The Suisun Marsh is the largest contiguous brackish water wetland in western North America, providing food and habitat for thousands of migratory birds and many species of plants, fish and wildlife. The combination of tidal wetlands, diked seasonal wetlands, sloughs and upland grassland comprises more than 10 percent of the remaining wetlands in California.

The marsh is where fresh water from the Sacramento-San Joaquin Delta meets salt water from San Francisco Bay.

Fed by runoff from the northern Sierra Nevada mountains and the southern Cascades, the Sacramento River flows south to meet the northbound San Joaquin River just south of the city of Sacramento where the Sacramento-San Joaquin Delta is formed.

Here, settlers transformed the marsh into agricultural and ranching land and later the area became a popular spot for hunting birds (Suisun Marsh is a critical part of the Pacific Flyway for migratory birds).

The rivers’ combined fresh water flows roll through the Carquinez Strait, a narrow break in the Coast Range, and into San Francisco Bay’s northern arm, forming the San Francisco Bay/Sacramento-San Joaquin Delta Estuary, commonly referred to as the Bay Delta.

Suisun Marsh and adjoining bays are the brackish transition between fresh and salt water. But the location of that transition is not fixed. Hydrologic studies suggest the Delta works like a giant washing machine agitator, with tidal forces and river flows constantly sloshing water back and forth.

Suisun Marsh Challenges

There are approximately 230 miles of levees protecting over 50,000 acres of marsh land in the Suisun Marsh. Only about a third of the Delta levees (385 miles) are part of a federal flood management project of the Sacramento and San Joaquin River systems and, as a result, are eligible for rehabilitation by the Corps. All of the Suisun Marsh levees – are local levees constructed and maintained during the past 130 years by local reclamation districts.

Delta salinity impacts the Suisun environment as well. Plant growth, for instance, can be affected, which in turn can affect waterfowl that feed on the plants. The State Water Project’s Suisun Marsh Salinity Control Gates near Collinsville modify tidal flows through Montezuma Slough, limiting upstream movement of salty tidal flows into Suisun Marsh.

California’s Department of Water Resources built the salinity control gates in the 1980s. According to DWR, the gates “control salinity by restricting the flow of higher salinity water from Grizzly Bay into Montezuma Slough during incoming tides and retaining lower salinity Sacramento River water from the previous ebb tide.”

This usually takes place from October to May and helps moderate salinity in marsh channels.

Pelagic fish declines and scientific surveys of the Delta and Suisun Marsh in 2005 revealed ongoing, sweeping population crash of native pelagic fish. The decline could not be explained by drought or any other easily identifiable cause.

The Bay Delta Conservation Plan is responsible for developing an in-Delta and Suisun Marsh habitat restoration program. The BDCP conservation efforts are in accordance with the federal and state Endangered Species Act and the state’s Natural Community Conservation Planning Act.