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Vital Statistics

Watershed Area 593 acres
Lake surface area 38 acres
Maximum depth 32 feet
Mean depth 18 feet
Public Park Y
Boat Ramp Car top boats
Fish Present bass, stocked rainbow


Volunteers monitored Fivemile Lake in the 1980s, from 1994 - 2008, and resumed in 2014. The lake is highly colored, but poorly buffered against pH change. It is moderately high in primary productivity (high mesotrophic) with fairly good water quality, which was steady over time. Phosphorus appears stable, but there may be a decreasing trend in total nitrogen. Thermal stratification is stable in summer, and sedimentary phosphorus release adds a small amount to deep water concentrations. Nitrogen to phosphorus ratios are nearly always above 25:1, which generally does not favor cyanobacteria. Fivemile Lake has no boat ramp, but car top boats may be launched from the county park on the eastern shoreline. Residents should keep a watch on aquatic plants growing near shore to catch early infestations of noxious aquatic weeds.

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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

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