ross Valley Flow Reduction Study Report (Flow Reduction Study)(CH2M, 2015). The CIP described plans and technical rationale for a suite of on-the-ground flood reduction measures, spread watershed-wide, that work together as a system to reduce flooding and seek to restore the ecological health and function of Corte Madera Creek and its tributaries. The Flow Reduction Study re-examined the flood control measures proposed in the CIP and added other new measures either not previously considered, or if considered, were ruled out based on prior screening criteria. One of the key goals of the Flow Reduction Study was to reexamine the proposed detention basin sites, as well as consider additional detention basin sites, to ensure every option was assessed before moving forward with complex flood reduction measures. The Study reaffirmed the need for detention basins to be implemented alongside creek improvement measures to achieve Program objectives.

The CIP and Flow Reduction Study utilized a computer-based floodwater model of the Ross Valley Watershed to examine numerous approaches to flood risk management. Collectively, the various recommended flood reduction measures would function collectively to contain the 100-year flood event along Corte Madera Creek and its contributing tributaries in Ross Valley.  The results of these studies form the foundation of the Program The Flood Control District is currently upgrading and expanding the hydrologic and hydraulic model to include additional portions of the watershed, incorporate climate change projections and model potential flood reduction measure options.   


Next Steps

The Notice of Preparation (NOP) for the Program wide Environmental Impact Report (PEIR) was issued on December 21, 2016, kicking off a 60-day scoping and comment period. The public can submit comments about the Program’s potential environmental effects (see table below) during the NOP period.  Comments can be submitted by mail, email and/or in-person at a public scoping meeting (see Events & Meetings for date and location) scheduled prior to the end of the scoping period on February 24, 2017. The scoping meeting will include an overview of the environmental review process, briefly outline the Program and provide an opportunity to receive oral or written comments from the public.  All public comments will be included and addressed in the PEIR. 

Subsequent to the PEIR scoping period, a Draft Program Environmental Impact Report (DPEIR) will be released and circulated. The DPEIR will include another opportunity to receive public comments and a public hearing on the adequacy of the DPEIR will be held by the Flood Control District Board of Supervisors during the DPEIR comment period. Comments will be responded to formally and published in a Final Program Environmental Impact Report (FPEIR). The FPEIR will also be circulated and any remaining public or agency comments will be responded to before the FPEIR is presented to the Flood Control District’s Board of Supervisors for certification of the FPEIR as adequate and complete in compliance with CEQA. No specific projects encompassed in the Program and evaluated in the PEIR will be approved as a direct outcome of the Board’s certification of the FP EIR. Subsequent CEQA analysis, as appropriate and regulatory permits are required to construct projects within the Program.

In parallel with the PEIR, a project-level environmental review for the San Anselmo Flood Protection Project will be prepared as shown in the figure below. A NOP for the San Anselmo Flood Protection Project EIR is expected in Spring of 2017 and will provide opportunities for public input. All of the environmental and subsequent CEQA processes will also include opportunities for public input.

*Tentative Timelines for Project and Program Environmental Impact Reports


Program EIR 

San Anselmo Flood Protection Project EIR

  Notice of Preparation/ Public Scoping

Dec 2016- Feb 2017* 

April 2017* 

Draft EIR Circulated

  Sept 2017 - Nov 2017* 

Sept-Nov 2017* 

Final Report

May 2018* 

Nov 2017-Jan 2018* 

  EIR Certification

June 2018* 

March 2018* 

                              *dates are tentative

Program EIR Information

The Marin County Flood Control and Water Conservation District will be preparing a Program Environmental Impact Report (EIR) for the Ross Valley Watershed Flood Risk Reduction Program (Project). The Project involves implementing various flood risk reduction measures throughout the Ross Valley Watershed to achieve a 100-year level of flood protection, located in central eastern Marin County. 

The Project proposes several components designed to reduce flood risk in the watershed. Proposed components include: 

  • Increasing creek and floodplain capacity to convey floodwaters. 
  • Enlarging some channels through the removal or modification of existing obstructions to flow.
  • Reducing peak discharge and attenuating the flows by increasing floodplain detention storage.
  • Flood preparedness including regular creek maintenance and structure modification
  • Community flood education including emergency alerts, stream monitoring, and flood response checklists and information 

The County conducted a public scoping meeting on Thursday February 16, 2017, and the Notice of Preparation and Powerpoint from the meeting are now available at links below:

Take a few minutes to watch this video about local residents telling their personal stories of flooding in the Ross Valley over the years. This video was featured during the July 13, 2013 Community Meeting.

Video of residents