A pipe leading to an air release valve broke, causing a sewage backup that led to a 1,000-gallon spill around 2 p.m., district officials said in a news release.
The spill occurred at Pump Station 13 near the Bon Air Center in Greenbrae.
The district recovered about 800 gallons of wastewater and directed it back into the sewer system, but approximately 200 gallons flowed from a manhole near the pump station into a storm drain that leads to the creek, the district's chief of operations John Clark said.
Crews shut down the pump station, drained the line and repaired the plastic pipe by 6 p.m. Tuesday, Clark said.
The district posted signs about the release and advised people to stay out of the creek, according to Robert Turner, a project manager with Marin County Environmental Health Services.
Water samples were taken Tuesday and Wednesday, and more will be taken today and Friday, Turner said. The samples will help determine the bacteria levels in the water.
The sanitary district informed all the regulatory agencies that must be notified about the spill, including the California Emergency Management Agency, county environmental health services and the Central Marin Sanitation Agency, Clark said.