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Lake Grades

Letter Grades Represent lake health

Beginning in 2023, RPBCWD added a letter grade ("A" to "F") to lake fact sheets as a simple way to convey overall lake health to the public. The letter grade is based upon lake monitoring standards set by the Metropolitan Council (see grading system table below). The letter grade is based upon yearly averages for Total Phosphorus, Chlorophyll-a (pigment in algae), and water clarity measured by Secchi disk.

The Metropolitan Council assigns letter grades to lake throughout the Metropolitan Areas, including within the District. However, the Metropolitan Council's letter grade for a lake may differ from that assigned by the District. This is because the dataset used by the Metropolitan Council is different than the dataset used by RPBCWD. The Metropolitan Council uses data collected through its local partners and Citizen-Assisted Monitoring Program (CAMP) of citizen scientists, while the District uses data collected by RPBCWD staff during the growing season (June 1 to September 30).

To see lake grades assigned by RPBCWD, check out the lake fact sheets.

Calculating an Overall Letter Grade

A lake's yearly average for Total Phosphorus, Chlorophyll-a, and water clarity is assigned a letter grade based upon the table below. If a lake received a "B" for all three parameters, then the lake's overall letter grade is a "B." If a lake receives at least one different letter grade among the three parameters, then staff average the three letter grades to calculate an overall letter grade for a lake.

Water Quality Grading System

The table below was developed by the Metropolitan Council as a simple way to represent overall lake health using letter grades. 

Grade Total Phosphorus (micrograms/liter) Chloroyphyll-a (micrograms/liter) Secchi/water clarity (meters)
A <23 <10 >3.0
B 23-32 10-20 2.2 - 3.0
C 32-68 20-48 1.2 - 2.2
D 68-152 48-77 0.7 - 1.2
F >152 >77 >0.7