Beaver Lake Water Lily Removal FAQ

Beaver Lake Lilies.jpg

Lily pads are also known as “fragrant water lilies” or “Nymphaea odorata”. Their legal status in King County is Class C noxious weed. They are non-native species that can be designated for control based on local priorities (RCW 17.10). The King County Control Weed Board recommends control of this species. They are native to the Eastern half of the U.S. and were introduced as an ornament in many parts of Western Washington. Left unmanaged, they can restrict lakefront access and hinder recreation. Drownings in King County have been attributed to lily pads (entanglements in dense stems). Stagnant mats create mosquito-breeding areas and increase water temperatures by absorbing sunlight. They also contribute to algal growth and water quality problems. For more information about fragrant water lilies, please visit this page.

Manual: Hand pulling can be successful for a small area if repeated on a regular basis; however, this method is impractical for large infestations. When using this method, one must make sure to get the entire rhizome. Also, one must make sure to remove all pulled/cut plant pieces from the water. It can be composted on land or placed in a yard waste bin. Another option is a carbohydrate depletion technique in which all emerging leaves are consistently removed. Typically it takes two to three growing seasons to kill the plants. All manual control methods that disturb lake bottoms, wetlands or streambeds require at a minimum a Hydraulic Project Approval (HPA) pamphlet permit from the Washington Department of Fish and Wildlife. Hand pulling of lily pads is not a realistic approach for large areas of infestation.

Mechanical: An opaque bottom barrier can be used to suppress growth in small areas such as a boat launch or around a swimming area. Underwater rototilling of the rhizomes using a backhoe mounted to a barge and cutting and harvesting using boat-mounted cutters are options for large-scale removal and control. These methods require approval from the Washington Department of Fish and Wildlife. Underwater rototilling and cutting and harvesting using boat-mounted cutters methods were thoroughly investigated for Beaver Lake and were deemed unsuitable. This is due to the detrimental effect of such methods on water turbidity and quality, the lack of access of the boat to most the shallow areas of the lake (essentially all the littoral of Beaver Lake 2 and all of Beaver Lake 1 and 3), and the prohibitive cost of this approach.

Chemical: Herbicides may be the most effective, cost effective option for eradication of large infestations. Professional, licensed contractors are required for this method. In Washington State specially formulated aquatic herbicides are only available for purchase to licensed applicators. Applying herbicides to water requires a permit from Washington Department of Ecology.

For additional information, read this.

An aquatic formulation of glyphosate will be used. This is NOT RoundUpTM. While glyphosate is the active ingredient in RoundUpTM, RoundUpTM contains ingredients that are toxic to aquatic organisms. It is ILLEGAL to use RoundUpTM in aquatic environments. Information about glyphosate can be obtained here.

Glyphosate is sprayed directly on the buoyant leaves of the lily pads. This allows for maximum exposure of the plants and minimal spillage in the water and the environment. Inevitably, some Glyphosate will be sprayed in the water.

Chemical treatment is planned during summer 2018 and 2019. Future years will be assessed for hand or mechanical removal.

The decision to treat Beaver Lake was made by the Advisory Board of the Beaver Lake Management District (BLMD). The BLMD’s mission is to track environmental conditions at Beaver Lake and to promote actions and behaviors among area residents that will minimize negative impacts to the lake and its surrounding ecosystems. The advisory board of the BLMD determines which projects and programs are most beneficial to the lake, based on current conditions and the overarching goals of the BLMD. Once the work program is determined, the City of Sammamish, acting upon the advice of the BLMD Board, contracts with public agencies or private contractors to complete the work.

Property owners with frontage on Beaver Lake and property owners whose properties drain to Beaver Lake are annually assessed through the Beaver Lake Management District Special Assessment. This fee appears on property tax bills processed by King County. The assessment also covers monitoring, outreach and education, and administration.

After Beaver Lake was treated in 2016 water samples were taken 48 hours after the treatment and analyzed for residual glyphosate in the water. Those samples were collected in locations where the herbicide was applied and all samples came back from the laboratory with NO detection of glyphosate in the water. The Washington State Department of Ecology conducted two studies on Spring Lake in 2003 and North lake in 2005 . In both case there was no detection of any pesticide 24 hours after application.

The Beaver Lake Community Club (BLCC) engaged the services of a licensed applicator to treat the lake with glyphosate in 2016. Given the level of high infestation of the lake, this treatment was the first step in a multi-year effort to control the lily pads. By design, not all the lily pads are treated at the same time to avoid a mass killing of the weed and the subsequent formation of mud mats that could cause significant turbidity and degradation of water quality.

Glyphosate is not banned in California. In July 2017, the Office of Environmental Health Hazard Assessment (OEHHA) added glyphosate to the list of chemicals known to the state to cause cancer for purposes of Proposition 65. The list contains a wide range of naturally occurring and synthetic chemicals that are “known to California to cause cancer or birth defects or other reproductive harm”. These chemicals include additives or ingredients in pesticides, common household products, food, drugs, dyes, or solvents. Listed chemicals may be used in manufacturing and construction, or they may be byproducts of chemical processes, such as motor vehicle exhaust. Glyphosate is not banned in Europe.

According to the Department of Ecology website for water use after glyphosate herbicide treatment, the following restrictions apply: Potable Water Use Restriction - Water in the treated area and within 1/2 mile of the treated area may be used for drinking 48-hours after treatment, or once the Glyphosate level is 700 ppb or less. Swimming & Water Contact Activity - No restrictions or advisories. Fishing - No restrictions or advisories. Irrigation Water & Home Lawn/Garden Use - No restrictions or advisories. Livestock/Domestic Animal Water - No restrictions or advisories.

Chris Knutson
Water Quality Planner/Project Manager II
King County Water and Land Resources Division
201 S. Jackson St, Suite 600
Seattle, WA 98104-3855
206-477-4739 (office)