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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


Cottage Lake was monitored between 1995 – 2008, and work resumed in 2014. This lake is lightly colored, moderately buffered against pH change, and high in primary productivity (eutrophic) with fair water quality, remaining steady over time. There is a lake management plan (King County, 1996) and a WDOE water clean-up plan (2004). A phosphorus reduction project began in late 2005 and ended in 2010. Phosphorus may be decreasing over time, but the data gap makes this difficult to verify. Thermal stratification is relatively stable through summer, and considerable sedimentary phosphorus release adds to deep water concentrations. Nitrogen to phosphorus ratios are commonly below 25:1 in late summer, which favors cyanobacteria. Cottage was monitored for cyanotoxins in 2009-2012, but levels have remained low to-date. Cottage Lake has no public access boat ramp, but car top boats may be launched through the county park. Eurasian watermilfoil has been found and treated.

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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

Friends of Cottage Lake

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