Estero Americano

Estero Americano is one of the prime examples of a linear estuary in the State (see also Estero de San Antonio). There are approximately 120 hectares (300 acres) of open water and 160 hectares (400 acres) of wetlands. The tidal influence extends some 5.5 kilometers (3.5 miles) inland except during the summer, when a sand bar blocks the mouth. Salinity varies greatly and diminishes inland; during the late summer, the lower reach becomes hypersaline, up to 67 parts per thousand.

Several plant communities are present, including brackish and freshwater marshes, bay groves, coastal strand and coastal prairie. In the open water, pond-weed, Potamogeton pectinatus, is common in the brackish and fresh water. Eel grass, Zostera marina, occurs near the mouth. Some 38 species of marine algae have been collected in the estuary.

Dominants in the brackish marshes are pickleweed, Salicornia virginica, Jaumea carnosa, and salt grass, Distichlis spicata; in the better-drained areas Frankenia salina and saltbush, Atriplex triangularis, are common. On the higher ground there is a small amount of freshwater marsh and some seeps. Vegetation here includes cattails, Typha latifolia, plantain, Plantago subnuda, and the non-native loosestrife, Lythrum hyssopifolium.

Inland, adjacent to the marsh and sheltered from the winds, are a few small bay groves with bay, Umbellularia californica, dominant, together with some buckeye, Aesculus californica; there is a rich assortment of species in the understory. Vegetation in the uplands includes coastal scrub and grasslands. On the sand at the mouth, a coastal strand community is found.

Just inside the mouth is a mudflat which is extensively utilized by feeding birds, shorebirds at low tide and diving ducks at high tide. Over 120 species of birds have been observed in the area and 65 of these are water or marsh oriented. A number of migratory species visit the area, including pintail, Anas acuta, baldpate, Mareca americana, canvasback, Aythya valisineria, and ruddy duck, Oxyura jamaicensis. Whistling swans, Olor columbianus, are winter visitors in the upper reaches, and the endangered bald eagle, Haliaeetus leucocephalus, has been observed.

Pacific herring, Clupea harengus, staghorn sculpin, Leptocottus armatus, tidewater goby, Eucyclogobius newberryi, and arrow goby, Clevelandia ios, are among the more common of the 21 fish species collected here.

There is an abundant invertebrate fauna, though the diversity is reduced by the hypersalinity.

The estero, which is fed by Americano Creek, is a drowned valley.

Integrity: Though the uplands are grazed and portions are impacted by cattle, the estuary is virtually pristine.

Use: Private

Ref: Madrone Associates, 1977. The Natural Resources of Esteros Americano and de San Antonio. Calif. Dept. of Fish & Game, Coast Wet. Ser. No. 20, 81+ pp.

February 1978