Water stargrass

Overview and Concerns

Water stargrass (Heteranthera dubia) is a species of perennial aquatic plant in the Pontederiaceae family. It lives in freshwater such as lakes, rivers, and irrigation canals; preferring calmer waters. It is a native species in the Yakima Basin, but its growth has exploded in recent decades. Historically, large floods and higher spring flows likely kept stargrass isolated to lower velocity parts of the river, but since the development of the Yakima River storage reservoirs, flood flows and the spring freshet have been significantly reduced (in order to store water in reservoirs for summer use). Prior to the 1990s, high fine sediment levels from irrigation runoff likely suppressed stargrass growth by blocking sunlight. Major efforts were made by irrigation districts and state and federal partners to reduce the erosion that caused high fine sediment levels in the summer, significantly cleaning up the river and reducing the associated levels of DDT and other toxins in the Lower Yakima- but also improving growth conditions for stargrass.

Today, water stargrass is prolific in most stretches of the lower 47 miles of the Yakima River, below Prosser Diversion Dam, and is becoming more noticeable upstream of there too. Water stargrass can form a bank to bank monoculture and grows from stream bottom to the water surface and then continue growing horizontally forming a canopy at the water surface. It also forms dense vegetative mats over stream substrates covering gravels cobbles. Water stargrass has many major negative impacts on the river:

• Stargrass degrades migration conditions for salmon, steelhead and lamprey by:
  • Blocking fish movement through fish ladders and at its worst, entire river channels,
  • Causing major daily fluctuations in dissolved oxygen and pH levels due to stargrass respiration and photosynthesis,
  • Improving habitat for warmwater predatory fish like smallmouth bass, increasing predation rates on salmon smolts (young fish travelling to the ocean), and
  • Changing local habitat conditions including temperature, sediment and insect populations in ways that reduce their value for juvenile anadromous fish.
• Stargrass has essentially eliminated fall chinook spawning in the lower river by growing over previously productive spawning areas and locking in spawning gravels and causing fine sediment deposition on top of them.
• Stargrass often creates stagnant water areas that support mosquito breeding, increasing human health risk from mosquito-borne illnesses and increasing the costs of mosquito control efforts.
• Drifting water stargrass often plugs or block irrigation intakes, making them hard to operate and requiring operators to do almost continual maintenance to keep diversions functional.
• Dense growth of water stargrass interferes with recreational activities such as swimming, boating and fishing and leads to the broad public perception that the river is 'gross'.
• Stargrass can make it harder to measure river flows (essential for river management) when it fills so much of the river that the surface level rises, requiring river gage operators to have to frequently visit and re-calibrate gages to ensure accurate flow measurement.

Stargrass Research and Monitoring

Multiple studies focused on water stargrass and its impacts on the Yakima River have been completed or are on-going, and new monitoring and research work is being developed to ensure that expanding treatment efforts are effective. Completed studies include:

• A 2009 report by the USGS and the South Yakima Conservation District detailed a 2004-2007 assessment of eutrophication in the lower 116 miles of the Yakima River. This was one of the first studies that characterized the proliferation of water stargrass in the Lower Yakima River.
• The 2011 Lower Yakima River Assessment completed by the Benton Conservation District includes a chapter on water stargrass and the issues it causes.
• Aaron Pelly's 2020 Master's Thesis focuses on water stargrass and its impacts on water quality in the Lower Yakima River.
• Pelly and others produced a 2024 report on water stargrass removal on dissolved oxygen, temperature, and water velocity in the Lower Yakima River.
• The USGS released a 2024 draft report on stargrass and water quality in the Lower Yakima River from 2018-2020. The data is available on the USGS website, link.

Ongoing monitoring efforts include:

Monitoring Gages

USGS and Benton Conservation District installed monitoring gages as part of the water stargrass research efforts. The gages were installed in the lower Yakima River at Prosser  (2018 - 2024), Benton City (Kiona; 2018 - 2025) and West Richland (Van Giesen; 2018 - 2025). The gage at Prosser was operated under funding from PNNL research from 2021 - 2024. Benton City and Van Giesen have operated through Department of Ecology and USGS funding from 2018 - 2025. The archived data for these sites can be retrieved from the USGS database:

• Prosser Gage

• Kiona Gage
• Van Giesen Bridge Gage

Biomass Surveys

• USGS and Benton Conservation District have collected biomass samples at Prosser (2018 - 2020) and Kiona and Van Giesen (2018 - present). Funding for these efforts has been through Department of Ecology Centennial funding and YBFWRB funding (2022). Samples are collected between June - September to document the rate growth and estimate the total annual biomass during the growing season. Data through 2020 are available in USGS 2024 report. Data 2021 - 2025 are in review and will be published by USGS in 2027.

Drone Surveys

• Benton Conservation District and Benton County Mosquito Control Board (BCMCB) are documenting the seasonal biomass using drone imagery at key locations on the lower river as part of the efforts to evaluate mechanical harvesting effectiveness as well as seasonal growth. Drone images are available upon request submissions to BCD.

