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B

School Lake

Chisago County, MinnesotaMesotrophic

School Lake comes in at a B on the grading rubric — a respectable showing for a Minnesota lake of its size and depth. Secchi readings are the weakest of the scored parameters, suggesting suspended sediment or algal biomass dominates the optical signal.

The lake sits in the mesotrophic zone — the band where most well-managed Minnesota lakes fall. The lake's maximum depth is not yet documented in state morphometric records — context for its physical structure remains limited. The lake's surface area is not consistently recorded across state datasets — physical context remains partial. Among the 31 graded lakes in Chisago County, School Lake ranks 6 — in the top quartile locally.

No invasive species are currently listed at School Lake — the lake remains off the Minnesota infested-waters roster. The state fisheries records do not list documented species for School Lake, which usually reflects a lack of formal fisheries survey work rather than an empty lake. No formal public access is documented at School Lake — most use is by shoreline residents and their guests. The grade is supported by multiple sampling years from Minnesota PCA volunteers and partner agencies, giving the letter a reasonably stable foundation.

Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2024-09-26. Grade methodology: LakeGrade methodology.

Reviewed by LakeQuality Editorial Team · Updated

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Swimming Safety

Good for swimming, clear water with low algae levels

Water Quality Grade: B, Good

Moderate clarity, visible to about 7.9 ft. Phosphorus level: 20 µg/L. Chlorophyll-a: 1.6 µg/L. Trophic State Index: 43.

MetricValueGrade
Water Clarity (Secchi Depth)7.9 ftC
Phosphorus20 µg/LB
Chlorophyll-a (Algae)1.6 µg/LA
Trophic State Index (TSI)43Mesotrophic

Moderate nutrients, good water quality

Water Quality Trend: Improving

Based on 5 years of monitoring data (2020-2025).

MetricTrendChange/YearYears
Water Clarity Improving+0.093 m/yr5
Phosphorus Improving-12.55 µg/L/yr5
Chlorophyll-a Declining+7.45 µg/L/yr3
See year-by-year chart →

Location

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County Ranking

Ranked #6 of 31 lakes in Chisago County

Cleaner Lakes Within 30 Miles

School Lake holds Grade B. One nearby lake has a higher grade.

See full comparison →

Nearby Lakes in Chisago County

EPA Impairment Status

School Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 4A).

Causes of impairment

Nutrients (phosphorus, nitrogen)

A Total Maximum Daily Load (TMDL) plan has been approved.

Source: EPA ATTAINS assessment unit MN13-0029-00 · Official waterbody report · Matched by proximity (1.48 km)

Data Sources

Water quality data from the EPA Water Quality Portal

Impairment status from EPA ATTAINS 303(d) database

Grading methodology based on Metropolitan Council standards

Most recent sample: 2024-09-26

Monitoring stations: 1