San Juan Creek


San Juan Creek, also called the San Juan River, is a stream in Orange and Riverside Counties, draining a watershed of. 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. State Route 74, the Ortega Highway, crosses the Santa Ana Mountains via San Juan Canyon.
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, development 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.
The San Juan watershed is home to sixteen major native plant communities and hundreds of animal species. However, the watershed is projected to be 48 percent urbanized by 2050. In addition, urban runoff has changed flow patterns in San Juan Creek and introduced pollutants to the river system. Although the main stem of San Juan Creek does not have any major water diversions or dams, some of its tributaries, including Trabuco and Oso Creeks, have been channelized or otherwise heavily modified by urbanization.

Course

San Juan Creek begins high in the Santa Ana Mountains southwest of Lake Elsinore, at the head of the steep and narrow San Juan Canyon, at roughly in elevation where Morrell Canyon Creek, draining the western Elsinore Mountains and southernmost Santa Ana Mountains, has its confluence with Bear Canyon Creek. From there, it flows steeply down a rocky gorge over rapids and waterfalls. San Juan Canyon forms the mountain pass for California State Route 74, which connects San Juan Capistrano to Lake Elsinore and the Inland Empire. San Juan Falls, a cascade, and the Ortega Falls are located along the headwaters of the creek. The creek then flows generally southwest through a canyon, receiving Hot Springs Creek and Cold Springs Creek from the right, and Lucas Canyon Creek from the left.
At Caspers Wilderness Park, the San Juan Canyon opens up into a fairly wide valley in the foothills of the Santa Ana Mountains. This reach of San Juan Creek is underlain by thick alluvial deposits and is still used for farming and grazing. It is joined by Bell Canyon from the right, and Verdugo Canyon Creek from the left. Trampas Canyon enters from the left and Cañada Gobernadora and Cañada Chiquita enter from the right. The creek flows through residential Rancho Mission Viejo, crosses under Interstate 5, and enters a concrete flood control channel, turning south and receiving El Horno Creek on the right. It receives its largest tributary, Arroyo Trabuco, from the right, then flows south toward the Pacific Ocean. The creek forms a fresh water lagoon at the northern end of Doheny State Beach, which overflows into Capistrano Bay during periods of high flow.

Tributaries

All direct tributaries of San Juan Creek, from mouth to source, are listed. The list also includes streams that join major tributaries.
  • Arroyo Trabuco
  • * Oso Creek
  • ** La Paz Channel
  • * Tijeras Canyon Creek
  • * Live Oak Canyon Creek
  • * Hickey Canyon Creek
  • * Rose Canyon Creek
  • * Falls Canyon Creek
  • * Holy Jim Canyon Creek
  • El Horno Creek
  • Cañada Chiquita
  • Cañada Gobernadora
  • * Wagon Wheel Canyon Creek
  • Trampas Canyon
  • Verdugo Canyon
  • Bell Canyon
  • * Crow Canyon Creek
  • * Dove Canyon Creek
  • * Fox Canyon Creek
  • Lucas Canyon Creek
  • * Aliso Canyon Creek
  • Cold Springs Canyon
  • Hot Spring Canyon
  • * Chiquito Spring Creek
  • Lion Canyon Creek
  • Bear Canyon Creek
  • Morrell Canyon Creek
  • *Long Canyon Creek
  • ** Arroyo El Cariso
  • *Decker Canyon Creek
Image:Bell_Creek_at_Caspers_Park.JPG|thumb|right|Bell Creek, the second largest tributary of San Juan Creek, flowing through Caspers Wilderness Park

Geology

The San Juan Creek watershed is crossed by a complex network of seismic fault zones, with streams tending to form canyons along fault traces. The Cristianitos fault runs northeast-southwest along Oso Creek, passing offshore south of the mouth of San Juan Creek. The Mission Viejo fault zone parallels the Cristianitos but ends much farther south, in San Diego County. The first recorded earthquake in the area partially destroyed Mission San Juan Capistrano in 1812, killing forty people in the adobe chapel when it collapsed. Other major quakes occurred in 1862, 1933 and 1938.
Soils in the San Juan watershed are mostly sedimentary rock and are highly erosive, resulting in large alluvial deposits along floodplains. Soil types in the San Juan watershed can be divided into the Metz-San Emigdio, Sorrento-Mocho, Myford, Alo-Bosanko, Cieneba-Anaheim-Soper, and Friant-Cieneba-Exchequer associations, in order from low to high elevations. Steep hills in the San Juan watershed are prone to collapse during heavy rainfall or seismic activity.
Over the past 1.2 million years the uplift of the San Joaquin Hills, a small coastal mountain range generally following the Pacific coast of Orange County, created a physical barrier for streams flowing off the Santa Ana Mountains. Climate change during the Ice Ages periodically made Southern California much wetter, most recently during the Wisconsinian Glaciation, when the area was home to a temperate rainforest climate that would receive rainfall in excess of per year. The increase in water flow in San Juan Creek allowed it to maintain its course as an antecedent stream, rather than being diverted to the east or west.

