San Gregorio Creek is historically significant as the campsite for Spanish explorer Gaspar de Portolà's expedition from October 24–27, 1769, when he overshot his goal and went on to Pacifica where he ascended Sweeney Ridge and discovered San Francisco Bay. The site is registered as California Historical Landmark 26. The creek was called Arroyo de San Gregorio in Spanish times and later, Arroyo Rodrigues in the 1850s.
Watershed
The San Gregorio watershed is located approximately south of Half Moon Bay and covers approximately. It is the second largest drainage in coastal San Mateo County, with approximately of "blue line" streams. The mainstem of San Gregorio Creek begins at the confluence of Alpine Creek and La Honda Creek in La Honda, and flows to its mouth at San Gregorio State Beach in San Gregorio. The creek ends in a lagoon primarily in the incised channel upstream from the Highway 1 Bridge. The lagoon at its seasonal largest, is about and deep, and serves as habitat for tidewater goby and rearing steelhead. Coho salmon do not rear in the lagoon but outgoing smolts use it to physiologically prepare for migration to saltwater. This river floods occasionally on its path down CA Route 84 towards the Pacific Ocean, most famously during the 1997 El Niño.
Ecology
The mainstem of San Gregorio Creek, in combination with its tributaries of La Honda, Alpine, Harrington, El Corte de Madera and Bogess Creeks, contains approximately of historical coho salmon rearing habitat. In the 1800s the creek had large enough salmon runs to support commercial harvest. As part of the southern range of the Central California Coast Coho Evolutionarily Significant Unit, the creek is considered an attractive site for re-stocking. In fact, in 1998 the Draft Strategic Plan for Restoration of Endangered Coho Salmon South of San Francisco Bay identified San Gregorio Creek and its tributaries as one of nine creeks in which recovery of coho salmon is a priority. South of San Francisco to Monterey Bay the coho salmon was listed by the State of California as endangered in 1995. The San Gregorio watershed has seen recently increasing residential development but remains primarily pastoral with cattle and sheep grazing, timber harvesting, and recreational trails being the main commercial uses. Because of the large private ownership and development potential, water diversions and low base flows are an important issue in this watershed. In 1993, water rights in the San Gregorio watershed were adjudicated and a minimum stream bypass flow was established. However, the prescribed bypass flows are too low to assure viable coho salmon populations. San Gregorio Creek is also part of the Central California Coast steelhead ESU and historically supported a run of 1,000 fish as recently as 1971. In 2007, the San Gregorio Creek watershed was targeted by the California Department of Fish and Game for salmon recovery. In a 2008 survey, both coho salmon and steelhead were noted in the creek but threatened by Highway 84 related barriers to fish passage, as well as pressure from sedimentation from residential development, grazing and logging. The primary threat to salmonids in this report were bridge culverts at three sites which impede fish passage during low creek flows, and one site where a culvert from a small tributary is completely impassable in any season. Four special-status animal species - California red-legged frog, coho salmon, steelhead, and tidewater goby are the focus of a June 2010 Watershed Management Plan. Western leatherwood, Santa Cruz manzanita, and King’s Mountain manzanita, which are all included in the California Native Plant Society's Inventory of Rare and Endangered Plants of California, have been documented in the La Honda Creek Open Space Preserve.
