The Loma Prietan - October 2015

Tough Choices for Searsville Dam and San Francisquito Creek

What to do after a reservoir becomes a marshland?

By Gabriel Lewis, Loma Prieta Intern

What should happen now that the dam has outlived its purpose?
What should happen now that the dam has outlived its purpose?

One grain of silt at a time, the problem has been approaching. Since its creation in 1892, the reservoir behind Searsville dam has been accumulating sediment; 2.7 million cubic yards later, the reservoir is nearly filled in. A number of solutions have been proposed, each weighing the risk of flooding in Silicon Valley, the agricultural demand for water, and the possibility of harm to threatened and endangered species. Whichever balance is struck, it will be precarious.

The stakes are high and the issues complex. San Francisquito Creek is formed by the confluence of Corte Madera Creek and Bear Creek just below the Searsville dam on Stanford University's Jasper Ridge Biological Preserve, and runs to San Francisco Bay past unincorporated Ladera, Stanford Weekend Acres, Stanford University,  Menlo Park, Palo Alto, and East Palo Alto. Long ago, the stream’s periodic floods deposited much of the soil upon which these communities are built, but now, such floods are damaging to the very cities that they made possible. Over 8400 homes and businesses are at risk from flooding today, and if sediment from Searsville reservoir is not managed properly, flooding could worsen.

The creek also provides some of the little remaining habitat for a number of rare and threatened species, including  tiger salamander and red legged frogs, as well as steelhead trout, which the dam currently obstructs from historic spawning habitat in the upper reaches of the stream. However, Searsville dam is not simply an environmental detriment. Sediment deposits upstream of the reservoir have created many acres of wetlands and riparian forest.  The fens and open water in the reservoir provide important habitat for migratory and resident bird and bat species, many of which are protected and some which are currently suffering declining populations.

In April, after four years of reviewing technical information and study of eight alternatives, Stanford’s faculty-staff Searsville Steering Committee recommended two alternative proposals. Their preferred option is to open a ground-level orifice through the base of the dam, ideally allowing for the free passage of spawning steelhead and other wildlife, and for the reservoir's water and lighter sediments to flow out into San Francisco Bay. However, researchers do not currently know to what extent the transported sediments will accumulate downstream, risking flooding; if such silting is likely, the Steering Committee instead recommends that the reservoir be allowed to silt in completely, becoming a marsh, with either a bypass stream channel or a fish ladder to allow the unrestricted passage of fish. The state of California division of dam safety has deemed the dam fully sound, and it is expected to remain so in either case.

Other options, including dredging out the reservoir to return it to full capacity, were reviewed and discarded as unacceptable; drying and hauling 150,000 truckloads of dredged material would be too detrimental to the surrounding ecosystems and communities, the Steering Committee concluded.

Some organizations contend that it would be better to remove the dam all at once. Other environmental organizations, including the Santa Clara Valley Audubon Society and the Committee for Green Foothills, support Stanford’s more measured approach one that includes consideration of impacts to endangered species and habitats throughout the watershed, as well as risks to downstream communities. 

While the Steering Committee has not ruled out the eventual removal of the dam, it wishes to begin with strategies that are cautious and reversible, given that the dam currently plays an important role in mitigating storm flows, and that any greater modification of the dam could have drastic and unpredictable effects upon the natural ecosystems above and below the dam.

The Sierra Club has not taken a position as to a preferred course of action.  We are at the beginning of a long process, and the Chapter will be weighing in as the necessary environmental studies and permits are being considered.

To learn more or get involved, contact Mike Ferreira, Chair Loma Prieta Chapter Conservation Committee His email is michaeljferreira@gmail.com.

                                   
author Bio: Gabriel Lewis is a Sierra Club intern, applying his BA in Economics to land use issues. He enjoys hiking and playing Spanish guitar.