The San Andres Creek place name is shown on the Rancho de las Pulgas 1856 plat map.

[12][13] Before European settlers arrived, much of the San Andreas Valley portion of the watershed was composed of wetlands.

However, the arrival of colonizers between the late 18th and mid 20th centuries led to a string of impactful environmental and ecological changes to San Andreas Creek and the rest of the watershed.

These dams completely deluged the naturally existing aquatic habitats, including the riparian ones, desiccating much of the wildlife.

However, modern American influences from the mid 20th century onwards have decreased the grassland area by about 70%, and this destruction was accelerated by highway construction and residential development.

These haven't been the only habitats to face destruction around the San Andreas creek and the surrounding area; furthermore, other environmental damages, such as Sudden Oak Death and the spread of plant pathogens, have exacerbate as a result of various human-created developments.

On Cahill Ridge (just west of San Mateo Creek and east of Pilarcitos Creek, at an elevation of 1,000 feet (300 m), Oberlander measured fog drip beneath tanoak (Lithocarpus densiflorus), Coast redwood and three Douglas fir trees, the latter 125 feet (38 m) tall.

The Douglas fir produced 7 to 17 inches (180 to 430 mm) of fog drip and appeared to provide unique conditions supporting the orchids giant helleborine (Epipactis gigantea) and phantom orchid (Cephalanthera austiniae), since these plants were found exclusively in these moist ridge tops.