Clearing the Floodplain, Adapting to Change

Elkhorn Slough conservation efforts began in the late 1960s and early 70s with work by The Nature Conservancy to purchase the first conservation lands here. By the late 1970s, the National Oceanic and Atmospheric Administration designated 1000 acres in the southeastern areas within the estuary as the Elkhorn Slough National Estuarine Research Reserve. This has grown to encompass over 1700 acres. The main channel of the slough became part of the Monterey Bay National Marine Sanctuary in 1992. The California Department of Fish and Wildlife owns and manages ESNERR, and has additionally designated this land as a State Ecological Reserve. CDFW also owns and manages the 872-acre Moss Landing Wildlife Management Area, along the north shore of the slough. The Nature Conservancy and the Elkhorn Slough Foundation have invested in protecting over 4,000 acres of land in the watershed. These multiple designations and strategic land acquisitions recognize the importance of Elkhorn Slough as a vital ecosystem, protecting significant natural resources.

Today, the Elkhorn Slough Foundation is the single largest landowner in the Elkhorn Slough watershed, with 3700 acres owed in fee and 300 acres protected by conservation easements. ESF conservation objectives aim to support resilient ecosystems, and, since the Foundation’s establishment in 1982, it has actively restored 450 acres of degraded land, eradicated non-native invasive jubata grass on 2,000 acres, and implemented best management practices on its working farmlands. Recently, cooperative planning efforts have focused on identifying climate change impacts to vulnerable tidal marsh areas.

Restoration is a guiding long-term management objective. To this end, the Tidal Wetland Strategic Plan describes the Slough’s unique habitats, characterizes the main human impacts causing loss and degradation and provides broad conservation and restoration recommendations. This project, led by the Elkhorn Slough Reserve and supported by ESF, emphasizes targeted habitat restoration, highlighting key strategies to enhance degraded wetland sites such as adding sediment to rebuild sunken marshes, sequestering carbon, restoring tidal creek networks, enhancing tidal exchange or improving upland best management practices.

ESF works to protect land and water resources in the Elkhorn watershed and surrounding areas in Monterey County, California. The Elkhorn Slough watershed consists of approximately 45,000 acres, and currently there are more than 8,000 acres – approximately 18% of the watershed – under conservation protection. The slough harbors California’s largest tract of tidal salt marsh outside San Francisco Bay. Additionally, Elkhorn Slough tidal habitats encompass extraordinary biological diversity, with over 340 species of birds identified in and around the slough, including more than 135 aquatic species, 550 species of marine invertebrates and 102 fish species. The Elkhorn Slough is also home to a healthy population of sea lions, harbor seals and California sea otters.

In addition to the rich diversity of flora and fauna in the tidal wetlands, the upland habitats within the watershed include three of the top ten most imperiled U.S. habitats: freshwater wetlands, coastal prairie and maritime chaparral. These systems are amazingly diverse, coastal prairie being the most species rich grasslands in North America. Along with providing habitat for a diverse range of life, wetlands provide a critical service to the environment. Wetlands provide a buffer from land to sea – protecting the water from soil erosion and the land by reducing the impact of flooding. As natural filters, wetlands can remove impurities from the water before it enters our streams and oceans. Wetlands also have been proven to be carbon sequesters — removing and storing greenhouse gases from the earth’s atmosphere, slowing the onset of global warming.

Estuaries like Elkhorn Slough are among the most threatened ecosystems in California, facing rates of habitat loss between 75 and 90 percent. As a result, a disproportionate number of rare, threatened and endangered species reside in these areas. In the Elkhorn Slough watershed, two-dozen species are included in these categories. Recognizing the value of these resources to the country, the National Oceanic and Atmospheric Administration designated areas of Elkhorn Slough as part of the Monterey Bay National Marine Sanctuary and as a National Estuarine Research Reserve. The complexity of the area provides a fascinating and useful living laboratory to conduct research on a variety of topics. The lessons on water quality, species impacts, tidal influence and climate change learned here at Elkhorn Slough have been widely applied.

The wetland environment of Elkhorn Slough contains approximately 2,690 acres of distinct habitat types. This includes 293 acres of subtidal channels and tidal creeks, 1,605 acres of mudflats and 796 acres of intertidal salt marshes and tidal creeks. These habitats provide a rich ecosystem for the diversity of species noted above. The climate, geomorphology and tidal hydrology have gradually shaped the spatial distribution of Elkhorn Slough’s estuarine habitats throughout the past 20,000 years.

Over the past 150 years, human actions have altered the tidal, freshwater, and sediment processes that are essential to support and sustain Elkhorn Slough’s estuarine habitats. Approximately 50 percent, or 1000 acres, of the tidal marsh in Elkhorn Slough has been lost since 1870 due to human activities. Major physical modifications to the estuary have caused and are currently causing high rates of habitat loss and degradation in Elkhorn Slough. Human impacts have resulted in ongoing marsh loss and estuarine habitat erosion, degraded water quality conditions, increased levels of pollution, eutrophication and increased numbers of invasive species. Almost 73,250 cubic yards of sediment are exported each year from Elkhorn Slough into Monterey Bay from habitat erosion. Bank erosion rates along the main channel of Elkhorn Slough range from 1 to 2 feet per year. These rapid changes not only affect the estuary’s animals and plants, but also impact neighboring private lands, public access sites, and railroad and road infrastructure.

