Superior Lake
Superior Lake in Cook County, Minnesota is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 15.1 ft visibility. Phosphorus: 4 µg/L. Chlorophyll-a (algae): 1.2 µg/L.
- Clarity
- 15.1 ft
- Typical in August
- 56°F
- Trend
- → Stable
- Public access
- Yes
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Why Superior Lake has no grade
We grade a lake only when the monitoring record supports a letter: at least three phosphorus or chlorophyll-a samples in the last ten years, scored against EPA National Lakes Assessment thresholds for this lake’s ecoregion. For Superior Lake, no EPA ecoregion for this lake, so the National Lakes Assessment condition thresholds cannot be applied. Sampling here has recorded water clarity, phosphorus and algae (chlorophyll-a).
Not Rated is not an F. It is EPA Integrated Reporting Category 3 — insufficient information. The water here may be excellent or poor; nobody has measured enough of it to say. Everything below that has been measured is still reported on this page.
Sampling is run by MPCA and volunteer programs, and published through the EPA Water Quality Portal. Read the full grading methodology.
Superior Lake is a lake in Cook County, Minnesota. Phosphorus level is declining over recent sampling years. Limited fish-species data available. Public access is available with 2 public boat landings.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2024-09-17. Grade methodology: LakeQuality methodology.
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Swimming Safety
Not ratedBacteria B
Based on E. coli testing, Superior Lake rarely exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 2.3% of 129 samples across 5 seasons.
This is a historical summary of past testing, not a current condition — bacteria levels change quickly, especially after rain. Always check Minnesota’s current beach and health-department advisories before swimming.
Water quality measurements for Superior Lake
These readings exist but are not enough for an A–F grade — see why.
Crystal clear, you can see 15.1 ft down. Phosphorus level: 4 µg/L. Chlorophyll-a: 1.2 µg/L. Trophic State Index: 32.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 15.1 ft | Not graded |
| Phosphorus | 4 µg/L | Not graded |
| Chlorophyll-a (Algae) | 1.2 µg/L | Not graded |
| Trophic State Index (TSI) | 32 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Where to stay near Superior Lake
Compare lakeside cabins, hotels, and vacation rentals near Superior Lake in Cook County, Minnesota — live availability and prices for your trip. Switch the map to Satellite to see the lake and how close each stay is to the water.
Top-rated places to stay near Superior Lake
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Planning a trip? Cabins and lodging at Superior Lake → · more Minnesota lake trips
Water Temperature
Superior Lake typically sits around 56°F at the surface in August. Summer average is about 49°F. Usually warmest in Sep (about 57°F) and coldest in May (about 40°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 1,298 field readings (2016–2024) via the EPA Water Quality Portal. See the full Superior Lake water temperature breakdown →
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for Superior Lake. 2 reports on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- —Fish Stocking History — SuperiorFish Stocking History · MN DNR LakeFinder
- —Public Water Access Sites — SuperiorPublic Water Access Sites · MN DNR LakeFinder
Recreation & Access
- Grand Marais Safe HarborPublic
- Grand Marais WestPublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Fishing Superior Lake
Invasive & Introduced Species
Red chips are ecologically harmful invasive species. Gray chips are stocked or introduced fish that aren't a current ecological concern.
Water Quality Trend: → Stable
Based on 2 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | -0.03 m/yr | 2 |
| Phosphorus | ↓ Declining | +0.5 µg/L/yr | 2 |
| Chlorophyll-a | ↑ Improving | -0.5 µg/L/yr | 2 |
Regulated Pollution Sources Nearby
1 EPA-permitted facility operate within about 5 miles of Superior Lake under Clean Water Act (NPDES) discharge permits. 1 facility has a reported Clean Water Act violation in the past three years.
- Grand Marais WwtpGrand Marais · 0.6 mi away · NPDES MN0020010Recent violation
Facilities permitted to discharge under the Clean Water Act within ~5 miles; proximity does not by itself mean a facility affects this lake. Source: EPA ECHO (ICIS-NPDES), as of 2026-07.
Graded lakes near Superior Lake
Superior Lake has no grade, but these lakes within a short drive do.
Nearby Lakes in Cook County
Frequently asked questions about Superior Lake
Where is Superior Lake?
- Superior Lake is a lake in Cook County, Minnesota. LakeQuality has not rated this lake (UNRATED) — no EPA ecoregion for this lake, so the National Lakes Assessment condition thresholds cannot be applied.
What is the water quality grade for Superior Lake?
- Superior Lake in Cook County, Minnesota is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 15.1 ft visibility. Phosphorus: 4 µg/L. Chlorophyll-a (algae): 1.2 µg/L.
Is Superior Lake safe to swim in?
- Based on E. coli testing, Superior Lake rarely exceeded the EPA safe-swimming threshold (235 MPN/100mL) — on 2.3% of 129 samples across 5 seasons. That works out to a bacteria swim-safety grade of B — a historical summary of past testing, not a current reading. Superior Lake is not rated for water quality — its monitoring record does not meet the evidence bar. Bacteria levels can spike quickly after heavy rain, so always check Minnesota's current beach and health-department advisories before you swim.
What is the water temperature of Superior Lake?
- Superior Lake typically sits around 56°F at the surface in August, from 1,298 EPA Water Quality Portal field readings (2016–2024). That is a typical value for the month, not a live reading.
Are there boat ramps on Superior Lake?
- Superior Lake has 2 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Grand Marais Safe Harbor, Grand Marais West. Public access is listed as yes.
Explore Superior Lake across LakeQuality
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Where this data comes from
124 measurements from 6 monitoring stations, 2023–2024. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Minnesota Pollution Control Agency - Ambient Surface Water
MNPCA124 samples · 2023–2024
Analytical methods
- Field measurement/observation, generic method
- Phosphorus by Two Reagent Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
Monitoring station identifiers (6)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated and data sources.
Other records on this page
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Named boat-access points from the USGS National Boat Ramps inventory