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Brazilian elodea has established a large population in the lake. Check boat trailers, fishing tackle, and boats carefully to ensure you don't transport Brazilian elodea to other lakes!

Vital Statistics

Watershed Area 580 acres
Lake surface area 22 acres
Maximum depth 31 feet
Mean depth 13 feet
Public Park Yes
Boat Ramp Yes
Fish Present rainbow trout, grass carp


Volunteers monitored Lake Fenwick in 1994 -1995, 2001, 2003-2004 and again in 2021.

The key takeaways from the 2021 monitoring season are:

• Lake Fenwick has fairly clear water with high nutrient concentrations and high algal growth. • In comparison to other lakes in the program, Lake Fenwick experienced some of the highest nutrient concentrations in 2021. • Nitrogen-to-phosphorus (N:P) ratios were below 25 for much of the monitoring season. This indicates the potential for algal blooms to be dominated by cyanobacteria (which have the ability to produce toxins). • Total phosphorus exceeded the lake’s TMDL (total maximum daily load) of 19 µg/L during each sampling event. • An algal bloom was sampled for toxin testing in September. No algal toxins were detected in this sample. • Lake Fenwick was last monitored in the Lake Stewardship Program from 2001 - 2004.

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

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For questions about lakes in King County, please contact or call the Water and Land Resources Division front desk at 206-477-4800.