The Elkhorn Slough Tidal Wetland Project is a collaborative effort to develop and implement strategies to conserve and restore estuarine habitats in the Elkhorn Slough watershed. It involves over a hundred coastal resource managers, representatives from key regulatory and jurisdictional entities, leaders of conservation organizations, scientific experts and community members. The main goals of the Tidal Wetland Project are to: (1) conserve existing high quality estuarine habitats, (2) restore and enhance degraded estuarine habitats and (3) restore the physical processes that support and sustain estuarine habitats. Particular emphasis in the restoration planning process has been placed on the first goal, which aims to stop the ongoing marsh loss and estuarine habitat erosion in Elkhorn Slough. The Parsons Slough Sill Restoration has been a key element of the overarching Tidal Wetland Project, and its recent completion is a testament to the leadership and dedication of the Elkhorn Slough Foundation and partners to protecting and preserving critical tidal marsh habitat.

Climate change and sea level rise have been increasing management considerations.

The major thrust began with a Coastal Impact Assistance Program grant to the Reserve to look at the issue of marsh loss. Cooperative climate impact assessments taking place at the Slough include examinations of marsh elevations, tidal dynamics, and sediment deposition to determine which, if any, salt marshes at Elkhorn Slough will be sustainable in the face of projected sea level rise. The Tidal Wetland Program works to promote resilient systems as well as adapt to future climate change impacts.

ESF promotes adaptation to climate change-driven sea level rise impacts by (1) “clearing the floodplain” and (2) building adaptive infrastructure.

Clearing the Floodplain

“Clearing the floodplain” is an approach to conservation that prioritizes land acquisition and management planning to allow for tidal marsh migration as well as protect and restore habitat diversity. Efforts to “clear the floodplain” are increasing in coastal communities across the country – for example, the New Jersey Sea Grant’s Manual for Coastal Hazard Mitigation emphasizes the importance of combined management interventions, and details mitigation techniques including beach nourishment, dune protection, and floodplain restoration, as well as coastal regulations aimed to reduce coastal hazards. The report notes local land use code updates can support floodplain conservation and restoration efforts by limiting construction and requiring enhanced mitigation measures in hazard prone coastal areas. In California, the ESF has used GIS mapping to identify land parcels intersecting with the 100-year floodplain, helping to prioritize acquisition and management of lands with current natural resource values and those that are at risk for sea level rise impacts.

In addition to using adaptive management to clear the floodplain, ESF is building adaptive infrastructure to reduce vulnerabilities to climate change impacts.

Building adaptive infrastructure

ESF and the ESNERR designed and constructed water control structures that can adapt to rising sea levels. The Parsons Slough Sill Project is a restoration project designed to conserve healthy habitat in Elkhorn Slough. The project involved constructing an underwater wall called a sill to maintain a healthy slough ecosystem and correct the problem of tidal scour, which is the erosion of soft muddy sediment in the slough due to the velocity of the water as it moves in and out with the tide. Historically the slough was a shallow estuarine lagoon and less prone to scour, but creation of a deep water opening to Moss Landing Harbor at the mouth of the slough, as well as increasing tidal extents have caused the slough channel to deepen. The sill is designed to slow the tide coming out of the Parsons Slough and reduce erosion in Elkhorn Slough from Parsons Slough to Monterey Bay. This adaptive management intervention aims to help to maintain the diverse range of habitats found in this area. This project was funded by a $4.5 million dollar ARRA grant through NOAA.

The sill is designed to accommodate increasing rates of sea level rise associated with climate change such that it will remain effective with a sea level rise of 1.6 feet, which is a median forecast of sea level for the year 2060. The sill is also designed to support additional weight, if, because of sea level rise, a future decision is made to make it higher. For more information on the ESF Parsons Slough Sill project, visit the FAQ and Project Information webpages.

Acquisition of new properties as well as monitoring and restoration efforts on existing properties is anticipated to continue.

The Elkhorn Slough Foundation works with a tremendously diverse community – in the large sense – so there is a diversity of opinions about climate change. The land trust has primarily communication about climate change impacts through publications and reports but also supports the Elkhorn Slough National Estuarine Research Reserve Coastal Training Program to facilitate discussion at community meetings where the issue and supporting data are presented. Additionally, the land trust’s approach of “clearing the flood plain” is communicated to funders and the board.

While opinions about climate change projections may vary, historic data demonstrates clear trends of ecological changes in land and aquatic systems – a paleo-ecological study published in Restoration Ecology concluded that marsh conditions in the Elkhorn Slough showed a significantly altered rate of sediment accumulation, suggesting a drowning marsh plain associated with sea level rise and anthropogenic land use activities such as erosion-prone agricultural activities. These impacts, as well as future risks, can be successfully addressed through cooperative planning.

Key partners

The Foundation works with agency partners and the Elkhorn Slough National Estuarine Research Reserve, one of 28 NERR field laboratories in the nation, to promote conservation in the Slough. The Elkhorn Slough NEER is administered by NOAA and managed by the California Department of Fish and Wildlife. The Tidal Wetlands Project involves over one hundred coastal resource managers, representatives from key regulatory and jurisdictional entities, leaders of conservation organizations, scientific experts and community members.