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Water Quality Monitoring

The headwaters of San Anselmo Creek, in Fairfax, have the watershed’s best spawning habitats.

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Water Quality Monitoring

Friends of Corte Madera Creek Watershed strives to ensure that we can safely use our creek for recreation, and that fish and wildlife have the high-quality habitats they require to thrive.  

Bacteria Monitoring

Water quality has improved as sewers and private laterals have been repaired, reducing the amount of raw sewage reaching the creeks. Presenting Friends’ reports completed some years back allow us to track changes in water quality over time. 

Sampling in 2003 – 2006 

Volunteers gathered water samples in Corte Madera Creek and its tributaries during six sampling periods: summer 2003, winter 2004, summer 2004, winter 2005, summer 2005, and summer 2006. EPA Region 9 Laboratory analyzed bacteria concentrations for four periods and the Marin County Public Health laboratory analyzed two. (Protocols followed Standard Operating Procedure for Volunteer Monitoring of Surface Waters for Bacteria #1106.) For more information on testing for bacteria, see:  

For several reasons, the bacterial sampling effort was discontinued. First, because of the complex nature of the watershed (tidal and non-tidal), we were not able to meet an early goal to identify the source(s) of the high bacterial counts. Second, it is time-consuming and difficult to organize a high quality, all-volunteer effort and we could not continue to provide that level of commitment. Third, we attempted to obtain funding to implement a professional sampling effort, designed to identify sources, but we were not successful.  

Sampling in 2009 

Friends collaborated with Ross Valley Sanitary District on site selection for the Healthy Waterways projects, a bacterial monitoring project that measured E. coli, total coliform, and several species-specific forms of bacteroides. Five non-tidal sites were sampled during September (dry season) 2009: Larkspur Creek, Corte Madera Creek, Sleepy Hollow Creek, Fairfax Creek, and San Anselmo Creek. The Ross Valley Sanitary District no longer funds E. coli testing.  

Sampling in 2020-21 

During the early days of the COVID-19 pandemic when there was a period of warm weather, residents were eager to explore local recreational activities. We received numerous phone calls and emails asking if it was safe to swim in Corte Madera Creek. After several years, and extensive work by Ross Valley Sanitary District to upgrade its sewer system and support property owners in repairing leaky laterals, we felt it warranted more testing in the tidal reach where people are more likely to swim.  Ross Valley Sanitary District General Manager Steve Moore arranged for the Central Marin Sanitation Association to both provide supplies and then test our samples for Enterococcus, the species of fecal bacteria that persists in saltwater.  Our volunteers gathered samples in September and October 2020 and April through October 2021. The results are in the two annual summaries of data. Generally, the water quality meets criteria for body contact, except after rains.  

Temperature & Dissolved Oxygen Monitoring  

A.A. Rich and Associates installed and monitored thermographs that recorded water temperature during the summer of 1999, as part of a fishery resource conditions study of most creeks in the watershed. That information is available in Education and Outreach/Reports. She found that temperatures were commonly too high for healthy salmonid populations.  

Volunteers measured temperature, pH, dissolved oxygen, and specific conductivity in San Anselmo Creek at Bridge Avenue from June through October 2008. For reports on the results of these projects see: BridgeT_DO.pdf (632 KB).  

Volunteers measured temperature, pH, dissolved oxygen, and specific conductivity in Ross Creek and Phoenix Lake from May through October 2008. For a report on the results see:  RossCreekPhoenixLake2008.pdf (644 KB).  

For a report of temperature and dissolved oxygen testing done during 2012, see WQ2012Report.pdf (940KB), and for several creeks during 2016, WQ2016Report.pdf; during 2017-2018, Friends WQ Report 2017-18.pdf; during 2019, Temp_DO_Report_2019.pdf.  

Because of the shortage of qualified volunteers, since 2020 we have measured temperature and dissolved oxygen only in Ross Creek. See the results for 2020 in Ross Creek Temp_DO_Report_2020.pdf. The report for 2021 through 2024 is in preparation.  

Iron & Manganese  

Oxidized iron and manganese color rocks orange and black near the base of Phoenix Lake Dam. Photo by Sandy Guldman.

During several summers, we noticed degradation of water quality in upper Ross Creek during the summer, when flows are low. During 2011, water samples were collected on three days, about a month apart, at four locations covering some 1,600 feet of Ross Creek. For comparison, one sample was collected from San Anselmo Creek at Creek Park, upstream of the confluence of Ross and San Anselmo creeks. Download the technical report describing the results here: FeMn_Report_2011.pdf (4300 KB).  

A summary was published in Creek Chronicles: https://friendsofcortemaderacreek.org//home/creekman1/public_html/new_site/wp-content/uploads/IronManganeseinRossCreek.pdf