Forbes Lake

Vital Statistics
Overview
Maps
Water Quality Data
Reports and Related Links

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Vital Statistics

Watershed Area	471 acres
Lake surface area	7 acres
Maximum depth	30 feet
Mean depth	feet
Public Park	Yes
Boat Ramp	No
Fish Present

Overview

The key takeaways from the 2020 monitoring season are:

• Forbes Lake continued to have high nutrient concentrations, with fairly clear water and algal growth. • While phosphorus concentrations have been increasing since 2018, long-term trends suggest that water quality in Forbes Lake has been improving over time, with decreasing nitrogen and deeper Secchi depths. • No algal blooms were reported for toxin testing in 2020.

Please see below for a link to the full 2020 Forbes Lake Monitoring Report.

Maps

Bathymetry map unavailable

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

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).

Reports and Related Links

2020 Forbes Lake Monitoring Report
2019 Forbes Lake Monitoring Report
2018 Forbes Lake Monitoring Report
2017 Forbes Lake Monitoring Report

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