Monitoring invasive weeds in the Sacramento-San Joaquin River Delta: Methods, effectiveness, and potential for restoration

Invasive aquatic weeds negatively affect biodiversity, river processes, and water related human activities. Aquatic plant management in the United States has long been a federal and state wide concern, costing over 110 million dollars per year in damages and control programs. In the Sacramento-San Joaquin River Delta in California, invasive macrophyte communities are actively managed for the control of water hyacinth (Eichhornia crassipes) and Brazilian waterweed (Egeria densa). Remote sensing can be a potential solution to species monitoring, providing insights on rates of spread and invasibility, and to elucidate restoration potential and assess its success. We have been applying remote sensing techniques to detect, quantify and monitor submerged Brazilian waterweed and water hyacinth throughout the Sacramento-San Joaquin River Delta from 2003 to 2006. Species range has fluctuated but invaded areas have been re-colonized every year, even after control methods were applied. Active restoration areas in the system have also been focus of monitoring. We estimated a turnover of invasive weeds shortly after restoration treatments were applied. Results suggest that: (i) remote sensing is a useful tool to map and monitor invasive species, (ii) control programs of invasive aquatic weeds in the Delta may need to adaptively improve their efficiency, and (iii) coordinated restoration efforts need to be undertaken to prevent the mere substitution of target species by alternative exotic species.