Invasive Aquatic Vegetation in the Sacramento–San Joaquin Delta and Suisun Marsh: The History and Science of Control Efforts and Recommendations for the Path Forward

Invasive aquatic vegetation (IAV) is a management challenge in the Sacramento–San Joaquin Delta and the Suisun Marsh that has commanded major resource investment for 4 decades. We review the history and supporting science of chemical, biological, and mechanical control of IAV in the Delta and Suisun March, and in flowing waters outside the region. Outside the Delta, there is a significant history of research on IAV control in lotic systems, but few studies come from tidal environments, and we found no investigations at a spatial scale like that of the Delta. The science of control efforts in the Delta is nascent but has seen marked growth over the recent decade. Since 1983, control of invasive submerged and floating species has been centralized within the California State Parks Division of Boating and Waterways (CDBW). The program relies on herbicides, with an annual budget that has exceeded $12.5 million since 2015. However, the results have been mixed because of the challenge of applying herbicides effectively in a tidal system. In parallel, biological control agents for water hyacinth (Eichhornia crassipes) and giant reed (Arundo donax) have been released but have not provided an appreciable control benefit, likely because they are not suited for the temperate Delta climate. Over recent decades, regulatory complexity has increased, hampering efforts to innovate alternative methods or respond quickly to new invaders. Control efforts for giant reed and common reed (Phragmites australis), the main invasive emergent plants, have not been coordinated under a central program, and studies to investigate control strategies have only recently been permitted. As a result, no local studies have been published on control outcomes for these species. Based on this history and our review of the science, we develop recommendations for leadership and science actions to proactively manage IAV.