Point Reyes Light - August 4, 2005
Surprises in study may guide fate of Bolinas Lagoon
By Chris Stafford
Mother nature is mostly to blame for sediment building up in Bolinas Lagoon, which is now expected to close the lagoon’s mouth in the next 50 years.
The great earthquake of 1906 greatly hastened the sedimentation, but erosion from Bolinas’ oceanside cliffs may be now the major source of sediment silting up the lagoon.
So said two consultants Tuesday, Dr. Roger Byrne, a geology professor from UC Berkeley, and Don Danmeier of Philip Williams and Associates in San Francisco
Hiring the consultants was a Project Reformulation Advisory Committee of Marin Open Space District.
Speaking to more than 85 county officials, environmentalists, and other residents at Civic Center, Dr. Byrne said he is "90 percent sure the origin [of most sediment these days] is the Bolinas cliffs."
The professor’s theory suggests that most silt entering the lagoon is being carried into the mouth by high tides and deposited there when tides go out.
The professor will check for barium in lagoon sediment to confirm his theory. The element is less common in the oceanside cliffs than on the slopes inland of the lagoon.
1906 earthquake is culprit
When he analyzed the strata of deposits on the lagoon floor, Dr. Byrne said, he was surprised by his findings. The professor said he "was expecting logging to be the biggest impact [in causing siltation]. In fact, it appears the earthquake of 1906," put more sediment in the lagoon than any other event.
The earthquake caused the north end of the lagoon to drop 1.5 meters, he noted, and the area was transformed from an intertidal mudflat to open water.
Hydrologist Danmeier’s data confirmed this. The earthquake also changed the tidal currents in the lagoon, changing the rate of sedimentation, he said.
In the years after the 1906 earthquake, the lagoon was deepest at its center and began to acquire sediment there – a pattern also seen in lakes, Dr. Byrne said.
Dr. Byrne has found that the rate of sedimentation in Bolinas Lagoon was faster in the twentieth century than in the 1800s although logging in the surrounding hills was then at its peak.
Supervisor Steve Kinsey asked if natural forces have shaped the present lagoon more than human activities. While Dr. Byrne emphasized natural causes for the lagoon’s siltation, he agreed that "humans have changed the equilibrium."
Lagoon could close off from sea
Hydrologist Danmeier compared current topographical surveys with surveys conducted in 1854 and 1929 to show how the geography of the lagoon has changed. Calling his conclusion "conservative," Danmeier said that at the current rate of sedimentation, the mouth of the lagoon will close in approximately 40 years.
Given natural forces, "it seems to me that human activity pales in comparison, that we’re nibbling around the edges," a Point Reyes Station resident said. "Why don’t we leave it alone?"
Replied professor Byrne, "Right now, I don’t see a need for dramatic surgery."
Gordon Bennett of Inverness asked if it is inevitable that the mouth of the lagoon will close permanently.
Danmeier said that sandbars across the lagoon mouth could breach naturally in winter when rain-swollen creeks could break through.