Stargrass Control Efforts

Over the last decade, the Benton County Conservation District (BCD) and partners have researched and piloted several techniques to control water stargrass in the Lower Yakima River. Starting with the 2022 water stargrass recommendation report and followed by the Management and Control Techniques for Water Stargrass are discussed in detail in this BCD report

In 2024, BCD and Mid-Columbia Fisheries completed an emergency drought response plan for control of water stargrass in low water years.

Starting in 2024, key partners came together to formalize the Water Stargrass Coalition and develop a shared strategy for addressing the negative impacts of water stargrass on the Lower Yakima. In the spring of 2025, the leadership group of the Yakima Basin Integrated Plan focused on highlighting the importance of reducing stargrass impacts and securing significant financial investments to accelerate work on the ground. As part of this effort, the Roza Irrigation District produced a white paper that provides a good overview of stargrass control strategies and efforts in the Lower Yakima River, the coalition is implementing several pilot projects in the summer/fall of 2025 and is working on developing a longer-term work plan to coordinate activities and secure support and funding for expanding control work and associated monitoring and research in the future.

Hand

The most effective approach to manage stargrass is to remove the plant material at the roots. The simplest and most straight forward method is hand pulling with or without tools. Several hand-pulling projects have been completed over the last decade, and have shown that treatments can improve water quality, reduce future stargrass growth, and restore conditions that support fall chinook spawning. Hand-pulling is relatively slow and works best in shallower areas and lower flow velocities, limiting its broad application throughout the river; it can be effective in clearing smaller priority areas.

Mechanical

Mechanical harvesting typically uses a floating barge/boat equipped with either a cutter head (which typically mow the plan above the roots) or a roller (which can pull the plants, including roots, up, resulting in longer term control). Harvested plants may be removed from the water for onshore disposal or left in river to be washed downstream (allowing much more rapid operations). Mechanical harvesting began in 2021, when BCD acquired an Ecoharvester. ,. The focus of removal locations has been in the Yakima Delta area, the Wanawish Dam pool, the Benton City area and Prosser Dam pool. Pilot work has help determine where the harvester can and cannot operate well, and has confirmed that when using the roller head, it can significantly reduce plant densities in a way that persists in subsequent years. The Benton Conservation District produced a report on water stargrass mechanical harvest operations from 2021-2023. Approximately 16 acres were harvested in 2024. Several additional harvesters of varying sizes have been ordered in 2025 and should be fully operational in 2026. Partners are also exploring the use of land-based and amphibious harvesting equipment, including a summer of 2025 pilot of a spyder excavator led by WDFW. Pilots are also underway to test the use of hydraulic jets and suction dredging to control stargrass by dislodging it from the stream bed,.

Herbicides

Potential aquatic herbicide treatments are being assessed; herbicides being considered include endothall, imazamox, carfentrazone-ethyl, and fluridone. Local partners are working closely with US Army Corps of Engineers and Bureau of Reclamation aquatic plant management specialists who have experience with similar work in other area to evaluate whether and how herbicide treatments could be piloted in the Yakima River.

Flow Management

Flow conditions have a major impact on stargrass growth. In years where high, cool and turbid flows last through late spring, stargrass growth is significantly reduced; in low flow years it starts much earlier and grows much faster. This is due to a combination of reduced light availability for growing plants and cooler temperatures slowing growth. Higher flows- typical of winter and spring floods, break off stargrass and may even remove entire plants, including the roots when velocities are high enough to scour the bed, reducing the extent of stargrass in the next growing season. There is also anecdotal evidence that late spring/summer pulse flows released by the Bureau of Reclamation to encourage sockeye and summer chinook migration dislodge and move significant amounts of stargrass downstream; while this can create short-term problems at fish ladders and irrigation diversions, it has also been reported to have reduced stargrass issues at these sites for weeks after. The Water Stargrass Coalition is working to identify flow and turbidity management strategies that can help reduce the impacts of stargrass on the river while also meeting other fish and water supply objectives.

Digital Resources

Benton Conservation District - Water Stargrass Management Page. BCD Water Stargrass Videos

Mid-Columbia Fisheries Enhancement Group - Water Stargrass Page

State of Washington Water Research Center Spring 2025 Seminar Series- Water Stargrass Video

UW Burke Herbarium - Water Stargrass page

Water Stargrass Coalition Pictures Includes an assortment of pictures ranging from hand-pulling efforts to conditions in the Lower Yakima River.

Aaron Pelly's Water Stargrass presentation at the Society for Freshwater Science Annual Meeting in 2025.

Media Coverage

Type	Source	Title	Date	Notes
TV	NBC	Crews Remove Water Stargrass from the Yakima River to Save Salmon	9/7/2025
News	KIMA News	WDFW project targets invasive aquatic plant to improve habitat in Yakima River	9/4/2025
Blog	WDFW	Collaboration in the Yakima Basin: Project kicking off in September will remove water stargrass, improve habitat in Yakima River	8/28/2025