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Vital Statistics

Watershed Area 4275 acres
Lake surface area 63 acres
Maximum depth 25 feet
Mean depth 15 feet
Public Park Y
Boat Ramp Car top, no gas engines
Fish Present bass, stocked rainbow


The key takeaways from the 2021 monitoring season are:

• Cottage Lake had less-clear water, with high nutrient concentrations and high algal growth. • While no significant long-term trends were detected, this year we saw a spike in chlorophyll concentrations while phosphorus concentrations peaked at the end of the monitoring season. This supports the algal blooms we are seeing every year. • Cottage Lake had a prolonged algal bloom with samples submitted for toxin testing in August and September. Toxin testing found elevated microcystin concentrations: 12.9 µg/L, above the Washington State Recreational Guideline of 8.0 µg/L.

Please see below for a link to the full 2021 Cottage Lake Monitoring Report.

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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.
This map shows the area of the watershed relative to the area of the lake. Generally speaking, the larger a watershed is relative to a lake, the greater the influence land use practices on lake water quality.

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 Cottage Lake Monitoring Report
2020 Cottage Lake Monitoring Report
2019 Cottage Lake Monitoring Report
2018 Cottage Lake Monitoring Report
Cottage 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.