L Thunder Lake
L Thunder Lake in Cass County, Minnesota is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 14 ft visibility.
- Clarity
- 14 ft
- Mean depth
- ~17 ft est.
- Trend
- ↓ Declining
- Public access
- Yes
- Ice-out
- Apr 18
Next lake in Cass County
Why L Thunder 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 L Thunder Lake, the only measurement available is water clarity, which is not a water-quality indicator on its own — EPA's National Lakes Assessment does not grade clarity. Sampling here has recorded water clarity, but not 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.
L Thunder Lake is a lake in Cass County, Minnesota with an estimated average depth of about 17 feet. Overall water quality 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 2024-09-02. Grade methodology: LakeGrade methodology.
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Swimming Safety
Not ratedLimited data
Not rated for swimming — there is not enough monitoring data to judge this lake
Water quality measurements for L Thunder Lake
These readings exist but do not meet the evidence bar for an A–F grade — see why.
Good clarity, visible to about 14 ft. Trophic State Index: 39.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 14 ft | Not graded |
| Phosphorus | No recent samples | |
| Chlorophyll-a (Algae) | No recent samples | |
| Trophic State Index (TSI) | 39 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Where to stay near L Thunder Lake
Compare lakeside cabins, hotels, and vacation rentals near L Thunder Lake in Cass 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 L Thunder Lake
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Lake Details
| Characteristic | Value |
|---|---|
| Average Depth | ~17 ft est. |
| Public Access | Yes |
Depth marked “est.” is a modeled estimate from HydroLAKES, not a measured survey. Minnesota has not published bathymetry for L Thunder 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
- Big Rice Lake (East)Public
- Little Thunder LakePublic
Fishing L Thunder Lake
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for L Thunder Lake. 1 report on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- —Public Water Access Sites — Little Thunder (West Bay)Public Water Access Sites · MN DNR LakeFinder
Water Quality Trend: ↓ Declining
Based on 5 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↓ Declining | -0.494 m/yr | 5 |
Ice Season
Historical ice data from Minnesota DNR (8 observations).
| Earliest | Latest | |
|---|---|---|
| Ice-Out | 2024-03-17 (2024) | 2018-05-01 (2018) |
| Ice-In | 2018-11-14 (2018) | 2017-11-20 (2017) |
Most recent ice-out: 2024-03-17
Graded lakes near L Thunder Lake
L Thunder Lake has no grade, but these lakes within a short drive do.
Nearby Lakes in Cass County
Frequently asked questions about L Thunder Lake
What is the water quality grade for L Thunder Lake?
- L Thunder Lake in Cass County, Minnesota is not rated for water quality — its monitoring record does not meet the evidence bar. Water clarity: 14 ft visibility.
Is L Thunder Lake safe to swim in?
- Not rated for swimming — there is not enough monitoring data to judge this lake. L Thunder Lake is not rated for water quality — its monitoring record does not meet the evidence bar. Always check local health department advisories before swimming.
How deep is L Thunder Lake?
- No published bathymetric survey covers L Thunder Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 17 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate.
When does L Thunder Lake thaw?
- L Thunder Lake typically has ice-out around Apr 18.
Are there boat ramps on L Thunder Lake?
- L Thunder Lake has 2 documented boat landings in the USGS National Boat Ramps inventory. Named access points include Big Rice Lake (East), Little Thunder Lake. Public access is listed as yes.
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Where this data comes from
29 measurements from 1 monitoring station, 2020–2024. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Minnesota Pollution Control Agency - Ambient Surface Water
MNPCA29 samples · 2020–2024
Analytical methods
- Field measurement/observation, generic method
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 size & watershed from HydroLAKES (HydroSHEDS)
Lake details from Minnesota DNR LakeFinder
Named boat-access points from the USGS National Boat Ramps inventory
Ice data from Minnesota DNR Climate