The Peyton Slough Marsh Complex

McNabney and Moorhen Marshes

Mt. View Sanitary District’s (MVSD) treated effluent flows into a series of marshes, rather than a deep-water outfall as most treated wastewater does. These marshes, enhanced by the reclaimed effluent, are rich and diverse in wildlife and easily accessible to students, birders, researchers, and the general public. Biologists have identified 116 bird species 69 plant species, over 20 mammals and at least 34 species of aquatic invertebrates making their homes in the wetlands. At least 15 species of birds use the marsh complex for nesting.


Marsh WrenMoorhen Marsh is a 21-acre constructed wetland, the first wetland on the entire West Coast to use treated effluent as its primary water source. Moorhen Marsh is located on the west side of I-680 near the MVSD Administration Building and Treatment Plant and immediately adjacent to the Interpretive Center used by the students participating in the Wetlands Field Trip Program. Download an aerial map of the marsh complex.


McNabney Marsh (formerly known as Shell Marsh) is a 130-acre restored, seasonally tidal wetland located east of I-680. The major area of McNabney Marsh lies southward and eastward of Peyton Slough and is referred to as McNabney Marsh East. The 20 acres comprising McNabney Marsh West lies west of Peyton Slough. McNabney Marsh West, combined with the 20 acres immediately adjacent to and east of the slough, makes up the 40-acre portion of the McNabney Marsh belonging to MVSD. The remainder of the marsh and adjoining uplands to the east are currently owned by the East Bay Regional Park District. Since 1988, MVSD has managed McNabney Marsh jointly with the East Bay Regional Park District, the Contra Costa County Mosquito and Vector Control District and the California Department of Fish and Wildlife.

In 2009, following a realignment of Peyton Slough and reconstruction of the Peyton Slough Tide Gate Structure by Rhodia, Inc., the Peyton Slough Tide Gates were opened and the Peyton Slough Marsh Complex was introduced to seawater for the first time in over a century. In the intervening years, McNabney Marsh has experienced a slow transition from a brackish wetland toward a tidally influenced wetland / tidal marsh. There have been a number of challenges and unanticipated consequences resulting from this transition.

Flooding and Mosquito Control The Peyton Slough Tide Gate Structure was originally constructed around the turn of the 20th Century to protect the property located upstream on Peyton Slough from flooding due to high tides. Historically, the Tide Gate was operated to allow flow out to Carquinez Strait only. This operation maximized drainage of Peyton Slough and minimized the amount of ponded water, thereby also minimizing impacts from mosquitoes.

When the tide gates were opened in 2009 at high tide, existing petroleum pipelines and roadways were threatened by flooding. To address these flooding risks, the pipeline have either been removed or elevated to minimize the flood risk. The Tide Gate operator and property owners adjacent to Peyton Slough work collaboratively to monitor water levels at low points adjacent to Waterfront Way, and other key locations. The Tide Gates are opened and manipulated to allow seawater into the upstream marshes at high tides while managing the flood risk.

The “Pumping Up” Effect The introduction of seawater into McNabney Marsh is desired for many reasons. The increased salinity associated with seawater has helped manage and curtail the growth of cattails which were threatening to take over much of the open water surface of McNabney Marsh. The introduction of seawater also improves the overall water quality within McNabney Marsh by increasing the amount of dissolved oxygen in the water of the marsh, thereby improving living conditions for all forms of aquatic life.

Unfortunately, while high tides provide sufficient energy to push large volumes of water up Peyton Slough and into McNabney Marsh, there is not enough slope or elevation differential for McNabney Marsh to drain sufficiently during low tides. The result is that over a period of several daily tide cycles with the tide gates open, McNabney Marsh will “pump up” and get full.

When the marsh is full, the upper end of the marsh tends to become stagnant. This stagnation contributed to an algae bloom in 2012 that resulted in a large algal mat on the water surface throughout the late summer of 2012. The algal mat did not dissipate until several rain storms helped break up the mat and, hopefully, also flushed much of the algal material out of the marsh and back out to Carquinez Strait.

