The San Gabriel River Arundo Removal project removes 80 net acres of Arundo donax, from the San Gabriel River and Rio Hondo at Whittier Narrows. Coexisting infestations of castor bean (Ricinus communis), perennial pepper weed (Lepidium latifolium), Tree of Heaven (Ailanthus altissima), and passion vine (Passiflora) will also be treated. Monitoring for treatment efficacy, missed invasives and resprouts, will continue past initial treatment for a total project duration of five years.
The San Gabriel River serves as the primary water source for many cities along the river through recharge of the Upper San Gabriel and the Central groundwater basins. The Los Angeles Flood Control District and the Water Replenishment District run a pair of spreading grounds below Whittier Narrows, one on the San Gabriel River and one of the Rio Hondo. Arundo transpires water at a much higher rate than locally native vegetation and Arundo control is expected to save water lost to evapotranspiration. Based on research by Giessow, et al. (2011,Table 4-4) Arundo control in the San Gabriel River could add as much as 1,000 ac-ft/yr of water to the groundwater supplies. Montebello Hills Habitat Reserve, adjacent to the project area, has been set aside as permanent native habitat. The combination of this upland sage scrub habitat with the riparian Rio Hondo habitat makes for a more diverse open space than usually found this close to the urbanized parts of the San Gabriel Valley. Removing the invasive Arundo from this stretch of the Rio Hondo helps restore this entire area to its natural habitat.