Stretching 7 miles inland from the center of Monterey Bay at Moss Landing, Elkhorn Slough is a tidal marsh whose waterway is part of Monterey Bay National Marine Sanctuary. It is known by visitors for its pickleweed, mud flats and hundreds of species of birds. A population of California sea otters also calls the slough home. Managers have tackled erosion from neighboring farms, many of which have been purchased by the Elkhorn Slough Foundation to implement erosion control and organic growing to protect the slough.
More than 100 years ago the Salinas River flowed through the mouth of the slough. The river was diverted to the south to prevent flooding of farm land and today there is only seasonal summer flow down the old river channel to the slough. After years of grazing and other agricultural uses made possible through diking and diverting water, the slough is being restored, parcel by parcel, to become largest tidal marsh south of San Francisco Bay thanks to efforts by many partners.
Amidst a constant struggle to balance tidal flow from the ocean ahead and sandy soil erosion from behind, slough managers are now working to protect it from a future defined by climate change, including a sea that is rising and changing.
I asked Mark Silberstein, director of the Elkhorn Slough Foundation and a key player in restoring the slough, about what’s happening to it and what measures are being taken.
Q: What are the threats that the slough faces as a result of likely climate change scenarios, and what will you have to do to keep the slough in reasonably good shape?
A: Sea Level Rise is the most obvious. Over 50 percent of the tidal marshes of Elkhorn Slough were lost to diking, dredging and draining. When tidal marshes are removed from circulation by diking, they sink or subside. In Elkhorn Slough diked marshes are two to five feet below their original elevation. Tidal salt marshes grow in the intertidal zone within a range of a few feet. If the plants are underwater longer, they drown and die.
As diked areas return to tidal exchange, either through the erosion and failure of levees or the purposeful opening of areas to increase circulation, the salt marsh plants have died and the areas converted to poor quality muddy tide flats. With sufficient sediment supply, marshes can keep up with rising sea levels. Researchers working in the slough have shown that the marshes continue to trap sediment but the rate of sedimentation is not sufficient to keep up w subsidence and sea level rise.
As sea level rises and marshes are lost, they could theoretically migrate onto land that is now just above the tides. If those lands are developed or protected by hard structures, or on steeply rising topography, there will be no accommodation space and these tidal marsh habitats will be lost, along with their concomitant values.
Ground water depletion and salt water intruding into aquifers, as we pump ground water below sea level, is one of the most serious issues we face in the central bay where we are entirely dependent on ground water. As sea level rises, we might assume that intrusion into aquifers will be exacerbated. This in combination with unknown effects on climate and rainfall, could lead to loss of freshwater supplies.
Q: What is the slough’s role in combating climate change, such as sequestration, now?
A: Emerging data on the carbon sequestration capacity of tidal marshes and sea grass meadows indicates that these coastal “Blue Carbon” zones can capture and store CO2 for thousands of years and, for a given comparable area, trap more carbon than temperate or tropical forests. These systems are also effective at trapping and transforming nutrients coming off the land before they impact near-shore environments. All of these areas also play key roles in providing habitat for a tremendous diversity of other plants and animals and in fisheries production.
Q: What projects do you have in the works in this area?
A: We are working with many partners on the Ridgeline to the Tideline initiative. Recognizing that to protect and manage these valuable coastal environments, we can’t stop at the water’s edge. The Elkhorn Slough Foundation has acquired upland properties that drain into the slough wetlands and that, because of past management and history, have been a chronic source of water quality degradation and habitat loss.
We’ve reconfigured the farm footprint, taking steep eroding hillsides out of cultivation and managing the productive areas of the farm with sustainable practices including converting them entirely to organic farming and installing infrastructure like sediment catch basins and vegetated buffer strips between fields and waterways. These changes have resulted in improved water quality and have reduced ground water extraction by 700 million gallons a year in a part of the bay that has been heavily over-drafted. In addition, we are restoring native vegetation on the hills in a way to capture more rainwater and increase infiltration into the ground water. At the same time, we are keeping seven farms in production, leasing to local farmers.
The first phase of the most ambitious restoration in the history of slough wetlands has been completed. Sixty-one acres of diked and sunken wetland was restored to an elevation to regrow salt marsh and designed to accommodate sea level rise in a way that will sustain the marshes. More than 200,000 cubic yards of sediment was brought in to elevate the sunken marsh plain up to the right level for the plants. We are initiating the second phase that will add an additional 30 acres of healthy marsh. The restored area is already being explored by otters and tremendous numbers of wetland bird species.
Both of these projects reflect the partnerships that have worked to conserve the slough over the past four decades including the Elkhorn Slough National Estuarine Research Reserve, California Department of Fish and Wildlife, The Nature Conservancy, the Elkhorn Slough Foundation, the National Marine Sanctuary, the U.S. Fish and Wildlife Service, the Coastal Commission, the State Coastal Conservancy, the California Department of Parks and Recreation, and a legion of scientists from MLML, UCSC, MBARI, MBA, CSUMB, Stanford, many of the State Universities and University of California, among a host of other partners. We recognize that the issues that impact the slough are global. We try to follow the old mantra: “Think Globally, Act Locally.”
Silberstein will address the Aptos Library Program at 7 p.m. Tuesday at the Rio Sands Community Room, 116 Aptos Beach Drive. For information, visit fscpl.org/3693-2/.
Ocean activist Dan Haifley can be reached at dan.haifley@gmail.com.
