A local agency has proposed creating a giant artificial lagoon in San Francisco Bay that could result in deadly algae blooms, destruction of wetlands, and other harmful impacts to the Bay. This unprecedented project would construct a 2.65-mile-long offshore barrier stretching from San Francisco Airport to the San Mateo border and walling off 670 acres of the Bay. Green Foothills is urging that the proposal be rejected.

Should we experiment with the health of the Bay?

In October 2023, the San Mateo County Flood and Sea Level Rise Resiliency District (known as “OneShoreline”) announced a proposed project to protect the shorelines of Millbrae and Burlingame from flooding and sea level rise. OneShoreline’s preferred option for this project is to construct a 2.65-mile-long offshore barrier out in the middle of San Francisco Bay, creating a 670-acre shallow lagoon. A tide gate would control the level of water inside the lagoon, and eventually, if sea levels continue to rise, the gate would be closed, permanently sealing the lagoon off from the rest of the Bay.

Let’s be clear: this proposed offshore barrier and lagoon is basically a giant experiment where the health of the Bay and Bay wetlands are at stake. It’s hard to know exactly what the impacts would be, since no one has ever walled off 670 acres of the Bay in this manner before, but it’s likely that there would be permanent and significant changes to the temperature, salinity, oxygen content, and other aspects of the water in the artificial lagoon, which could have drastic impacts to the wildlife that live in and around the lagoon area.

Will the lagoon turn into a warm, stagnant basin where algae blooms like those that killed thousands of fish in recent years would be a persistent problem? Will freshwater from creeks flowing into the lagoon change the salinity of the water, killing off fish and other aquatic species for which this area currently provides habitat? Will sediment washed into the lagoon from those creeks start to fill up the lagoon, requiring costly and environmentally harmful dredging? Will the offshore barrier be stable in the event of an earthquake, or will the Bay mud on which it would be constructed liquefy and cause the entire structure to fail?

One thing we definitely know is that the offshore barrier, as planned, would destroy existing wetland habitat. The barrier would require placing fill in the Bay on top of mudflat habitat, and according to OneShoreline’s Notice of Preparation, one end of the barrier would be built within a tidal marsh that provides habitat for an endangered bird (the California Ridgway’s Rail). For these reasons alone, the offshore barrier project should be abandoned.

This proposal is unpleasantly reminiscent of the 1800’s and early 1900’s, when diking off and filling in the Bay for development was a common practice. Although the purpose of this project would be to protect the Millbrae and Burlingame shorelines, the attitude is similar: a willingness to experiment with untested, artificial alterations to natural landscapes. We are seeing now how those experiments are failing – structures intended to protect the coast are impacting bluffs and beaches, Delta wetlands drained for agriculture are sinking, and development built on Bay mud is at risk of liquefaction. If Californians have learned nothing else in the past two centuries, we ought to have learned that tampering with nature is a dangerous idea.

There are other options to protect against sea level rise

Studies of the Millbrae and Burlingame shoreline area indicate that there are several natural options that could be implemented here to help protect against sea level rise. Although this shoreline has been extensively developed and there are few natural areas left for nature-based solutions like marshes, other strategies like creating coarse beaches and planting underwater aquatic vegetation can slow down storm surges and reduce flood risk. Levees or flood walls may need to be part of the solution for protecting this shoreline, due to the fact that commercial and industrial development has been built in some areas right up to the water’s edge. Ultimately, here as in many other urban areas around the world, we will need to have difficult conversations about whether and when to relocate development away from the shoreline and out of the path of sea level rise.

OneShoreline’s plans recognize that alternatives to the offshore barrier need to be considered. However, the California Environmental Quality Act (CEQA) requires that one preferred alternative be chosen to evaluate thoroughly, with the other alternatives receiving much less study. If OneShoreline continues on its path of choosing the offshore barrier as the preferred alternative, significant taxpayer funding will be spent analyzing an unwise and risky project that ultimately may never get approved by state and federal agencies.

Green Foothills, together with numerous other environmental groups, is urging OneShoreline to abandon the proposal for an offshore barrier and lagoon, and instead explore other flood and sea level rise adaptation strategies. We should not be risking the health of the Bay and Bay wetlands on a never-before-tried experiment.