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
- Mean depth
- ~481 ft est.
- Trend
- → Stable
- Public access
- Yes
Next lake in Cook County
Why Superior Lake has no grade
We grade a lake only when its monitoring record clears the evidence bar: 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 with an estimated average depth of about 481 feet. 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: LakeGrade methodology.
Watch Superior Lake water quality
Subscribe for LakeQuality updates by email. No spam, unsubscribe anytime.
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 do not meet the evidence bar 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
Availability and prices from Stay22 partners (Booking.com, Vrbo, Expedia). We may earn a commission from bookings, at no extra cost to you.
Planning a whole trip? See where to stay at Minnesota’s top lakes →
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 →
Lake Details
| Characteristic | Value |
|---|---|
| Average Depth | ~481 ft est. |
| Public Access | Yes |
Depth marked “est.” is a modeled estimate from HydroLAKES, not a measured survey. Minnesota has not published bathymetry for Superior Lake.
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth here is modelled, not surveyed — HydroLAKES v1.0 estimates lake depth from surrounding terrain rather than from soundings. Treat it as an approximation, and prefer a state bathymetric survey where one exists.
Recreation & Access
- Grand Marais Safe HarborPublic
- Grand Marais WestPublic
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.
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
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
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.
How deep is Superior Lake?
- No published bathymetric survey covers Superior Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 481 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate.
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 LakeGrade
Fish species and top-graded lakes across Minnesota
Fish species and best lakes in Cook County
Species activity outlook by water temperature and season
Where to stay at Minnesota's top lakes, each with its grade
Lakes in the Lake Superior Basin (MN+WI), ranked by water quality
Minnesota's cleanest and most-polluted lakes, ranked
More Minnesota Lake Rankings
State Parks Near Superior Lake
Other Lakes in Minnesota
Oligotrophic Lakes in Minnesota
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)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Lake details from Minnesota DNR LakeFinder
Named boat-access points from the USGS National Boat Ramps inventory