We are doing the first major salt marsh restoration project in Elkhorn Slough, by adding sediment to a subsided marsh, and are in the midst of thoughtful experimental design to ensure we have a high, sustainable marsh that will be more sustainable in the face of sea level rise than our other low marshes. The Elkhorn Slough Tidal Marsh Restoration Project was a primary focus of TWP in 2014. Much of the year was spent evaluating the existing conditions and planning the project. This effort was led by ESA-PWA and included H.T. Harvey, Moffatt and Nichol, Moss Landing Marine Laboratories and ENGEO. Select the highlighted links to view the various reports. The Existing Conditions Report characterizes the current hydrology, water quality, geology, soils, biological resources, land use history and other environmental resources in and around the project site. This was pared down and formed the basis for the Restoration Plan. The Restoration Plan (5MB, and Appendices 18MB) covers a number of aspects of the project including; design criteria, project alternatives, sediment suitability, construction sequencing and methods as well as experimental design and adaptive management. We have enlisted a Salt Marsh Working Group, a group of scientists with expertise in marsh restoration both locally and worldwide. They have helped us with critical questions on the marsh elevation, slope and tidal channel configuration. CEQA is currently underway with the California Department of Fish and Wildlife (CDFW) as the lead. Two multi-agency meetings were held in 2014 (October and November) to gather agency input on regulatory compliance issues.

CDFW released the Initial Study Mitigated Negative Declaration. The public review period runs from July 10 – August 24, 2015. Permit applications are in preparation. It is anticipated that at least 10 permits will be needed from various regional, state and federal entities. Fundraising for construction is underway, we still need to raise about $2.5M. Construction is currently slated for 2016.

Restoration Plan for the Tidal Marsh Restoration Project at the Elkhorn Slough Reserve (24MB, PDF).

General Information

One hundred years ago, extensive tidal marsh occurred on the shores of Elkhorn Slough. By the 1930s and 40s much of the salt marsh on the southern and eastern shores had been diked and drained. Decades later, these dikes began to fail, reintroducing tidal waters to the reclaimed lands. However, while the wetlands were drained, the soils consolidated and decomposed and the land subsided. The land surface dropped by up to several feet. This made the soil too low and wet to support salt marsh when the tides returned. This extra room (accommodation space) also contributes to a shortage of sediment in the slough that affects the viability of tidal marsh elsewhere.

The Elkhorn Slough Tidal Marsh Restoration Project will add sediment to these former marshes, returning the land to the elevation necessary for marsh plants to thrive. If fully successful, the healthy plants will create and capture sediment in the future, jumpstarting the process that sustains healthy tidal marsh habitats as sea level rises.

The sediment addition will also reduce Elkhorn Slough’s tidal prism, the volume of water that flows in and out of the slough each tidal cycle. The tidal prism today is larger than it was historically, and the additional flowing water has eroded banks and soft mud habitats.

The goals of the project are to:

Goal 1: Increase the extent of tidal marsh in Elkhorn Slough.
Goal 2: Reduce tidal scour in Elkhorn Slough.
Goal 3: Protect and improve surface water quality in Elkhorn Slough.
Goal 4: Provide resilience to climate change to Estuarine ecosystems in Elkhorn Slough
Goal 5: Increase understanding of how best to create salt marsh

Detailed Reports

These reports are data rich and because of that they are also very large.
Choose the one that interests you the most, or read them all!

Project overview and frequently asked questions:

Elkhorn Slough Tidal Wetland Project Strategic Plan:
Elkhorn Slough Tidal Wetland Strategic Plan – low resolution (ESTWP_PLAN_050207_lres.pdf, 2.23MB)
Elkhorn Slough Tidal Wetland Strategic Plan – high resolution (ESTWP_PLAN_050207_hres.pdf, 20.4 MB)

Parsons Slough Wetland Restoration Plan:
A Wetland Restoration Plan for Parsons Slough was completed in 2010. That plan focused on the large scale addition of sediment to restore tidal marsh to the area. It includes a Draft Initial Study for a 2-million cubic yard sediment addition project.

Parsons Slough Sill Final Initial Study/ Mitigated Negative Declaration:

About Tidal Marshes

Elkhorn Slough’s salt marshes are vegetated wetlands that are subjected to tidal action.  They are dominated by pickleweed, and in many parts of the Slough the marshes are thousands of years old.  California’s salt marshes have become a rare habitat in recent history and the various laws in place protecting them are evidence of their value.

Elkhorn Slough’s salt marshes are home to to dozens of native plant species, and are used by invertebrates, fish, reptiles, birds and mammals, including sea otters (see videos below) and for resting, feeding, breeding and refuge.  Tidal salt marshes undergo the processes of subsidence and sediment accretion, which often allow them to maintain a stable elevation in pace with the natural rate of sea level rise.   The process of accretion both traps and creates sediment, and this helps slow water velocity to control erosion, reduce tidal channel widening, maintain important subtidal habitats, and provide shoreline protection.  Marshes have also been shown to improve nutrient filtration, which provides a health benefit to the estuary’s aquatic life forms and to humans by reducing eutrophication and the transmittal of pathogens.  Furthermore, the unique sediment conditions of salt marshes allow them to store disproportionate quantities of soil carbon and help them to remove nitrogen from the hydrosphere.

Learn more about the value of local salt marshes, download this technical report: Woolfolk, A. and Labadie, Q. 2012. The significance of pickleweed-dominated tidal salt marsh in Elkhorn Slough, California. Elkhorn Slough Technical Report Series 2012:4

Fifty percent of the tidal salt marsh in Elkhorn Slough has been lost in the past 70 years. This habitat loss is a result of past diking and draining, and increased tidal flooding, which “drowns” the vegetation. The Elkhorn Slough Tidal Marsh Restoration Project is designed to plan for the restoration of up to 100 acres of lost marsh.  We plan to add sediment to some of these former marshes, returning the land to the elevation necessary for marsh plants to thrive. If fully successful, the healthy plants will create and capture sediment in the future, jumpstarting the process that sustains healthy tidal marsh habitats as sea level rises.


Otters using the salt marsh in Elkhorn Slough