Thunder Lake
Thunder Lake in Cass County, Minnesota has a water quality grade of A (Excellent). Water clarity: 16.5 ft visibility. Phosphorus: 10 µg/L. Chlorophyll-a (algae): 2.4 µg/L. Maximum depth: 95 ft. Surface area: 1346.87 acres.
- Clarity
- 16.5 ft
- Typical in August
- 63°F
- Max depth
- 95 ft
- Area
- 1,347 acres
- Type
- Oligotrophic
- Trend
- ↑ Improving
- Public access
- Yes
- Ice-out
- Apr 21
- Open water
- 231 days
Next lake in Cass County
Free interactive depth-contour map of Thunder Lake, reaching a maximum surveyed depth of 80 ft. Toggle Depth contours to overlay the bathymetric survey lines. Contour data from Minnesota DNR.
Thunder Lake is a 1,347-acre cold, clear oligotrophic lake in Cass County, Minnesota with a maximum depth of 95 feet (37-foot average). Water quality is graded A based on 2 sampling years through 2025. Overall water quality is improving over recent sampling years. The lake supports 17 documented fish species including black crappie, bluegill, and bowfin (dogfish). Public access is available with 1 public boat landing. Compared to the 103 other monitored lakes in Cass County, Thunder Lake ranks #37 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2025-09-29. Grade methodology: LakeGrade methodology.
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Swimming Safety
Grade onlyLow algae
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.
Excellent for swimming, crystal clear water with minimal algae
Water Quality Grade: A (Excellent)
Crystal clear, you can see 16.5 ft down. Phosphorus level: 10 µg/L. Chlorophyll-a: 2.4 µg/L. Trophic State Index: 38.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 16.5 ft | Not graded |
| Phosphorus | 10 µg/L | Good |
| Chlorophyll-a (Algae) | 2.4 µg/L | Good |
| Trophic State Index (TSI) | 38 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Where to stay near Thunder Lake
Compare lakeside cabins, hotels, and vacation rentals near 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.
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Water Temperature
Thunder Lake typically sits around 63°F at the surface in August. Summer average is about 56°F. Usually warmest in Sep (about 68°F) and coldest in Jun (about 51°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 58 field readings (2023–2025) via the EPA Water Quality Portal. See the full Thunder Lake water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 95 ft |
| Average Depth | 37 ft |
| Surface Area | 1,347 acres |
| Shoreline Length | 15.9 mi |
| Littoral Zone | 17% |
| Public Access | Yes |
Recreation & Access
- Big Thunder LakePublic
Fish Species
Click a species to see every graded lake where it is documented.
Fishing Thunder 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 Fisheries Survey Summary
21 surveys on file from MN DNR Fisheries. Most recent: 2025-09-10 (Targeted Survey).
Top Species by Catch Rate
| Species | Avg CPUE | Avg Weight |
|---|---|---|
| Yellow Perch | 23.31 | 0.15 lb |
| Walleye | 22.05 | 1.03 lb |
| Bluegill | 20.66 | 0.14 lb |
| Largemouth Bass | 17.68 | 1.01 lb |
| Bluntnose Minnow | 10.04 | — |
| MMS | 8.65 | — |
CPUE = catch per unit effort, averaged across surveys (excludes juvenile shoreline seining). Higher CPUE = more abundant in standardized sampling.
Length Distributions
Number of fish caught at each inch class in the most recent survey that recorded lengths. Red dashed line marks an approximate trophy threshold for that species.
