Cass Lake
Beltrami County, Minnesota15,958 acresView mapDirections
Clarity, phosphorus, and algae all sit in the cleanest band for this region.
- Oligotrophic
- zebra mussel
- #59 of 85 in the county
- 120 ft max depth
- Public access
- Limited data
Cass Lake in Beltrami County, Minnesota has a water quality grade of A (Excellent). Water clarity: 18.5 ft visibility. Phosphorus: 11 µg/L. Chlorophyll-a (algae): 3.2 µg/L. Maximum depth: 120 ft. Surface area: 15958.25 acres.
- Clarity
- 18.5 ft
- Typical in September
- 55°F
- Max depth
- 120 ft
- Area
- 15,958 acres
- Type
- Oligotrophic
- Public access
- Yes
- Ice-out
- Apr 24
- Open water
- 224 days
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Free interactive depth-contour map of Cass Lake, reaching a maximum surveyed depth of 120 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Minnesota DNR.
Cass Lake is a 15,958-acre cold, clear oligotrophic lake in Beltrami County, Minnesota with a maximum depth of 120 feet. Water quality is graded A based on 3 sampling years through 2025. The lake supports 21 documented fish species including black bullhead, black crappie, and bluegill. Public access is available. Compared to the 84 other monitored lakes in Beltrami County, Cass Lake ranks #59 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2025-10-15. Grade methodology: LakeQuality methodology.
What Cass Lake is good for
From access and monitoring records, not editorial picks.
- Clear water18.5 ft of visibility
- Fishing21 species recorded
- Ice seasonIce-out around Apr 24
- Deep water120 ft surveyed max depth
- Big water15,958 acres
Swimming Safety
Grade onlyLimited data
Based on clarity and algae grades from past sampling — we do not hold live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
The A is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
Water Quality Grade: A (Excellent)
Crystal clear, you can see 18.5 ft down. Phosphorus 11 µg/L. Chlorophyll-a 3.2 µg/L. TSI 39. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 18.5 ft | Not graded |
| Phosphorus | 11 µg/L | Good |
| Chlorophyll-a (Algae) | 3.2 µg/L | Good |
| Trophic State Index (TSI) | 39 | Oligotrophic |
Low nutrients, clear water. The letter is not a swimming clearance.
Water Temperature
Cass Lake typically sits around 55°F at the surface in September. Summer average is about 57°F. Usually warmest in Jul (about 60°F) and coldest in May (about 48°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 293 field readings (2024–2025) via the EPA Water Quality Portal. See the full Cass Lake water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 120 ft |
| Surface Area | 15,958 acres |
| Shoreline Length | 45.4 mi |
| Littoral Zone | 20% |
| Public Access | Yes |
Fish Species
Click a species to see every graded lake where it is documented.
Fishing Cass 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.
Ice Season
Historical ice data from Minnesota DNR (16 observations).
Estimated open water season: 224 days
| Earliest | Latest | |
|---|---|---|
| Ice-Out | 2021-04-06 (2021) | 2008-05-12 (2008) |
| Ice-In | 2008-11-24 (2008) | 2024-01-07 (2024) |
Most recent ice-out: 2025-04-27
County Ranking
Ranked #59 of 85 lakes in Beltrami County
Nearby Lakes in Beltrami County
Frequently asked questions about Cass Lake
Where is Cass Lake?
- Cass Lake is a lake in Beltrami County, Minnesota. LakeQuality grades it A for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Cass Lake?
- Cass Lake in Beltrami County, Minnesota has a water quality grade of A (Excellent). Water clarity: 18.5 ft visibility. Phosphorus: 11 µg/L. Chlorophyll-a (algae): 3.2 µg/L. Maximum depth: 120 ft. Surface area: 15958.25 acres.
Is Cass Lake safe to swim in?
- The A is nutrients and algae, not a swimming clearance. Check local advisories before you swim.
What is the water temperature of Cass Lake?
- Cass Lake typically sits around 55°F at the surface in September, from 293 EPA Water Quality Portal field readings (2024–2025). That is a typical value for the month, not a live reading.
How deep is Cass Lake?
- Cass Lake has a maximum depth of 120 feet. The lake covers 15958.25 acres.
What fish are in Cass Lake?
- Fish species found in Cass Lake include black bullhead, black crappie, bluegill, bowfin (dogfish), brown bullhead, burbot, greater redhorse, hybrid sunfish, lake whitefish, largemouth bass, muskellunge, northern pike, pumpkinseed, redhorse, rock bass, shorthead redhorse, tullibee (cisco), walleye, white sucker, yellow bullhead, yellow perch. Cass Lake is located in Beltrami County, Minnesota.
When does Cass Lake thaw?
- Cass Lake typically has ice-out around Apr 24. Ice typically forms around Dec 4. The open water season is approximately 224 days.
Explore Cass Lake across LakeQuality
Depth, room, public access, and the state wake-sport rules
Fish species and top-graded lakes across Minnesota
Fish species and best lakes in Beltrami County
Species activity outlook by water temperature and season
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Low nutrients, clear water. The letter is not a swimming clearance.
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Where this data comes from
23 measurements from 1 monitoring station, 2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Leech Lake Band of Ojibwe
LEECHLAK_WQX23 samples · 2025
Analytical methods
- Phosphorus by Two Reagent Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
- Nitrate-Nitrite Nitrogen by Colorimetry
Monitoring station identifiers (1)
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
Lake details from Minnesota DNR LakeFinder
Ice data from Minnesota DNR Climate