Collaboration in the Yakima Basin: Project kicking off in September will remove water stargrass, improve habitat in Yakima River
Updated for accuracy Sept. 5, 2025
Starting Sept. 2, 2025, the Washington Department of Fish and Wildlife (WDFW), Yakama Nation Fisheries, Benton Conservation District, and other partners will lead a project to remove water stargrass in two areas of the lower Yakima River. The project will help to improve fish passage, spawning habitat, water quality, and water movement.
This pilot project will occur near Prosser and Benton City, specifically in the area of the Prosser City Wastewater Plant to the Yakama Nation Prosser Hatchery. Then the work will shift to just above the Benton City Unit of WDFW’s Sunnyside-Snake River Wildlife Area downstream to Benton City Park and the WDFW water access area. Work could continue through Sept. 30 at the latest.
Benton Conservation District and Yakama Nation Fisheries have been removing water stargrass over several seasons and this collaborative, pilot project builds on that critically important work.
Water stargrass and its impact on salmon survival
Water stargrass (Heteranthera dubia) is a native aquatic plant in the Yakima River basin that recently saw explosive growth and now dominates the lower part of the river. This impacts salmon, irrigation, and recreational river use. Historically, the presence of water stargrass was minimal due to lower water temperatures in the river and higher flow levels. Low flows resulting from multiple years of drought contributed to increases in water stargrass. The combined challenges of drought and stargrass can be deadly for returning adult salmon.
Water stargrass can be especially concerning because it displaces fall Chinook salmon spawning in their historic areas of the Yakima River, particularly in the reach below Prosser Dam, where hundreds of redds (or salmon nests) used to be active. Salmon are unlikely to successfully construct redds due to the thick mats formed by the roots of the plant. The plant’s density slows salmon down, forcing them to spend more time in warmer waters and to move further upriver to spawn, exposing their offspring to predators and deadly temperatures.
The Yakima Basin is experiencing drought for the third year, which exacerbates water stargrass impacts to spawning fall Chinook and migrating steelhead and coho. Last summer, water stargrass contributed to a significant sockeye salmon kill in the lower Yakima River. When salmon are forced to spend excessive time in warm, shallow water that has been depleted of dissolved oxygen, it can be fatal to them.
In addition to the impacts to salmon, water stargrass compromises infrastructure for irrigation and municipal water supplies as it restricts the flow of water, requiring continuous costly equipment and facilities cleaning during the summer. The grass also gets caught in irrigation structures’ fishways and becomes a complete barrier to fish passage.
Compounded with the stargrass issue in the Yakima River, multiple years of drought in the region have heavily impacted salmon, steelhead, bull trout, and lamprey and resulted in degradation of water quality in the river ecosystem. During spring in drought years, juvenile fish (smolts) experience water conditions that are too warm and shallow to journey to the ocean. Over the years, this loss of juvenile fish has a cascading effect on salmon population numbers. The 2015 drought reduced out-migrating salmon from 1 million to 200,000 fish, and the fishery, which is managed by Yakama Nation Fisheries and WDFW, is still recovering.
Water stargrass removal
The project included coordination between federal, state, and local government agencies to acquire the necessary permits for in-water work, ensuring it doesn’t cause additional damage to the river’s habitat or species.
“The work is not likely to impact salmon spawning since it won’t take place during October or November when spawning occurs,” says Trevor Hutton, WDFW South Central Fish Program Manager. “WDFW biologists will be on site to monitor the presence of fish in the area before, during, and after the work is completed.”
Project participants will use an excavator to rake and remove the stargrass, entering the river and working from upstream to downstream. Excavator operators will temporarily place pulled plants onto the riverbank to dry and then haul them away to compost. Work timeframes are dependent on weather and other conditions.
After removal, WDFW biologists will survey the treated areas to determine if the removal was effective in rehabilitating historical spawning areas.
Collaborative partnerships
This work is occurring in alignment with the Yakima Basin Integrated Plan (YBIP). YBIP is a coalition of local, state, and federal agencies, the Confederated Tribes and Bands of the Yakama Nation, and non-governmental organizations that are collaborating on projects to protect water for farms, fish, and communities in central Washington. Stargrass removal is one step in restoring the Yakima River for salmon and other important species.