Yellow Perch
Walleye
Bluegill
Largemouth Bass
From the 2025-09-10 survey
Walleye fingerlings are currently stocked into Thunder Lake in odd numbered years at the rate of two pounds per littoral acre (452 pounds). Night electrofishing was conducted on Thunder Lake on September 10th, 2025, to monitor the abundance of young-of-the-year (YOY) Walleyes and evaluate the contribution of natural…
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for Thunder Lake. 4 reports on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- —Fisheries Lake Survey — ThunderFisheries Lake Survey · MN DNR LakeFinder
- —Aquatic Plant Report — Thunder (1488)Aquatic Plant Report · MN DNR LakeFinder
- —Fish Stocking History — ThunderFish Stocking History · MN DNR LakeFinder
- —Public Water Access Sites — ThunderPublic Water Access Sites · MN DNR LakeFinder
Water Quality Trend: ↑ Improving
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.338 m/yr | 6 |
| Phosphorus | → Stable | -0.16 µg/L/yr | 6 |
| Chlorophyll-a | ↑ Improving | -0.26 µg/L/yr | 6 |
Ice Season
Historical ice data from Minnesota DNR (13 observations).
Estimated open water season: 231 days
| Earliest | Latest | |
|---|---|---|
| Ice-Out | 2015-04-13 (2015) | 2013-05-14 (2013) |
| Ice-In | 2002-11-29 (2002) | 2004-12-14 (2004) |
Most recent ice-out: 2022-05-07
Reservoir Info (USACE NID)
Thunder Lake is a man-made reservoir impounded by the Thunder Lake (completed 1934), built primarily for recreation on the Willow River-TR; earth-type dam, 18 ft tall.
- Surface area
- 1,347 ac
- Normal storage
- 7,750 ac-ft
- Max storage
- 8,640 ac-ft
- Drainage area
- 16.8 sq mi
- Hazard class
- Low
- Owner
- MNDNR-Fisheries
Source: USACE National Inventory of Dams, NID ID MN01091 · Operator website
EPA Impairment Status
Thunder Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 4A).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit MN11-0062-00 · Official waterbody report
County Ranking
Ranked #37 of 104 lakes in Cass County
Nearby Lakes in Cass County
Frequently asked questions about Thunder Lake
What is the water quality grade for Thunder Lake?
- Thunder Lake in Cass County, Minnesota has a water quality grade of A (Excellent). Water clarity: 16.5 ft visibility. Phosphorus: 10 µg/L. Chlorophyll-a (algae): 2.4 µg/L. Maximum depth: 95 ft. Surface area: 1346.87 acres.
Is Thunder Lake safe to swim in?
- Excellent for swimming, crystal clear water with minimal algae. Thunder Lake has a water quality grade of A (Excellent). Algae concentration (chlorophyll-a) is 2.4 µg/L. Always check local health department advisories before swimming.
What is the water temperature of Thunder Lake?
- Thunder Lake typically sits around 63°F at the surface in August, from 58 EPA Water Quality Portal field readings (2023–2025). That is a typical value for the month, not a live reading.
How deep is Thunder Lake?
- Thunder Lake has a maximum depth of 95 feet and an average depth of 37 feet. The lake covers 1346.87 acres.
What fish are in Thunder Lake?
- Fish species found in Thunder Lake include black crappie, bluegill, bowfin (dogfish), brown bullhead, burbot, hybrid sunfish, largemouth bass, northern pike, pumpkinseed, rock bass, shorthead redhorse, smallmouth bass, tullibee (cisco), walleye, white sucker, yellow bullhead, yellow perch. Thunder Lake is located in Cass County, Minnesota.
When does Thunder Lake thaw?
- Thunder Lake typically has ice-out around Apr 21. Ice typically forms around Dec 8. The open water season is approximately 231 days.
Are there boat ramps on Thunder Lake?
- Thunder Lake has 1 documented boat landing in the USGS National Boat Ramps inventory. Named access points include Big Thunder Lake. Public access is listed as yes.
Explore Thunder Lake across LakeGrade
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Where this data comes from
218 measurements from 3 monitoring stations, 2020–2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Minnesota Pollution Control Agency - Ambient Surface Water
MNPCA218 samples · 2020–2025
Analytical methods
- Field measurement/observation, generic method
- Phosphorus by Two Reagent Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
Monitoring station identifiers (3)
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
Impairment status from EPA ATTAINS 303(d) database
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