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Little Portage Lake

Cass County, MinnesotaOligotrophic

Little Portage Lake earns an A — every scored parameter (clarity and phosphorus) rates among the best monitored waters in Cass County. The two scored sub-grades — clarity and phosphorus — track close together, so neither one is dragging the average down.

The lake's low TSI puts it in oligotrophic territory — the cleanest of the four trophic classes, but also the most vulnerable to nutrient-driven shifts. 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. Little Portage Lake ranks 19 of 132 in Cass County, putting it in the upper tier of locally monitored lakes.

No invasive species are currently listed at Little Portage Lake — the lake remains off the Minnesota infested-waters roster. The state fisheries records do not list documented species for Little Portage Lake, which usually reflects a lack of formal fisheries survey work rather than an empty lake. No formal public access is documented at Little Portage Lake — most use is by shoreline residents and their guests. The lake has a partial ice record — 5 observed ice-outs, centered near Apr 12. The grade is based on limited monitoring — fewer than three independent measurement years contribute, so future updates may shift the letter.

Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2024-09-16. Grade methodology: LakeGrade methodology.

Reviewed by LakeQuality Editorial Team · Updated

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Swimming Safety

Excellent for swimming, crystal clear water with minimal algae

Water Quality Grade: A, Excellent

Crystal clear, you can see 16.4 ft down. Phosphorus level: 11 µg/L. Trophic State Index: 38.

MetricValueGrade
Water Clarity (Secchi Depth)16.4 ftA
Phosphorus11 µg/LA
Chlorophyll-a (Algae)No data
Trophic State Index (TSI)38Oligotrophic

Low nutrients, clear water, excellent for swimming

Water Quality Trend: Improving

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

MetricTrendChange/YearYears
Water Clarity Improving+0.102 m/yr5
Phosphorus Improving-1.5 µg/L/yr5
See year-by-year chart →

Ice Season

Historical ice data from Minnesota DNR (5 observations).

Typical Ice-Out
Apr 12
EarliestLatest
Ice-Out2006-04-11 (2006)2022-05-06 (2022)

Most recent ice-out: 2022-05-06

Location

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County Ranking

Ranked #19 of 132 lakes in Cass County

Nearby Lakes in Cass County

DNR Reports & Resources

Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for Little Portage Lake. 1 report on file.

EPA Impairment Status

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

Causes of impairment

Mercury

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

Source: EPA ATTAINS assessment unit MN11-0308-02 · Official waterbody report · Matched by proximity (2.44 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

Ice data from Minnesota DNR Climate

Most recent sample: 2024-09-16

Monitoring stations: 1