San Mateo Creek (California)

Category: Inland Aquatic Biomes.

Geographic Location: North America.

Summary: A relatively small but prolific wilderness area adjacent to urban areas on the Pacific coast.

San Mateo Creek is the most pristine intact coastal stream in southern California. (It is not to be confused with the San Mateo Creek in the San Francisco Bay area.) This creek is approximately 22 miles (35 kilometers) long, and its watershed covers about 139 square miles (360 square kilometers) of relatively undeveloped terrain. In part, it serves as the boundary between Orange and San Diego counties. Its watershed also includes Riverside County, where its headwaters arise, draining from the Santa Ana Mountains of the Cleveland National Forest. From this roadless wilderness area, runoff from rain flows westward and travels down steep canyons, through an alluvial plain, and into the Pacific Ocean.

The San Mateo Creek ecoregion encompasses areas of both mediterranean climate, to the north, and semiarid climate to the south and east. As a result, it is often described as an arid mediterranean, as well as semiarid steppe, environment. Average annual rainfall is between 15 and 20 inches (38 and 50 centimeters).

The watershed contains two distinct topographical regions: the upper and the coastal. The upper region lies mostly within San Mateo Canyon Wilderness in the Cleveland National Forest, with elevations up to about 3,500 feet (1,070 meters); its lower reaches run through Camp Pendleton Marine Corps Base and San Onofre State Beach (where the popular surfing beach, Trestles Natural Wetlands Preserve, is located).

Following a decreasing elevation gradient, the ecosystem starts at the ridgeline with mixed pine-oak forests descending to chaparral, coastal sage scrub, and finally to riparian habitats. It is home to many endemic (not found elsewhere) species, some of which are federally protected under the United States Endangered Species Act. This biome, though small, is extremely valuable.

Flora and Fauna

The vegetative community of the San Mateo Creek watershed is spectacular. Rainwater is sufficient to maintain a highly diverse floral ecosystem; spring rains bring an abundance of wildflowers blooming throughout the sage and chaparral. Five riparian plant communities have been identified in the Trestles Natural Wetlands Preserve alone. These include: coastal sage scrub or coastal bush scrub (scrub wetland); willow woodland (forested wetland); sycamore-cottonwood (forested wetland); freshwater marsh (emergent wetland); and jaumea meadow (emergent wetland).

Some of the individual plants found here are: larger trees and shrubs including coastal live oak (Quercus agrifolia), coastal scrub oak (Quercus dumosa), lemonade berry (Rhus integrifolia), Christmasberry (Heteromeles arbutifolia), California sage (Artemisia californica), ceanothus (Ceanothus spp.), poison oak (Toxicodendron diversilobum), and greasewood or chamise (Sarcobates spp.).

Fauna found in the region include 139 bird species, 37 mammal species, 46 reptile and amphibian species, and seven species of fish. The range features black bear, mule deer, bobcat, coyote, gray fox, American badger, ring-tailed cat, long-tailed weasel, bats, spotted owl, golden eagle, western pond turtle, toads, frogs, lizards, rattlesnakes, coyotes, skunks, and mice; the elusive mountain lion is here but rarely seen.

The San Mateo Creek ecoregion is unique in that much of its wildlife is on either the federal or State of California Threatened and Endangered Species lists. Some of these listed species have become the source of widespread media attention—for example, the California gnatcatcher (Polioptila californica), an 0.35-ounce (10-gram) bird that was not even a recognized taxon until 1993, when it was listed as threatened on the United States Threatened and Endangered Species list. The bird was featured on the cover of the January/February 1995 issue of Audubon magazine, with the headline, “Can this Bird Save California?” By the late 2010s, about 85–90 percent of the gnatcatcher’s habitat was overtaken by human development. However, conservationists have found some success in establishing new habitats for the gnatcatcher.