UPRR Railroad Bridge One of the constraints to draining McNabney Marsh may be at the location where Peyton Slough becomes very narrow as it crosses under the Union Pacific Railroad (UPRR) bridge. The Peyton Slough channel is reduced from about 30 feet wide to about 10 feet wide under the railroad. Replacing the UPRR bridge has been a long time goal of the Peyton Slough Wetlands Advisory Committee. In 2011, Mt. View Sanitary District began an effort to raise the funds necessary to replace the bridge and widen Peyton Slough to improve drainage of McNabney Marsh. With significant help from the San Francisco Bay Area Water Quality Control Board, the California Department of Fish and Wildlife, and the Dorothy M. Sakazaki Environmental Endowment Fund, Mt. View Sanitary District has raised a little over $1.1 million toward the estimated cost of approximately $2 million to replace the UPRR bridge. The District has initiated a preliminary design study to confirm the hydraulic parameters and to evaluate design and construction alternatives to identify the most cost-effective project. As this project continues to develop, more information will be available.

Adaptive Management The current management strategy for McNabney Marsh is best described as adaptive management. Each step of the transition to a tidal marsh has challenged what we thought was known about the marsh and the District and the Peyton Slough Wetlands Advisory Committee continue to work to improve the habitat values and recreational opportunities associated with this very valuable wetland.

RuddyDuckHabitat and Recreational Opportunities The marsh complex is an important stop over along the Pacific Flyway and a number of species use the wetlands as nesting habitat including American Avocet, Black-necked Stilt, Canada Goose, Mallard, Marsh Wren, and Ruddy Duck. Download the MVSD Bird Checklist to help you identify these different species of avian visitors to our marshes. If you would like to make special arrangements to bring a group of people out for a birding field trip, please contact the District at mailto:info@mvsd.org or 228-5635.

Visitors also have access to both McNabney Marsh and Moorhen Marsh Monday – Friday from 7:00 am to 5:00 pm excluding holidays. Please check in at the Administration Building before heading out. The observation platform along the plant road in the south end of McNabney Marsh is also open the first Saturday of every month from 7:00 am to 5:00 pm. Download driving directions and map here.


9-30-2010-044Fall 2010: Six interpretive panels were installed throughout Moorhen Marsh by the MVSD Board of Directors.Topics covered in the panels include: the historical significance of Moorhen marsh, mosquito control in wetlands, the Peyton Slough Watershed, and marsh denizens. Come out and take a look!

The tide gates on Peyton Slough in Rhodia Marsh were closed on October 21st. We anticipate the gates being re-opened around May 1, 2011.


Wild Birds Unlimited of Pleasant Hill held a bird walk on Saturday, October 2nd and MVSD held its annual Fall Bird Walk on Saturday, October 23rd. Forty-five species were seen on the October 23rd trip including Belted Kingfisher, Common yellow-throat, American White Pelican, and Green Heron. The highlight of the day was a female Cooper’s Hawk that flew from the direction of I-680 towards McNabney Marsh and struck and killed a Killdeer near the ground in view of the participants on the observation platform. After the hawk caught its breath, it picked up its prey and flew back over the freeway towards Moorhen Marsh. Cooper’s Hawks are not common in wetlands so this was a very unusual sighting!


otter wideRiver Otter sightings
A family of nine river otters has been repeatedly spotted in both Moorhen Marsh (Peyton Slough, Ponds A2, C, D, and E) and McNabney Marsh (Peyton Slough and West Channel) from mid-October through mid-November. Otters have been seen actively fishing, grooming, and walking across levees.


International Bird Rehabilitation Rescue Center (IBRRC) in Suisun City has released 4 species in the marsh complex this fall including: Common Moorhen, Gadwall, Mallard, and Wilson’s Snipe.  For more information regarding the IBRRC  follow the link.