New evidence: dredging Bolinas Lagoon would be futile

Scientists for the Bolinas Lagoon "Restoration" Project on Aug. 2 presented new results showing that Bolinas Lagoon will not fill up and become a meadow. The findings vindicate the Sierra Club's contention that major dredging is not needed to "save" or "restore" the lagoon.

For over a decade some local interests have promoted a project to dredge the lagoon. They claimed that activities in the watershed such as logging and over-grazing have filled the lagoon with sediment and so disturbed the ecosystem that the lagoon would fill up to become a meadow. Dredging was necessary, they argued, to enhance natural tidal processes that would flush out watershed sediment, retain open water, and restore the ecosystem.

New evidence, however, presented by Roger Byrne (UC Berkeley) based on extensive coring samples going back over 1,000 years, shows that the vast majority of the lagoon's sediment is not from the watershed and cannot be flushed out. Instead the corings show that Bolinas Lagoon is dominated by ocean sand flushed in by natural tidal processes. Dan Danmeier (Phil Williams Associates) indicated that lagoons dominated by ocean-sediment sources may fill in, but they do not fill up all the way to become meadows because they tend to reach an equilibrium that preserves open water.

Scientists reported that major earthquakes in 1906 and in the 1200s had lowered the floor of the lagoon, causing sedimentation to increase, as ocean sands flushed in to cover the lowered floor. In the centuries after each earthquake, the evidence shows, sedimentation gradually declines and reaches a long-term equilibrium that preserves open water. Dredging, like an earthquake, would simply cause more sediment to be flushed in to refill the dredge site by stripping adjoining beaches of sand and undermining the already-eroding Bolinas Bluffs. Thus, dredging not only is futile and unnecessary, but is a threat to adjoining properties.

Some local interests still express concern that without intervention the lagoon mouth will close, cutting off boat access. Scientists, however, said they can find no evidence that the mouth of Bolinas Lagoon has ever closed. Navigational guides from prior to 1906 depict Bolinas Lagoon as "bare at low tides and filled with small islets [1869]" and describe the lagoon's mouth as "a very contracted channel having only one foot upon its bar at low water [1889]." These accounts mirror earlier Spanish reports. Further, the corings show that the lagoon maintained open water and an open mouth for 700 years after the 1200 earthquake. Bolinas Lagoon clearly has great resiliency as a shallow, muddy lagoon, and since the temporary deepening in 1906 the lagoon has been recovering to a natural equilibrium with both open water and an open mouth - without the need for dredging.

Although the mouth of Bolinas Lagoon is likely to remain open, records of other lagoons dominated by ocean sediments indicate that temporary closures may occur. Wildlife has evolved with and adapted to summer closures of varying duration, which are part of the natural functioning of lagoons all along the Pacific Coast. Closures lasting a few hours to a few days change little in these lagoons. Longer closures favor birds adapted to more brackish water and may displace harbor seal summer haulouts, but do not affect salmon. For example, Pescadero Lagoon and Abbotts Lagoon, closed all summer, have abundant bird life. The small lagoon behind Muir Beach also closes all summer, yet still provides winter passage for salmon to upstream breeding habitat.

While noting that sedimentation declines as the lagoon approaches equilibrium, the scientific team did compute a closure projection based on continuation of current sedimentation rates. This projection suggests that infrequent combinations of strong storms without rain in El Niño years occurring during neap tides might cause the lagoon mouth to temporarily close about once every 10 years, beginning 40 years from now. But even in this unlikely but natural situation, there would be no need to dredge the mouth to facilitate natural ecological processes.

Because Bolinas Lagoon is part of the Gulf of the Farallones National Marine Sanctuary, dredging is not permitted for the purpose of facilitating boat access. Fishing boat access to Bolinas Lagoon will remain a problem due to channel depth and location.

As the Sierra Club and other environmental groups have long maintained, there is neither need nor justification for massive intervention to "save" Bolinas Lagoon or "restore" its ecosystem. This is not to say that absolutely nothing can or should be done, and further research, expected this fall, will indicate whether there may be some opportunity for modest "tinkering" that could enhance the lagoon or its wildlife. The Sierra Club would support such tinkering if it is scientifically justified, adaptively managed, and carefully monitored to demonstrate ecosystem benefit. If some of those ecosystem benefits incidentally turn out to benefit boat access, then so much the better. In the meantime, enjoy Bolinas Lagoon! - it's going to be around for a long time.

Gordon Bennett, chair, Marin Group