Also of note was the proposed modification of State Route 241, a planned extension of an existing toll road that would have terminated at San Onofre State Beach; road opponents feared the loss of an important and unique habitat. In September 2021, California passed a bill prohibiting the expansion of the road into the San Onofre Beach area. Another species of concern is the southwestern willow flycatcher (Empidonax trailli extimus), whose breeding habitat is located in moist riparian vegetation, often containing willows. Another federally listed species is the least Bell’s vireo (Vireo bellii pusilus), a subspecies impacted by habitat loss. Additionally, a small population of southern steelhead trout (Oncorhyncus mykiss) was found in the watershed; it survives because of the flow of freshwater, and is federally listed as endangered.

Human Impact

The early Native American inhabitants of the coast and the Santa Ana Mountains included the Kumeyaay, Luisaños, Cahuella, and Capeño. These groups fished the streams and found an ample food supply among the abundant plant life. The explorers Vizcaíno and Cabrillo reported that the native population did considerable burning of the brushlands, but the overall impact was probably not very great.

European inhabitants did not form a permanent presence until the mid-to-late 1700s, with the construction of the mission network and an extensive agricultural and irrigation system. The Spanish missions initiated changes to the rivers with the introduction of irrigation; they prospered until the separation of Mexico from Spain in 1821. By the mid-1800s, large portions of land had been opened to settlement by private ranchers. The land within the watershed was parceled out in the form of large Mexican land grants. Ranching became the predominant activity in the watershed.

Large numbers of cattle and sheep were brought in, and they grazed the grasslands of the lower San Mateo Creek drainage basin, drastically altering the native landscape. Widespread overgrazing throughout the area destroyed native vegetation; rancheros cut brush and trees and cleared underbrush with fires. The introduction of nonnative invasive plants displaced native grasslands, which probably was the single most destructive assault on the landscape.

In the late 1860s, gold miners from northern California flowed into the area, bringing about ecological destruction in the form of logging (for mine timbers) and mining for gold, lead, zinc, and silver. Subsequently, uncontrolled fire events threatened the water supplies of the surrounding region. In response, the California Forestry Commission voiced the necessity for special protection of the watershed to prevent fires and subsequent erosion. The Forest Reserve Act was signed by President Benjamin Harrison in 1891 to curb illegal timber cutting and mining. This Act established the boundaries of the Cleveland National Forest, which included a majority of the San Mateo upper watershed.

In the 2020s, the San Mateo Creek ecoregion supports more than 3 million nature visitors per year, mostly day picnickers and tent campers. The chief environmental concerns presently being studied and addressed for the ecosystem are: urban encroachment, groundwater depletion, erosion, military activities, the coming decommissioning of the San Onofre nuclear power plant, and the proposed toll road. Climate change also presents challenges, such as sea-level rise, higher temperatures, and ocean acidification. Impacts of these and related effects could include damage to trout and salmon population, streambank erosion and saltwater intrusion, increased wildfire frequency, and more invasive pests and disease.

Bibliography

“California State Route 241.” Sierra Club, 2021, www.sierraclub.org/angeles/sierra-sage/SR241. Accessed 1 Sept. 2022.

Lang, John S., Bruce F. Oppenheim, and Robert N. Knight. Southern Steelhead Oncorhynchus Mykiss Habitat Suitability Survey of the Santa Margarita River, San Mateo and San Onofre Creeks on Marine Corps Base, Camp Pendleton. Arcata: Coastal California Fish and Wildlife Office, 1998.

Palmer, Tim. California Wild: Preserving the Spirit and Beauty of Our Land. Minneapolis, MN: Voyageur Press, 2004.

San Mateo Creek Conservancy. “Trestles Natural Wetlands Preserve.” http://trestleswetlands.org/Default.aspx.

Wynns, Samantha. “Species Spotlight: The California Gnatcatcher.” National Park Service, 27 Sept. 2018, www.nps.gov/cabr/blogs/species-spotlight-the-california-gnatcatcher.htm. Accessed 1 Sept. 2022.