California Historical Landmark 26 commemorates the passage of Spanish Explorer Captain Gaspar de Portolá through this area in October 1769. Rancho San Gregorio, consisting of 17,752 acres, was granted to Antonio Buelna in 1839. The rancho extended from Tunitas Creek in the north to the mouth of Pomponio Creek and encompassed the entire lower San Gregorio watershed.
The estuary is home to many birds and small animals. Common shrubs include, coyote brush, bush lupine, California sagebrush (Artemisia californica), and lizard tail. One rare plant is found there—the Salt Marsh Milk Vetch.
Five native plant communities are represented: riparian, coastal strand, northern coastal scrub, annual grassland, and freshwater marsh.
The coastal strand community, consisting of beach and dune vegetation, is limited to a few small areas near the mouth of San Gregorio Creek and the upper portions of the beach, where spring high tides wash up against the coastal bluffs. Vegetation consists almost entirely of annual and perennial herbaceous plants. Sea rocket, beach bur, mustard (Brassica sp.), and several annual grasses are common. Coastal scrub is the predominant vegetation. It occupies the terrace between Highway 1 and the bluffs and is also found on all upland ridges of the inland portion east of the highway.