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See weekly bacterial counts and temperatures on the King County swim beaches website.

Vital Statistics

Watershed Area 328 acres
Lake surface area 67 acres
Maximum depth 38 feet
Mean depth 21 feet
Public Park Yes
Boat Ramp Yes, no gas engines
Fish Present bass, stocked rainbow


Volunteers monitored Lake Wilderness in Maple Valley from the 1980s-2014. The lake water is very clear, moderately buffered against pH change, and low to moderate in primary productivity (low mesotrophic) with good water quality. Productivity has been stable since 1998, but phosphorus has been decreasing over time. Concurrently, total nitrogen has been increasing. Thermal stratification is stable in summer, and sediments release phosphorus to deep water, but without ammonia build-up. Nitrogen to phosphorus ratios rarely drop below 25:1, which should not favor cyanobacteria. However, the lake was monitored for cyanotoxins in 2009-2011 and several toxic episodes were recorded. Scums accumulate downwind and can be a health hazard at access points. Lake users should take care to avoid all scums on the water surface. Lake Wilderness has a popular park with a beach and a boat ramp. There is a history of Eurasian milfoil infestation with control activities.

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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

City of Maple Valley
Lake Wilderness Arboretum

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