Birds, butterflies and wetland vegetation thrive at Oxford Basin, a decade after its restoration
Ten years ago, the Los Angeles County Department of Public Works renovated the Oxford Retention Basin (ORB), a stormwater retention basin built during the late 1950s in conjunction with Marina Del Rey.
Prehistorically, the basin area had been part of the Ballona Wetlands, which were dredged and filled for the Marina's construction.
Above: A 1950s aerial photo of the Ballona Wetlands, prior to Marina Del Rey and other developments. The red arrow shows the Oxford Basin location.
Connected to the Marina by a single tide gate offering limited water circulation, the ORB was often a smelly, algae-filled eyesore, but even then provided bird nesting habitat. Non-native trees around the basin added leaf matter to the water, promoting unnatural biological growth.
Renovation began in 2014 and included removing non-native trees and plants, replacing them with native species. The renovation also included the construction of two tide gates to replace the old single gate and the construction of a vegetated berm that separates the gates and aids in circulation.
Above: Interpretive panel at a viewpoint along Washington Blvd. The twin tides gates are visible at the upper right.
This improved water circulation by letting flood tidewater in through one gate and ebb tide flow out the other gate. However, because the gates do not allow tides to reach their maximum height within the basin (to prevent street flooding), water circulation is muted.
Above: The ORB shortly after construction. Note the circulation berm. Rising tides enter on the far side of the berm and return to the Marina from the near side during ebb tide.
The circulation design minimizes stagnation within the basin (which helps decrease odors), but water quality within the basin is still subject to existing polluted runoff from Venice streets during rainfall events and dry weather. During dry weather, the basin water quality mirrors that in Marina Del Rey due to daily tidal exchange.
Above: Monitoring data showing increasing native and decreasing non-native vegetation at the ORB over time.
In the ten years since renovation, the native vegetation surrounding the ORB has grown in substantially (see chart above). Prior to renovation, a few local activists opposed the removal of several non-native eucalyptus trees adjacent to the basin. They argued that the removals would harm nesting birds and the monarch butterfly, which were known to rest there during their migration.
Experts know that birds simply nest in other nearby trees when a tree is removed. In fact, the bird variety and abundance at the ORB has increased from about 20 species observed pre-construction to 57 different species observed over the post-renovation monitoring period.
Above: The Oxford Retention Basin today.
As for the monarch butterfly, they also find other trees in which to rest during their journey, and the ORB now supports native milkweed, on which the monarch depends for its life cycle.
Above: Western regional Monarch butterfly count from 1997-2024. Source: Xerxes Society
The basin renovation project also added public access features, including a footpath along the southern border and observation decks along the northern shoreline adjacent to Washington Boulevard.
Above: Plenty of eucalyptus trees still line the path outside of the ORB fence line
Happy 10-Year anniversary, Oxford Retention Basin!
Above: A Great Blue Heron rests at the ORB
Enjoy your Ballona Wetlands!
References:
Western Monarch Count. Xerxes Society for Invertebrate Conservationhttps://westernmonarchcount.or...
Oxford Retention Basin Multiuse Enhancement Project. Post-Construction Monitoring 5-Year Summary. Final Report. Prepared For: Los Angeles County Flood Control District 900 S. Fremont Ave. Alhambra, California 91803 Contract No.: PW15261 Task Order No.: 13434.200.006 November 2021. Weston Solutions.
Disclosure of author's affiliations:
Dr. David W. Kay served on the Board of Directors of the non-profit Friends of Ballona Wetlands from 2007 until 2015, and served as Board President in 2012-13. He presently serves on the Board of Ballona Discovery Park in Playa Vista. Dr. Kay is a staunch advocate for the state of California's plans to restore the Ballona Wetlands Ecological Reserve.
From 1984 until 2022, Dr. Kay was employed by Southern California Edison Company, exclusively in the company's environmental services organizations. His many responsibilities included restoration of the 440-acre San Dieguito Wetlands near Del Mar. He retired in 2022 as Senior Manager for Major Project Environmental Management at the company, after 38 years of service.
Dr. Kay earned bachelor and masters degrees in biology and a doctorate in environmental science.