Case Study: Central Arroyo Seco Stream Restoration Near Downtown Los Angeles

Arroyo Seco is a sub-watershed of the Los Angeles River. The Arroyo Seco channel runs in a deeply incised canyon that begins in the San Gabriel Mountains and drains to the Los Angeles River near downtown Los Angeles. Below Devil's Gate Dam, a flood control facility located in Pasadena, most of the stream has been channelized with a straightened concrete-lined rectangular configuration. Prior to channelization, stands of alder, willow, and sycamore lined a perennial, trout-filled stream. Most of the remaining aquatic and riparian habitat is located above the dam. Below the dam natural habitats and native vegetation have been largely replaced by urban development and exotic plant species. Chaparral, which covers much of the land mass in the adjacent Angeles National Forest, has encroached significantly upon the Arroyo Seco riverine corridor. Remaining riparian and aquatic habitat now comprises only 15 percent of the total land mass within the watershed.The few natural channel reaches below the dam are subject to flash flood events due to the highly impervious upstream-urbanized watershed, operational releases from the Devils Gate Dam, and the hydraulic efficiency of the concrete-lined channel reach. Flash flooding has adversely impacted physical habitat in the natural reaches by eliminating structural diversity and depositing large quantities of sediment. Without physical habitat such as backwater pools, fish and other aquatic species are at risk for being washed through to the downstream concrete-lined channelized portions of the Lower Arroyo Seco that are completely devoid of habitat and unable to sustain any form of life.The Central Arroyo Seco stream restoration, near the Rose Bowl, is a pilot project to re-establish habitat for native fish populations with the arroyo chub as the target indicator species. Resting areas, juvenile rearing, and spawning grounds were created for the arroyo chub and other native fish through the construction of backwater pools, riffles, and a series of wing deflectors and snags to increase resting, foraging, spawning, and protective habitat features. Local materials—including river run rock and boulders, fallen trees and logs, and root wads—were utilized in the construction of stream channel enhancements. Construction of these stream improvements, along with water quality related improvements to trails, banks, upland areas, adjacent parking lot, and citywide storm drains, serve as a model for stream restoration in the community of Pasadena, the Los Angeles River watershed and the state of California. The California Department of Fish and Game is utilizing the restored reaches of the Central Arroyo Seco in a pilot program to assess the sustainability of relocated arroyo chub populations.