San Luis Rey Flood Risk Management

Winter 2016 Ecesis, Volume 26, Issue 4

Active restoration area post passive restoration efforts and giant reed removal, January 2013.

Active restoration area post passive restoration efforts and giant reed removal, January 2013.

In the sunny City of Oceanside, California, there is a bike/maintenance path on the levee of the San Luis Rey River between Interstate 5 and College Avenue. It is along this path that you can peer into the river channel and see the riparian habitat that supports a number of federal and state endangered species, two of which are the least Bell’s vireo (Vireo bellii pusillus) and the southwestern willow flycatcher (Empidonax traillii extimus). 

This 7.2-mile stretch of riparian habitat is the project area of the San Luis Rey River Flood Risk Management Project. For years, this portion of the San Luis Rey River was overgrown with many invasive non-native plants like giant weed (Arundo donax), salt cedar (Tamarix sp.), and castor bean (Ricinus communis). The high abundance of these exotics diminished the quality of the habitat for the federally listed species and added to the dense vegetation that could lead to the increase of flood risk.

When faced with a complex system such as this one, choosing a restoration strategy can be difficult. The San Luis Rey River Flood Risk Management Project aims to find a balance between protecting the biological resources and maintaining flood-risk reduction. To achieve this balance, the project includes phased vegetation removal, sediment removal, active and passive restoration, and employs an adaptive management strategy that provides opportunities for learning and adapting to change when needed.

The phased vegetation removal consists of mowing and shredding the riverine riparian vegetation in a designated alignment on a regular basis. Areas that will never be mowed are passively and/or actively restored. 

The passive restoration effort involves removal of invasive non-native vegetation and allows for natural recruitment of native species. Active restoration involves removal of invasive non-native vegetation as well as planting native container plants and pole-cuttings to restore riparian habitats. 

Within the project area, the initial approach for determining the use of passive restoration in a particular area is based on the density, shape, size, and accessibility of each patch of invasive non-native that has been treated. Easily accessible large patches are earmarked for active restoration; while disjointed areas — where there are small and scattered populations of invasive non-native vegetation, where densities of weeds are low, or in remote areas that are difficult to access — are deemed suitable for passive restoration. Passive restoration is the predominant management strategy used for vegetation and habitat-type restoration on the San Luis Rey River and is the precursor for areas that are being actively restored.

Passive restoration began in 2006 with intensive treatment of invasive non-native invasive vegetation through herbicide application. Once the large stands of giant reed died out from multiple herbicide treatments, the dead stands were cut and mowed. These large areas of treated and mowed vegetation made way for active restoration to begin. In those areas slated for active restoration, few, if any, of the native species remaining were capable of recruiting into the area. 

Within the project area, over 100 acres have been treated passively. By significantly reducing the density of non-native vegetation, we are stopping the activities that degrade the natural system and are allowing natural processes to return. Since the intensive herbicide treatments, large stands of giant reed and salt cedar have been effectively reduced to a small fraction of their former size. Where stands of giant reed once had thousands of living shoots, only a small handful of shoots have emerged. 

In the areas chosen for active restoration, over 56,000 container plants were planted 2013. By spring 2014, one year later, least Bell’s vireo were found nesting within the active restoration areas. Currently, over 30 acres are in the process of being actively restored.

Throughout areas in the project that have been treated passively, there has been an abundance of dead and dying trees and shrubs —  negative effects from the drought conditions that have been prevalent in southern California over the last four years. The high mortality of riparian vegetation did not provide suitable nesting substrate and supporting habitat needed for successful nesting.

In order to tackle this issue head on, additional restoration strategies were evaluated for use in these stressed areas; mechanical weed control, supplemental watering, and other measures were implemented to boost natural recruitment. In the event these measures are not effective, then active restoration may be triggered. 

A close watch is kept on this ecosystem in order to determine the best management approach for each area of the project. Utilizing this type of adaptive management strategy and implementing both passive and active restoration efforts allows the project to restore this ecosystem as effectively as possible while meeting the overall project objectives. — by Karyl M. Palmer, Environmental Analyst/Biologist, RECON Environmental, Inc., kpalmer@reconenvironmental.com