page_title: San Juan Creek
section_title: River modifications
hierarchical_section_title: San Juan Creek / River modifications
caption_attribution_description: English: Drop structure on Trabuco Creek after a rain.
context_page_description: San Juan Creek, also called the San Juan River, is a 29-mile long stream in Orange County, California, draining a watershed of 133.9 square miles. Its mainstem begins in the southern Santa Ana Mountains in the Cleveland National Forest. It winds west and south through San Juan Canyon, and is joined by Arroyo Trabuco as it passes through San Juan Capistrano. It flows into the Pacific Ocean at Doheny State Beach. San Juan Canyon provides a major part of the route for California State Route 74.
  Before Spanish colonization in the 1770s, the San Juan Creek watershed was inhabited by the Acjachemen or Juañeno Native Americans. The Juañeno were named by Spanish missionaries who built Mission San Juan Capistrano on the banks of a stream they named San Juan Creek. The watershed was used mainly for agriculture and ranching until the 1950s when residential suburban development began on a large scale. Since then, the human population has continued to encroach on floodplains of local streams. Flooding in the 20th and 21st centuries has caused considerable property damage in the San Juan watershed.
context_section_description: Although the San Juan Creek watershed is less heavily developed than other coastal Orange County watersheds, extensive works have been constructed to control floods, reduce erosion, and provide reclaimed water for irrigation. A growing amount of urban runoff flows into the creek and its tributaries, creating a dry season "nuisance flow". Historically, only San Juan Creek and Arroyo Trabuco were known to contain water for most or all of the year. Oso Creek was formerly a seasonal stream, but it now has a permanent flow due to urban runoff. Runoff has caused Doheny Beach to rank in the ten most polluted beaches of California.
  The Upper Oso Reservoir and Lake Mission Viejo, both on Oso Creek, are the largest impoundments in the watershed, holding about 7,500 acre feet (9,300,000 m³) combined. While Lake Mission Viejo was built solely for recreation, the 115-acre (0.47 km²) Upper Oso Reservoir collects Oso Creek water and diverts it for irrigation use. The reservoir is occasionally used by air tankers to combat wildfires in the Cleveland National Forest. The Santa Margarita Water District is currently proposing a new 5,000-acre-foot (6,200,000 m³) reservoir in Verdugo Canyon, another tributary of San Juan Creek, to collect and store reclaimed water.
  The lower 3.5 miles (5.6 km) of San Juan Creek are channelized between levees, from a point immediately upstream of the Interstate 5 bridge to Doheny Beach. Arroyo Trabuco is only channelized for several hundred yards above its confluence with San Juan Creek. Oso Creek is the most heavily modified, flowing in an artificial channel for almost its entire length. Bell Creek and other eastern tributaries have retained their natural characteristics. The USACE describes the San Juan and Arroyo Trabuco levees as providing a "fairly high level of protection currently", though flooding in 1996 and 2005 caused significant damage. As a result, failure scenarios of levees in the San Juan watershed have been extensively studied. To protect against future flooding, work has begun on a new west-bank levee replacement to be finished in 2013.
  A few check dams exist on small upper tributaries of San Juan Creek, mostly inside the Cleveland National Forest. A larger gabion-type dam is located on the middle part of San Juan Creek near the Cañada Gobernadora. Although the dam has silted in and is no longer used for water storage, its roughly 3-to-4-foot (0.91 to 1.22 m) drop still poses a problem for migrating steelhead. There are a few water diversion weirs that exist on San Juan tributary streams to divert water for irrigation, ranching and limited municipal uses, but due to limited flows and polluted water, the usefulness of these structures are limited.
  A number of drop structures (small dams used to control water velocity and erosion) exist on tributaries of San Juan Creek. On Arroyo Trabuco, there are eight drop structures, mostly built of riprap. The largest are a 30-foot (9.1 m) cascade immediately downstream of a Metrolink bridge, and a concrete drop structure at the terminus of a culvert that crosses underneath Interstate 5. There are also seven drop structures on Oso Creek, mostly built of riprap. There are no such specifically constructed structures on San Juan Creek itself.
caption: Drop structure viewed during rainfall, near the San Juan-Trabuco confluence
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