Marina Lagoon, a remnant of a tidal slough that was diked and dredged, serves the City as a flood control basin, recreation area, aesthetic amenity, and ecological resource, and is managed to optimize these benefits. The lagoon meanders in a northerly direction from its inlet at the Belmont city limits to its outlet into Seal Slough, a distance of about 4 miles. The lagoon ranges from 300 to 400 feet wide, and averages a depth of 6 feet at mid-channel during the summer. Much of the lagoon is designed with side slopes of 5:1 for optimal bank stability. An earthen levy and concrete slide gate structure controls inflow from O’Neill Slough at its entry from Belmont, and at the north end a concrete dike impounds and separates the lagoon from Seal Slough and San Francisco Bay.
The lagoon’s primary water source is tidal flow from San Francisco Bay through O’Neill Slough, at a rate of approximately 52 million gallons per day annualized. Bay water is augmented by perennial low volume fresh water inflow from Laurel Creek and lesser drainage sub-basins within a 10.3 square mile watershed, but comprises only about 0.3 percent of total annual inflow. During the wet season, stormwater runoff can comprise a larger proportion of inflow over the short-term, depending upon the size of the storm event.