About this data
Sanitary collection systems throughout California are required to report sanitary sewer overflows (SSOs) and spills to the State Water Resources Control Board's California Integrated Water Quality System Project (CIWQS). This data is then made available for public download under the 'Interactive SSO Report' section of CIWQS.
Data represented here is limited to collection systems located in Region 2 of the State Water Board, which generally encompasses the nine-county Bay Area. Combined sewer systems are not required to report to the system, which in our region excludes only San Francisco.
Collection systems commonly report SSOs by calendar year, though data here is presented by water year. As defined by the United States Geological Survey (USGS), 'water year' is the 12-month period starting October 1 through September 30 of the following year. The water year is designated by the calendar year in which it ends. Thus, the year ending September 30, 2015 is called the '2015 water year'.
Sewer overflows typically occur during storms due to aging pipes or illegal connections that permit stormwater to enter the sanitary sewer and exceed the design capacity of the pipes which were generally designed only to carry sanitary waste. During dry years, such as the last four, considerably fewer SSOs are recorded in comparison to wet years.
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