A marsh is a type of wetland, an area of land where water covers ground for long periods of time. Marshes are usually treeless and dominated by grasses, rushes, or sedges. Suisun Marsh is considered as the largest brackish water marsh on west coast of the United States consist of 116,000 acres. The marsh is home to many species of wildlife, and it located west of Sacramento San-Joaquin Delta as a part of San Francisco Bay estuary (Figure 1).

Figure 1. Location of Suisun Marsh

The area alternately flooded and drained with the rise and fall of the tides. In winter, the ponds (Figure 2) supported high numbers of migratory waterfowl. In the past, 1880s until the 1930s, the marsh was extensively used by market hunters to provide fresh waterfowl and feathers to San Francisco markets. From then, this area was gradually converted to agriculture, made possible by the construction of levees and dikes to hold back the water. But increasing soil salinity made cultivation and even cattle grazing not feasible.  Most of the marsh was then managed by public and private interests as habitat for waterfowl, mainly to support hunting.

Figure 2. Suisun Marsh ponds

To maintain the wetlands, the area preserved from residential or commercial development. Today, Suisun Marsh supports a diversity of fish, wildlife, and plants, including several native fish species, and birds.

There are about 230 miles of levees in the Marsh prevent salinity intrusion into the freshwater of the Delta used by 22 million people for drinking water. Between October to January, managed seasonal wetlands are flooded to a depth of 8 to 12 inches to attract waterfowl. The rest of the year, the ponds are flooded and drained on a schedule designed to optimize conditions for plants which provide seeds preferred by waterfowl.

Salinity control using gates

The wetland managers for both the private hunting clubs and the state’s public land take water from major and minor sloughs throughout the marsh. Montezuma Slough (Figure 3), one of the largest, is open at both ends, and its flood tide current is longer and stronger than its ebb tide current, causing a net west-to-east flow which draws higher saline water eastward from the Grizzle Bay.

Figure 3. Grizzle Bay, Montezuma Slough, and Salinity control gate location

The flood tide pushing through the slough takes half an hour longer to traverse the marsh than does the matching flood tide following the more direct route in the main Suisun Bay channel. Thus, high tide at the east end of the slough arrives out of phase with high tide in the main channel, and rather than being pushed back, as it would be in the main channel or in a dead-end slough, the slough water keeps flowing eastward, drawing more saline water with it.

To meet the salinity requirements, the California State Water Project and the federal Central Valley Project built the Montezuma Slough Salinity Control Gates. They began operation in 1989. The gates span Montezuma Slough near the Roaring River intake (Figure 3) and are periodically operated from October to May to meet the salinity standards, to block the salty flood tide from Grizzly Bay (Figure 3) but allow passage of the freshwater ebb tide from the mouth of the Sacramento-San Joaquin Delta.

Because the Salinity Control Gates are more effective than anticipated other proposed salinity control measures were abandoned. The gates operate as needed from October through May.

Figure 4.  Suisun Marsh salinity Control Gates

Salinity Control Gate operation criteria

Suisun Marsh Salinity Control Gates consists of three radial gates, when operates open and closed with the tide cycle. When under typical operations, the gates are opened during ebb tide (low) and closed during flood tide (high) to tidally pump low salinity water into Suisun Marsh from the Sacramento River (Figure 5).

Figure 5. Salinity Control Gate operation

Results from modeling studies show that salinity control gates operations reduce the Suisun Marsh salinity by significant amount.

By timely water management and gate operation, Suisun Marsh is managed as a good recreation and wildlife sanctuary. It also can be used as a great source in the carbon credit market which is going to be discussed in a future article.

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