The colored ponds in south San Francisco Bay are salt evaporation ponds used for the commercial extraction of salt from the bay’s seawater.
In a process spanning several years, seawater is allowed to flow into shallow enclosed ponds on the shoreline. In the south bay’s sunny, breezy, climate, the water gradually evaporates, leaving behind increasingly concentrated saltwater, which is pumped on to a series of neighboring ponds, each one progressively saltier as evaporation continues, until salt crystals form.
A striking feature of the solar salt ponds is their vibrant colors at certain times of year, caused by organisms that can survive the increasingly salty conditions. In particular, brine shrimp, the micro-algae Dunaliella salina, and colored halobacteria can impart blue-green to red hues to the water as its salinity increases, creating a brightly colored patchwork visible from above.
A conservation program to return the man-made salt ponds back to natural tidal marsh, has seen commercial salt production in the Bay Area scaled back significantly since 2003, with the remaining operations mostly focused around the Cargill salt refinery in Newark.