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Vital Statistics

Watershed Area 324 acres
Lake surface area 12 acres
Maximum depth 55 feet
Mean depth 22 feet
Public Park Y
Boat Ramp No; no gas engines
Fish Present


Volunteers monitored the north basin of Beaver Lake in Sammamish from 1997-2014. The lake water is highly colored, poorly buffered against pH change, and high in primary productivity (eutrophic), with fair water quality. Sensitivity of the lake to development is discussed in the Beaver Lake Management Plan (see links). While overall productivity is not increasing over time, total phosphorus has been increasing slowly since 2002. Thermal stratification is stable in summer, and sediments release phosphorus to the deep water, while ammonia build-up indicates low oxygen. Nitrogen to phosphorus ratios are often near or below 25:1, which is favorable for cyanobacteria. Anabaena planctonica made a large population in late summer 2011, which was not toxic. No toxic events have been reported to-date. Beaver-1 can be accessed by a channel connecting to Beaver-2. Residents should track aquatic plants to catch early infestations of noxious 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

Beaver Lake Management Plans

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