San Juan Creek - Watershed

Watershed

The Santa Ana Mountains occupy most of the north, east and south parts of the approximately 133.9-square-mile (347 km2) San Juan Creek watershed, while the San Joaquin Hills and one of their small subranges border the watershed on the southwest, east, and northwest. The three major tributaries, San Juan, Trabuco and Bell Creeks, all originate in the Santa Ana Mountains. There are four main alluvial river valleys in the watershed, drained by San Juan, Trabuco, Oso and Bell Creeks. The San Juan Creek valley occupies the south portion of the watershed, running roughly south before spreading wider near the coast, and running northeast until it turns sharply to the southeast to meet the Pacific Ocean at the city of Dana Point. The Trabuco Creek valley runs west from the northeasternmost portion of the watershed, then gradually becomes wider in a section called the Plano Trabuco then it joins the smaller, urbanized valley of Oso Creek, which runs primarily south. The combined valley then runs southward to merge with the San Juan Creek valley in San Juan Capistrano. The Bell Creek valley is narrower, beginning a few miles south of Trabuco Canyon, California, and running in an almost perfectly straight southward course to San Juan Creek, which at this point is shortly out of the uppermost reach of its watershed, San Juan Canyon. There are 19 other major creeks in the watershed, including Tijeras Canyon Creek and El Horno Creek, both of which drain parts of the foothills of the Santa Ana Mountains.

San Juan Creek is considered as part of the 500-square-mile (1,300 km2) San Juan Hydrologic Unit, which is a coastal region running from central-south Orange County to the north extent of San Diego County, which borders Orange County on the south. The hydrologic unit also covers the watersheds of Aliso Creek, Salt Creek, Prima Deshecha Cañada, Segunda Deshecha Cañada, and San Mateo Creek, in order from north to south. Elevation above sea level ranges from 0 to 5,700 feet (0 to 1,700 m), and precipitation in the hydrologic unit ranges from 12 to 16 inches (300 to 410 mm) annually. Except for San Mateo Creek, all of the other streams in the unit have a relatively developed watershed. San Juan Creek receives extra runoff from agriculture, urban activities, commercial irrigation, and other human-induced sources, which creates an extra unnatural flow (consisting of urban runoff and agricultural return flows). This runoff is often polluted by trash, heavy metals, and oil received while running over pavement, and pesticides and fertilizer if draining farmland. A specific point of concern in the watershed is Oso Creek. A Southern California environmentalist, Michael Hazzard, said after diving into Upper Oso Creek Reservoir to retrieve an outboard motor:

I spent three days diving to retrieve an outboard motor and my skin broke out in hives and boils and my gallbladder suffered and I later had six operations over a 21⁄₂-year period.