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Why is the A12 pond pink?

A pink salt pond with white salt crystals with a blue sky and golden hills in the background.

Frequently asked questions

  • What is Pond A12?
    • Pond A12 and its surrounding former salt evaporation ponds are part of the Don Edwards San Francisco Bay National Wildlife Refuge. These ponds are important for migratory birds, which travel thousands of miles to stop here for a safe place with food and shelter. We ask that you respect the wildlife and stay on the Alviso Loop Trail.
  • Why are these ponds PINK?
    • Don’t worry, it’s natural. Tiny microscopic organisms specialized to live in very salty water live here, including the microscopic algae known as Dunaliella salina and halobacterium.  
  • Why is this pond so salty?
    • The U.S. Fish and Wildlife Service manages these ponds for a variety of migratory and resident birds. Bay water from Alviso Slough enters the ponds through a water control structure in Pond A9 and water circulates through a series of ponds before discharging back into Coyote Creek. In a normal year of operation, we would also move water into the ponds closest to Alviso Marina. However, in 2021, Valley Water, the State Coastal Conservancy and the U.S. Army Corps of Engineers (USACE) began construction of the South San Francisco Bay Shoreline Project (Phase I) between the Alviso Slough/Guadalupe River and Coyote Creek. The levee construction requires very low water levels, therefore we did not replenish water into these ponds as they naturally evaporate throughout the summer. As a consequence, Pond A12 got saltier and pinker.  This represents an example of how salt is made. KQED article for more background about these ponds. 
  • Why is the South Bay Shoreline Project significant?
    • The Project will provide FEMA certifiable coastal levees, help our community manage flood risk and adapt to sea level rise, restore tidal marsh habitat, connect to our regional trail system, and provide pedestrian bridges and observation platforms. Shoreline Project Factsheet

Join us in protecting Alviso’s wetlands

By staying on the trail, you help us protect the Don Edwards San Francisco Bay National Wildlife Refuge. Walking to the pond’s edge, dogs, and drones are prohibited to protect migratory birds, endangered species, and other wildlife. Salt pond A12 looks particularly pink right now because levee construction for the South Bay Shoreline Project requires water levels to remain temporarily low. Once completed, the levee will protect our community from coastal flooding and sea level rise.

You help protect wildlife when you stay on the trail, take dogs no further than the Alviso Marina parking lot, and do not fly drones. Flying drones can stress birds and potentially cause them to abandon their nests.

Thank you for your curiosity and cooperation in protecting Alviso’s wetlands for wildlife and future generations.