2004 Big Bow Lake Monitoring 

Description of the Site

Big Bow Lake is located in the city of Rock Island, Douglas County, Washington. The lakes were once sloughs left over from an oxbow of the Columbia River. In the 1970’s, a powerhouse was built at the Rock Island Dam which raised the water level causing the sloughs to increase by approximately six feet, creating larger lakes. Big Bow Lake, along with three other lakes, at one time was heavily utilized for fishing, boating, and swimming, and also provided excellent habitat for wildlife. Since the 90’s, Eurasian watermilfoil (Myriophyllum spicatum) has been present in the lakes and is now so abundant it poses safety concerns for swimmers, inhibits fishing and boating, negatively impacts fish and wildlife habitat, and has reduced the aesthetic appeal of the lakes.

The city of Rock Island Lake Enhancement Committee (LEC) was formed several years ago to address these issues. They applied for and received a grant from the Department of Ecology to develop an Integrated Aquatic Vegetation Management Plan (IAVMP). The primary goal of the LEC was to control Eurasian watermilfoil. The IAVMP recommended treating half of the littoral zone of Big Bow Lake (24.5 acres) with the aquatic herbicide diquat in 2004 and stocking grass carp in the lake. A second diquat treatment is planned for 2005.


Big Bow Lake was treated with the aquatic herbicide Reward® (active ingredient Diquat) on July 27, 2004. The herbicide was applied by subsurface injection at an application rate of 1.5 gallons per surface acre (the maximum application rate is 2 gallons per surface acre). A total of 24.5 acres were treated which included the eastern portion of the lake near the boat launch facilities and the western portion of the lake near a second boat launch area. A section of the southern boundary of the lake was treated also. The treated areas were selected to include the heaviest areas of milfoil infestation and high use areas. Surface water samples for diquat residues were taken from inside a treated area and from outside of the treated areas prior to herbicide application, and 1-day, 5-days and 10-days after application. The samples were collected by a water quality consultant from a small non-motorized boat. The samples were sent to an Ecology- accredited laboratory the day of collection or held overnight packed in ice and shipped the next business day. The laboratory analyzed the samples using EPA method number 549.2/3535.

Big Bow Lake. The red areas are the treated areas of the lake. The yellow areas on the photograph indicates where the samples were taken.


City staff monitored the condition of Big Bow Lake as the water quality samples were collected. They noticed a dramatic and immediate difference in the lake. Although only a portion of the lake was treated, the effects of the treatment were noticeable throughout the entire lake. Algae were also more apparent on the surface of the lake about 15 days after the application. This may be partially due to the increased temperatures that occur in July. It was evident to staff that the diquat treatment was very effective in removing milfoil.

Summary of Diquat (Reward™) Residues

Diquat levels in the treatment area were above the EPA drinking water standard of 20 ppb one day after treatment (40.38 ppb), but were below the drinking water standard after five days (11.02 ppb), and ten days (2.62 ppb) after treatment. There is a three-day drinking water restriction after diquat treatments so this higher level at one day after treatment is not unexpected. Only minimal concentrations of diquat were detected in the untreated area of the lake with the highest concentration measured at 1.14ppb, well below the drinking water standard.

Time Western End of the Treated Area Untreated Area in the Middle of the Lake
Pre-Treatment Non Detect Non Detect
1 Day after Treatment 40.38 ppb 0.75 ppb
5 Days after Treatment 11.02 ppb 1.14 ppb
10 Days after Treatment 2.62 ppb 0.80 ppb