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Vital Statistics

Watershed Area 166 acres
Lake surface area 19 acres
Maximum depth 31 feet
Mean depth 18 feet
Public Park No
Boat Ramp Yes, no gas engines
Fish Present bass, stocked rainbow


Volunteers monitored Neilson (Holm) from 1997-2008 and resumed in 2014. This lake is lightly colored, poorly buffered against pH change, and moderate in primary productivity (mesotrophic) with good water quality. While both productivity and phosphorus concentrations appear stable, total nitrogen may be increasing over time. Thermal stratification is stable in summer, and sedimentary phosphorus release adds to deep water concentrations, while ammonia increases as well, suggesting lack of oxygen. Nitrogen to phosphorus ratios in the upper water are generally above 25:1, which does not favor cyanobacteria above other algae. No toxic events have been reported to-date.. Neilson Lake has a public access boat launch, and pioneering infestations of Eurasian milfoil were observed in the summer of 2001 (King County 2002). Residents should watch the near shore environment for further infestations of milfoil or other 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

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