Background & History

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Background

The U.S. Army Corps of Engineers, in partnership with the State Coastal Conservancy and the San Francisco Bay Conservation and Development Commission, is working to restore the former Hamilton Army Airfield and the adjacent North Antenna Air Field back to a natural tidal wetland. Since being diked off from the Bay and protected by levees, the site has dropped below the elevation of surrounding properties.

In order to restore the land to natural wetlands, the site will be raised to an appropriate elevation in order to re-create the wetlands through the placement of dredged material. Dredge materials are sediments found at the bottom of harbors and other waterways that are suctioned up by large machinery.  Each year, three to six million cubic yards of sediment must be dredged to maintain safe navigation in and around the San Francisco Bay. In order to responsibly manage such large amounts of sediment, the San Francisco Bay Long Term Management Strategy for Dredging was established as the “regional dredging team” for the San Francisco Bay Area. The LTMS ensures that the dredged materials are being used in mutually beneficial situations.

The wetlands will be restored using this dredged material, as well as natural sedimentation. Before dredged material is placed in the area, perimeter levees will be constructed. The bayward levee will then be breached to allow for tidal flow. The surface area disturbed by the levee breaches and pilot channels will total 5.4 acres. Excavating the levee breaches and pilot channels will affect 1.8 acres of grassland and 3.6 acres of coastal salt marsh. One important advantage of using dredged material is the reduction in the amount of time necessary for restored wetlands to become fully functional. 

Site Information

The Hamilton Wetlands Restoration Project site was historically dominated by tidal salt marsh habitat but was converted in the late 1800s to agricultural land. In 1931, funds were appropriated for the construction of the Hamilton Army Airfield, which was operational until 1974. Currently, the site is mostly grasslands, seasonal wetlands, and residentially developed areas. 

The only remaining salt marsh in the project area is near the center of the dike that defines the developed portions of both area sites. A moderately large number of vertebrate species have been identified in this area, including some special-status species; however, relatively few species of reptiles and amphibians have been found to be present. A significant variety of birds have been identified at the site, but the number of birds actually using the project site is limited. Species present include ducks, shorebirds, wading birds, passerines (perching, mainly song birds), and many species of raptors (birds of prey) that forage across the entire site.

Tidal Marsh Habitat
The project site includes 120 acres of high pickleweed marsh. The pickleweed dominated tidal salt marsh along San Pablo Bay provides habitat for a number of bird species, including several special status species that are dependent on such habitats, such as the California clapper rail. Shorebirds, generally present during winter as well as spring and fall migration, feed on mudflats at low tide or around the marshes adjacent to ponds and sloughs. Some water birds occur in both fresh water and saline wetlands, including dabbling ducks and wading birds. Although no surveys for the salt marsh harvest mouse have been conducted, it is likely that the tidal marsh supports a population of the mouse.

Seasonal Wetland
There are 35.5 acres of seasonal wetlands on the project site. The dominant seasonal wetland species at the site include salt grass and alkali heath (Frankenia salina). Common wetland plant species on include cattail (Typha spp.), salt marsh bulrush (Scirpus maritimus), and curly dock (Rumex crispus). Seasonal wetlands commonly provide resting areas during high tide for shorebirds. In addition, the aquatic invertebrates that inhabit the seasonal wetland pools provide forage for shorebirds.

Brackish Marsh
Cattail and bulrush take up a total of four acres of marshy sections along the perimeter drainage ditch. Common species in the perimeter drainage ditch include threespine stickleback (Gasterosteus aculeatus), mosquito fish (Gambusia affinis) and red-winged blackbirds.

Grassland
Much of the site is grassland. This habitat is dominated by weedy upland plants such as bristly oxtongue (Picris echioides), yellow star thistle (Centaurea solstitialis), wild radish (Raphanus sativa), and curly dock (Rumex crispus). Additionally, non-native grasses such as Mediterranean barley (Hordeum marinum) and perennial ryegrass (Lolium perenne) are common throughout the project site. Grassland and vegetation around the project site supports relatively few bird species except where coyote bush (Baccaris pilularis consanguinea), blackberry (Rubus spp.), or patches of dense, tall herbaceous vegetation are present.

Developed Areas
284 acres of the project site are developed areas consisting of concrete, asphalt, buildings and bare ground. These areas provide minimal habitat for wildlife. The buildings were surveyed in 1997 for use by special-status bat species and none were present. 

Public Access
In the immediate area surrounding the Hamilton Wetlands Restoration Project site, there are several existing, and a few proposed, public access facilities that this project will connect with. The proposed public access for this project will include a 2.66 mile long length of the Bay Trail spine, a short spur trail to the existing Ammo Hill trail and five interpretive overlooks associated with the Center that would include 10-20 parking spaces, restrooms and information kiosks.

For more information about public access plans for the Hamilton Wetlands Restoration Project, please refer to the “HAMILTON WETLAND RESTORATION PROJECT - DRAFT PUBLIC ACCESS PLAN.”

History

Early Settlement

The Hamilton Wetlands Restoration Project site was initially part of the muddy grounds and adjacent tidal marshes that surround the mouth of Novato Creek. During the period from 1853 to 1884, mining for gold in the Sierra Nevada foothills caused substantial amounts of sediment to enter into the Bay system, resulting in the build up of the shoreline to the east of the project site. Around the turn of the last century, marsh lands on the project site were diked to accommodate dry land farming. 

The Hamilton Army Airfield

In July 1932, construction began on what is known as the Hamilton Army Airfield, named for Lt. Lloyd Hamilton, the first American pilot to fly with the Royal Flying Corps during WWI. Construction was completed in 1935, turning the site into a large enough base to accommodate four bomb squadrons and their personnel, as well as headquarters for the 1st Wing of the Army Air Corps – one of only three bases established for this purpose in the nation at the time. 

The base played a significant role during World War II in training and national defense by serving as an overseas staging area. Hamilton Army Airfield’s strategic location in the Bay Area was an ideal point of departure for Pacific-bound air troops. The base’s strategic location also allowed crews to closely monitor the coast’s air and naval traffic. 

After the Hamilton Army Airfield’s significant contributions to WWII, the base was reassigned several times until buildings and land were transferred to the Navy, Army and Coast Guard in 1985.

The airfield remained operational until 1974 and has been in the process of closure under the Defense Base Realignment and Closure Act since 1988. Consequently, the United States Army Corps of Engineers, the State Coastal Conservancy and the San Francisco Bay Conservation and Development Commission teamed to address the technical aspects of the restoration and to determine how to best restore the wetland habitats in a variety of beneficial ways.