Climate Ready: Marin Municipal Water District

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Management Priorities

Responsibilities and Jurisdiction

The MMWD was established as the first municipal water district in California, with the mission to manage local natural resources in a sustainable manner and to provide customers with reliable, high quality water at a reasonable price. MMWD provides drinking water to 188,200 customers in central and southern Marin County. MMWD’s water supply is provided by three integrated water sources: district reservoirs, imported water, and recycled water. On average about 75 percent of Marin County’s water supply comes from rainfall collected in the district’s seven reservoirs located on Mt. Tamalpais and in west Marin. The water supply provided by four of these reservoirs—Kent, Alpine, Bon Tempe, and Nicasio—is used annually. Water supply from the other three reservoirs—Phoenix, Lagunitas and Soulajule—is held in reserve. Water from the district’s reservoirs is conveyed to the district’s San Geronimo Water Treatment Plant or Bon Tempe Water Treatment Plant for treatment before entering the distribution system.

Historically, approximately 25 percent of MMWD’s water is imported from the Sonoma County Water Agency (SCWA). SCWA water originates from rainfall that flows into Lake Sonoma and Lake Mendocino and is released into the Russian River. The Russian River water is filtered naturally through 80 feet of sand beds adjacent to the river. The Russian River water is blended with MMWD’s reservoir water in the distribution system.

MMWD was also the first water supplier in California to use recycled water for car washes, air conditioning cooling towers, and commercial laundries. Up to two million gallons a day are recycled and distributed via a separate pipeline system to more than 350 customers in northern San Rafael during the warmer months. Recycled water is used for irrigation, toilet flushing, and other non-drinking purposes.

The watershed lands owned and protected by MMWD stretch over 21,635 acres, including more than 18,900 on Mt. Tamalpais and 2,700 adjacent to Nicasio and Soulajule reservoirs in west Marin. An additional 35,000 acres of privately owned watershed drains into those two reservoirs. In addition to being a valuable source of water for customers, the Mt. Tamalpais watershed is a natural wild-land of great biological diversity and a popular recreational destination (MMWD 2015).

Climate-related Concerns and Management Priorities

MMWD’s priority concerns are potential climate change impacts on lake operations and water demand, availability of surface water supply, drought risks, and stewardship of the Mt. Tamalpais watershed in terms of vegetation management and fire risk.

Marin Municipal Water District is currently developing the Water Resources Plan 2040, a long-term plan for improving water supply resiliency under water shortage conditions, thereby providing consistent water to customers into the future. MMWD staff may apply the Climate Ready North Bay data findings to this analysis as they work to select water supply resiliency alternatives for further study, in order to ultimately recommend a preferred alternative.

MMWD staff is also considering using Climate Ready North Bay products as communication tools to inform the MMWD Board of Directors. An objective for communications would be to foster understanding of why each climate scenario is different and what the implications of each may be. MMWD’s goal is to clearly illustrate the challenge of climate change adaptation in the context of historical data.

MMWD’s management concerns are grouped into three resource areas: 1) Regional Rainfall Annual Variability; 2) Marin County Surface Water Supply (including drought risks, demand, and fisheries habitat); and 3) Land Cover and Fire Risks.


Vulnerability Assessment Results


Key Vulnerability Assessment Findings for the North Bay Region

  • Rising temperatures across the region will generate unprecedented warm conditions for both summer and winter seasons
  • Rainfall is likely to be more variable in the future in term of both low and high annual extreme
  • The North Bay region is becoming more arid (subject to drier soil conditions) due to rising temperatures
  • Runoff may be increasingly flashy, with rates of groundwater recharge relatively less variable over time
  • Protecting available recharge areas will be critical to water supply sustainability
  • Water demand for agriculture may increase on the order of 10%
  • Fire frequencies are projected to increase on the order of 20%, requiring additional readiness planning and more aggressive fuels management
  • Vegetation may be in transition, meriting additional monitoring and consideration of a more drought-tolerant planting palette for restoration

See Climate Ready North Bay: Region-wide Findings and Applications for more about the vulnerability assessment findings for the Northbay Region.


Potential Climate Ready Marin County Watershed Data Applications

  • Use of localized climate temperature and rainfall data to inform the MMWDs Water Resources Plan 2040
  • Presentations to raise public awareness regarding the benefits of greenhouse gas reduction (mitigation) and the need to plan for adaptation
  • Use of hydrologic data to inform partner agencies’ long-term planning for surface water supply
  • Integration of potential vegetation transition risks and fire hazards into natural resource management plans and fire mitigation planning
  • Use of hydrologic assessments to evaluate potential high value resource streams and riparian zones at risk, as well as development of strategies to build adaptation into maintenance and restoration planning


Future Analyses


Additional management questions were identified in the Climate Ready North Bay stakeholder process that the team determined were beyond the scope of this study, and therefore were not addressed as part of the Climate Ready North Bay study. However, they are included here to provide a starting point for subsequent climate adaptation work.

  • What are the implications of potential temperature increases for water demand? Historic and projected monthly temperature data have been provided to the MMWD to complete this analysis. Historic temperature data can be compared with the historic demand record to see if there is a strong enough correlation to merit extrapolation of future demand as a function of monthly temperatures.
  • What are the implications of potential temperature increases on water quality?
    An original project goal was to try to correlate temperature with algal blooms, but apparently there was an insufficient historic record of bloom timing to facilitate a correlation. However, concerns remain regarding potential impacts of rising temperatures on water quality in reservoirs and storage tanks.
  • Can assessments of sub-areas of interest for vegetation transitions and/or fire address how the frequency of “red flag” days that trigger maintenance restrictions due to fire weather might be impacted? Can a correlation of “red flag” days tracked since 2004 be made to climatic water deficit or other climate indicators?
    There is currently a companion project at UC Berkeley looking at controls on red flag days and the potential to model these under future climate scenarios.
  • What are the flooding issues specific to Corte Madera?
    The Climate Ready North Bay team recommends exploring the development of higher temporal resolution climate-hydrology products that can look at the interface of watershed runoff and sea level rise projections presently under development for the Marin coast.
  • What are the implications of climate change for site-specific riparian vegetation and restoration projects?
    The Climate Ready North Bay team recommends investigating the Point Blue Conservation Science climate smart planting palette developed for the Students and Teachers Restoring Watersheds (STRAW) program.
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