Lagoon System

The Foster City Lagoon, as a drainage detention basin, is designed to successfully withstand a storm of 100-year return frequency or a storm of such severity that it is likely to occur only once each century. The lagoon therefore provides maximum drainage security for Foster City. Stormwater collected throughout the City flows to the Foster City Lagoon. All storm water enters the storm drain system through curb inlets and catch basins and drains into the lagoon from which it is pumped into the bay.

How well does the Foster City Lagoon system work as a drainage catch basin? During the El Niño rainstorms of 1997/1998, which were very close to a 100-year frequency severity, the City of Foster City experienced no flooding while surrounding cities in San Mateo County experienced major drainage problems. Thanks to the effectiveness of its lagoon system, Foster City has never experienced major flooding.

It is unlawful to dispose of any substances that may contaminate the lagoon and storm water system or cause blockages of the storm drainage system. In the event of flooding due to a blockage of the storm drain system, as well as any lagoon problems such as algae or weed growth, please contact the Foster City Public Works Maintenance - Streets Division.  Beach Monitoring information can be viewed on the San Mateo County website here: smchealth.org/beaches

Why Do We Adjust the Lagoon Levels Seasonally?

Our primary reasons for seasonal adjustment are:

• To provide adequate storm water controls;
• To allow storm water to runoff to prevent flooding;
• To maximize recreation potential during weekends and holidays;
• To minimize nutrient concentrations in the lagoon;
• To minimize detrimental bacteria growth;
• To comply with the operational requirements of the permit issued by the Regional Water Quality Control Board; and
• To help maintenance personnel clean over 1,000 storm catch basins throughout the city.

Lagoon Facts

• The surface area of the Foster City Lagoon is approximately 212 acres.
• The average depth of the lagoon is approximately six feet (6').
• The average temperature of the lagoon is approximately 60° in the winter and 69° during summer.
• Maintenance staff uses an environmentally safe pond dye that blocks sunlight disrupting photosynthesis, which impedes the growth of aquatic weeds. The secondary effect is the beautiful blue color, which is purely aesthetic.
• In the summer, it is expected that algae will become more noticeable, particularly near beaches and shallow areas of the lagoon. Treatment methods, such as dye, are utilized to restrict sunlight and limit the algae’s ability to photosynthesize.
• Please note that per Foster City Code Chapter 9.52 PROPERTY MAINTENANCE (codepublishing.com) (R) it is the responsibility of the property owner to maintain those portions of lagoon or waterfront properties directly adjacent to or observable from the water or land.  

Lagoon Levels

The City's Lagoon Management Plan sets standard lagoon water levels for winter and summer periods.

Raising Levels:

Around mid-March, water in the lagoon is raised to summer operating levels. The water level will usually be maintained between one or two feet from the elevation 100, which is the design height of most lagoon bulkheads. The water level will be generally higher on weekends and holidays to accommodate recreational uses, and the water level will be lower mid-week. This is done to promote maximum water exchange in the lagoon.  

Lower Levels:

In mid-November, the lagoon level is lowered to prepare for frequent winter storms. If a major storm is forecast during summer, the levels will be lowered to accommodate the anticipated runoff.