San Juan Creek - River Modifications

River Modifications

Although most streams in the San Juan watershed are less developed than those in the neighboring Aliso Creek and Salt Creek watersheds, San Juan, Oso and Trabuco Creeks have undergone an extensive series of modifications designed to control floods, reduce excessive erosion, and provide reclaimed water for irrigation. A growing amount of urban runoff flows into the creek and its tributaries, adding to the excessive perennial flow referred to often as a "nuisance flow" or "dry season nuisance flow". Historically, only San Juan and Trabuco creeks were known to be perennial, while most of the tributaries, except for spring-fed ones such as Holy Jim Creek, were ephemeral. Oso Creek was formerly ephemeral, but currently it can flow in excess of 100 cubic feet per second (2.8 m3/s) in up to ten percent of an year. Finally, the long series of human-made modifications to the creek have resulted in its mouth, Doheny State Beach, placing tenth in the most polluted beaches of California.

San Juan Creek is channelized for approximately 3.5 miles (5.6 km) from immediately upstream of the Interstate 5 bridge to the mouth. Trabuco Creek is only channelized for several hundred yards above its confluence with the larger stream. In sharp contrast is Oso Creek, which is channelized along nearly its entire length, while Bell Canyon Creek is free-flowing. Two large reservoirs, Upper Oso and Lake Mission Viejo, are built on Oso Creek, with a total storage capacity of roughly 7,500 acre feet (0.0093 km3). While Lake Mission Viejo is primarily for recreation, the 115-acre (0.47 km2) Upper Oso Reservoir collects Oso Creek water and diverts it for irrigation use, while any low-flow spillage that occurs over its dam is collected at a small downstream dam that feeds a pipe that pumps it back to the reservoir; in this way the Upper Oso Reservoir can be considered as a pumped-storage operation, although it does not produce hydroelectricity. A notable aspect of the reservoir is that it was vital in providing water to extinguish a large wildfire in October 2002. The dam is rockfill, 800 feet (240 m) long and 142 feet (43 m) high.

A few check dams exist on small upper tributaries of San Juan Creek, mostly inside the Cleveland National Forest, as well as a small, abandoned gabion structure lower on San Juan Creek. The structure, which was formerly a small dam, is now out of operation, but its roughly 3-to-4-foot (0.91 to 1.2 m) drop still poses a problem for migrating steelhead trout. 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, which are small dams used to control water velocity, exist on streams in the San Juan watershed, primarily also on Trabuco and Oso creeks. On Trabuco Creek, 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; however, neither is inventoried by the Orange County Flood Control Division. There are also about seven drop structures on Oso Creek, most of which are also built of riprap. The mainstem San Juan Creek does not have any notable drop structures, dams or water diversions.

As with channelization, levees are only found in the lower San Juan watershed. San Juan Creek is bounded with levees from upstream of Interstate 5 to very near its mouth at Doheny Beach, while Trabuco Creek has levees from upstream of Del Obispo Street (a continuation of the Ortega Highway) downstream to its mouth at San Juan Creek. Oso Creek has some raised banks, but it is not certain if those are levees. Smaller tributaries, such as El Horno Creek and Bell Canyon, do not have levees. Although the San Juan and Trabuco Creek levees are said to have a "fairly high level of protection currently", failure scenarios of levees in the San Juan watershed have been extensively studied, and they have been determined to possibly fail for a variety of reasons, some of which have already been demonstrated in the floods of 1996 and 2005. In response to the danger of another flood, work has begun on a new west-bank levee replacement that will finish in 2013.