King County logo
Lake Photo Image
Go to another lake:
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


Water quality monitoring began at Green Lake in 2005 after an alum treatment for phosphorus control, continuing through 2014. This lake in Seattle is clear and lightly buffered against pH change. It is currently moderate in primary productivity (upper mesotrophic), with overall good water quality, but has increased significantly over the last two years. The lake does not stratify thermally in summer, so there are no increases in phosphorus or ammonia in the deep water. Nitrogen to phosphorus ratios are generally below 25:1, which favors cyanobacteria. Accumulations along downwind shorelines have produced toxins, and scums tested in 2012-14 have produced values that could be harmful to children and pets. Green Lake is surrounded by a very popular city park and is open to car-top boating. There are established infestations of Eurasian milfoil and fragrant water lily that have been the targets of past control activities.

Back to top


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

Back to top

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).

Back to top

Reports and Related Links

Friends of Green Lake

Back to top