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Avoid all areas of algae accumulation along the shoreline; they could contain toxins harmful to people and pets.

Vital Statistics

Watershed Area 1875 acres
Lake surface area 259 acres
Maximum depth 30 feet
Mean depth 13 feet
Public Park Yes
Boat Ramp No, car top boats OK
Fish Present bass, carp, stocked RB trout


The key takeaways from the 2021 monitoring season are:

• This year, Green Lake had less-clear water, with high nutrient concentrations and moderate algal growth. • Nitrogen-to-phosphorus (N:P) ratios were below 25 throughout the monitoring season. This indicates the potential for algal blooms to be dominated by cyanobacteria (which have the ability to produce toxins). • Average 2021 chlorophyll, nitrogen, and phosphorus concentrations were noticeably higher than values observed in 2018. Although chlorophyll and nitrogen values are not as high as 2019 averages, these concentrations are all still noticeably higher than values observed before the alum treatment that occurred in spring 2016. • An algal bloom was sampled for toxin testing in October. Toxin testing found elevated microcystin concentrations: 16.1 µg/L, above the Washington State Recreational Guideline of 8 µg/L. There was a Toxic Algae Warning on the lake for a couple weeks in October.

Please see below for a link t

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This bathymetric map shows contour lines of equal depth, similar to a topographic map would for mountains and valleys. The red 'X' marks the location where water quality samples are taken.
Watershed map unavailable

Click image to enlarge

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Water Quality Data  

Through a combination of efforts by volunteer lake monitors and King County staff, data relating to physical, chemical, and biological aspects of the lake have been collected for most lakes. The King County Lake Stewardship Program analyzes data to track long-term water quality trends in small lakes in western King County.

View or Download Data
Use this tool to view or download data from the lake in tabular format. You can define date ranges and select which parameters to view or download.

Chart Data
Use the charting tool to look at graphs showing single parameters for a single water year at a time.

Water quality over time
A common method of tracking water quality trends in lakes is by calculating the “trophic state index” (TSI) (Carlson, 1977) and testing the values for positive or negative trends over time. TSI indicators predict the biological productivity of the lake based on water clarity (Secchi) and concentrations of total phosphorus (TP) and chlorophyll a (Chlor). Generally at least 8 years of data without major lake management activities are needed to have confidence in a significant trend.

The average of these three TSI indicators during the growing season can be used to place lakes in one of three broad categories:
<40 = oligotrophic (low productivity),
40 to 50 = mesotrophic (moderate productivity)
>50 = eutrophic (highly productive).

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Reports and Related Links

Lake Stewardship Program: 2022 Monitoring Update
2021 Green Lake Monitoring Report
2020 Green Lake Monitoring Report
2019 Green Lake Monitoring Report
2018 Green Lake Monitoring Report
Friends of Green Lake
Green Lake Hydrologic Data

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For questions about lakes in King County, please contact lakes@kingcounty.gov or call the Water and Land Resources Division front desk at 206-477-4800.