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Moorhen Marsh

In 1974, MVSD created Moorhen Marsh, the first wetlands on the west coast dependent solely on treated effluents as its primary water source. The 21-acre constructed wetland saved the District’s rate payers millions of dollars by avoiding the construction of a deep-water outfall to transport MVSD’s treated wastewater to the Carquinez Strait, all while creating critical habitat for wildlife. Native wildlife species dependent on the wetland for habitat include the western pond turtle, North American river otter, mink, North American beaver, and many species of birds.

In the San Francisco Bay Area, where most freshwater wetlands have been lost, Moorhen Marsh is a shining example of a community success story. It provides important open space in a heavily industrialized area, while also offering educational opportunities for over 1600 elementary, secondary, and college students every year. Birders, photographers, hikers and families also enjoy Moorhen Marsh. Moorhen Marsh also functions in the treatment process by reducing ammonia as the effluent travels through the wetland before flowing into Peyton Slough and eventually out to the Carquinez Strait.

The threat of nutrient pollution in the form of nitrogen and phosphorus from human waste, human activities and urban run-off can cause severe, negative impacts on aquatic ecosystems, the economy and human health. Looking for an environmentally friendly solution to remove these nutrients, MVSD installed 42 floating islands, fully stocked with native plant species, in Moorhen Marsh to act as a natural treatment system, taking advantage of the natural processes of plants and robust microbial activity in the roots. Additionally, the riparian zones throughout the marsh were planted with native species to establish a fully native habitat as a beneficial use for desired animal species. MVSD would like to thank the Contra Costa County Fish and Wildlife Committee for their support of the floating island project in 2022-2023.

Moorhen Marsh is completely dependent on MVSD’s advanced secondarily treated effluent for its water. Without the approximately 1.2 million gallons of effluent provided by MVSD’s treatment plant every day, this historic marsh would cease to exist.

The marsh is open by appointment only during MVSD business hours, Mondays-Thursdays, 7am-4pm, Fridays, 7am-3pm. To make an appointment contact the Public Outreach Coordinator at There is no access on the weekends or during observed holidays. Special arrangement requests may be made through the Public Outreach Coordinator.


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