Hydrology and groundwater

The U.S. Geological Survey measured the discharge of San Juan Creek in the city of San Juan Capistrano for two periods, from 1928 to 1969 and from 1969 to 1985. Stream flows for the earlier period are considerably different from the later period due to increased volumes of urban runoff. For 1928–1969, the average discharge was, and the peak flow was on February 25, 1969. During the 1969–1985 period, the average flow was, and a peak flow of was recorded on March 4, 1978. After 1985, the USGS stopped measuring discharge but continues to monitor water level in real-time at the La Novia Street bridge. According to the city of San Juan Capistrano, the largest flood of record occurred on January 11, 2005 with an estimated discharge of.
According to the California State Water Resources Control Board the San Juan Creek Groundwater Basin has a total volume of roughly. Natural groundwater recharge in the San Juan basin is estimated at per year. This amount has been reduced due to extensive urbanization of the lower watershed which results in more water running off to the Pacific Ocean. However, water from irrigation run-off and other human activities is responsible for recharging an additional per year. The Christianitos and Mission Viejo fault zones split the watershed into distinct "Upper" and "Lower" groundwater basins. The groundwater mostly lies in alluvium, which ranges from a depth of in the lower elevations to almost none in the high elevations. Groundwater in this basin at the San Juan Capistrano reach is considered of high quality.
Although San Juan Creek contains water for most of the year, it is highly seasonal, with strong flows during the wettest months of January through March, and shrinking to a trickle during the other months. In poor rain years, the stream can often dry up completely in its lower reaches. The total natural surface outflow from the San Juan basin into the Pacific is estimated at per year. Agricultural and urban runoff significantly increased the average outflow, to as of 1993. The maximum annual outflow is. Although the use of local surface water and groundwater is increasing, local groundwater levels have not been affected significantly by human use, due to the relatively high natural recharge rate.
Due to the lower amount of urbanization in the San Juan watershed as compared with other watersheds in the county, the 100-year flood inundation risk is also significantly lower than in other parts of Orange County. It has been calculated that a 100-year flood in the watershed would only affect a roughly wide area for the lower reaches of San Juan Creek inside San Juan Capistrano, while for Arroyo Trabuco, only a wide area would be affected, mainly due to considerable entrenchment of the river bed. However, the U.S. Army Corps of Engineers considers many levees in the San Juan Capistrano area to be inadequate for handling the 100-year flood, and that such a flood would cause $149 million of property damage.

Watershed

The San Juan Creek watershed covers in south Orange County and a very small portion of Riverside County. The Santa Ana Mountains occupy most of the north, east and south parts watershed, while the San Joaquin Hills border the watershed on the southwest. Along with San Juan Creek, the two largest tributaries – Trabuco and Bell Creeks – both originate in the Santa Ana Mountains. Although more than half of the watershed is undeveloped land, it also includes parts of the cities of Dana Point, Laguna Hills, Laguna Niguel, Mission Viejo, Rancho Santa Margarita, San Clemente, San Juan Capistrano, and the unincorporated communities of Trabuco Canyon and Rancho Mission Viejo.
There are four main alluvial river valleys in the watershed. The San Juan Creek valley occupies the south portion of the watershed; the heavily urbanized lower portion is located in the cities of San Juan Capistrano and Dana Point, while the largely rural portion extends well into the foothills of the Santa Ana Mountains. The Arroyo Trabuco valley forms a large alluvial plain called "Plano Trabuco" in the north part of the watershed. The Oso Creek valley is narrower, running south through Mission Viejo and Laguna Hills, and is mostly urbanized. The Bell Creek valley is the least urbanized of the four, being mostly located in the Cleveland National Forest, Starr Ranch Preserve and Caspers Wilderness Park. There are 19 other major tributaries in the watershed.
The California Department of Water Resources includes San Juan Creek in the San Juan Hydrologic Unit, which includes the coastal watersheds of Aliso Creek, Salt Creek, Prima Deshecha Cañada, Segunda Deshecha Cañada, and San Mateo Creek, which share a similar range of elevations and climate. Elevation in the Hydrologic Unit ranges from sea level to at Santiago Peak. The climate is Mediterranean, with hot, dry summers and cool, wet winters. Almost all the precipitation occurs between October and April; the average rainfall is, although mountain areas often receive higher amounts. San Juan Creek receives extra runoff from agriculture, urban areas, and other human sources, resulting in unnatural dry season flows that carry trash, heavy metals, oils, pesticides and fertilizer into the creek. The heaviest source of pollution is Oso Creek. Michael Hazzard, a Southern California environmentalist, said after diving into Upper Oso Creek Reservoir: "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 2-year period."