Thunder Lake
Cass County, Minnesota1,347 acresView mapDirections
Minnesota PCA publishes the water-quality measurements for Thunder Lake. Here is what has been measured.
- Oligotrophic
- zebra mussel
- 95 ft max depth
- Public access
Thunder Lake in Cass County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. 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.
Minnesota DNR's Targeted Survey on July 26, 2023 caught 0.64 walleye per hour of electrofishing at Thunder Lake. Length samples in the latest survey that recorded them ran 6–6 inches. DNR survey details →
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). 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. Methodology: how these readings are compared.
Thunder Lake covers about 1,300 acres in Cass County. It has one public boat landing. The median ice-out date is April 21, which is when the open-water season begins. Vrbo is where to look for cabins and lake homes around Thunder Lake, and Booking.com and Expedia search hotels near the lake for your dates.
What Thunder Lake is good for
From access and monitoring records, not editorial picks.
- Clear water16.5 ft of visibility
- Boat launch1 documented ramp
- Fishing17 species recorded
- Ice seasonIce-out around Apr 21
- Deep water95 ft surveyed max depth
- Big water1,347 acres
Swimming conditions
Readings onlyLow algae
Based on clarity and algae readings from past sampling — there is no live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
Whether Thunder Lake is safe to swim in today is a call for the Minnesota Pollution Control Agency and the Minnesota DNR and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Thunder Lake
These are the measurements Minnesota PCA publishes for Thunder Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Crystal clear, you can see 16.5 ft down. Phosphorus 10 µg/L. Chlorophyll-a 2.4 µg/L. TSI 38.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 16.5ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 10µg/L | Good GoodFairPoor |
| Chlorophyll-a (algae) | 2.4µg/L | Good GoodFairPoor |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 38 | Oligotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Low nutrients, clear water. No reading is a swimming clearance.
Ask about Thunder Lake
Answers come straight from the public measurements and survey records on this page.
Water Temperature
Thunder Lake typically sits around 68°F at the surface in September. 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.
Column height runs from this lake’s coldest month (51°F) to its warmest (68°F).
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 →
Minnesota DNR's Targeted Survey on July 26, 2023 caught 0.64 walleye per hour of electrofishing at Thunder Lake. Length samples in the latest survey that recorded them ran 6–6 inches.
A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Thunder Lake’s water-quality measurements separately, and the water-quality trend over 6 years of monitoring is improving. Catch rate is not a swim clearance and is not a weekly bite report.
DNR Fisheries Survey Summary
21 surveys on file from MN DNR Fisheries. Most recent: 2025-09-10 (Targeted Survey).
21 surveys on file · most recent 2025-09-10 (Targeted Survey)
| Species | Catch rate (CPUE) | Size | Avg weight |
|---|---|---|---|
| Yellow Perch | 23.31 | 5–11 in 2023-06-12 | 0.15 lb |
| Walleye | 22.05 | 6 in 2025-09-10 | 1.03 lb |
| Bluegill | 20.66 | 3–8 in 2023-06-12 | 0.14 lb |
| Largemouth Bass | 17.68 | 8–18 in 2023-06-12 | 1.01 lb |
| Bluntnose Minnow | 10.04 |
CPUE = catch per unit effort, fish caught per net per set in a standardised survey. It measures abundance, not size, and is not a weekly bite report.
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
Lake Details
Max depth
95 ft
Average depth
37 ft
Surface area
1,347 acres
Shoreline
15.9 mi
Littoral zone
17%
Share shallower than 15 ft
Public access
Yes
Listed by Minnesota PCA
Recreation & Access
- Big Springs ResortSemi-private
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Stays via Stay22
Stays 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.
Top-rated places to stay near Thunder Lake
Prices for Oct 9–11, 4 adults. Change dates on the map below.
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 →
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Fish Species
Click a species to see every 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.
Water Quality Trend: Improving
Based on 6 years of monitoring (2020–2025).
Water clarity
Improving6 years
Phosphorus
Stable-0.16 µg/L/yr6 years
Chlorophyll-a
Improving-0.26 µg/L/yr6 years
Ice season
Typical ice-out
Apr 21
Earliest Apr 13, 2015 · Latest May 14, 2013
Typical ice-in
Dec 8
Earliest Nov 29, 2002 · Latest Dec 14, 2004
Most recent ice-out
May 7
2022
13 observations · Minnesota PCA ice records
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
Nearby Lakes in Cass County
Frequently asked questions about Thunder Lake
Where is Thunder Lake?
- Thunder Lake is a lake in Cass County, Minnesota. Its water-quality measurements are published by Minnesota PCA and shown on this page as reported.
What is the water quality of Thunder Lake?
- Thunder Lake in Cass County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. 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?
- Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
What is the water temperature of Thunder Lake?
- Thunder Lake typically sits around 68°F at the surface in September, 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.
What did the DNR fisheries survey find at Thunder Lake?
- Minnesota DNR's Targeted Survey on July 26, 2023 caught 0.64 walleye per hour of electrofishing at Thunder Lake. Length samples in the latest survey that recorded them ran 6–6 inches. A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Thunder Lake’s water-quality measurements separately, and the water-quality trend over 6 years of monitoring is improving. Catch rate is not a swim clearance and is not a weekly bite report.
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 Springs Resort. Public access is listed as yes.
Can I fish at Thunder Lake?
- Thunder Lake has 1 documented boat landing in the USGS National Boat Ramps inventory. Seasons, size and bag limits, and whether you need a license are set by Minnesota's fish and wildlife agency, so check its current regulations before you go.
Explore Thunder Lake across LakeQuality
Depth, room, public access, and the state wake-sport rules
Fish species and the cleanest lakes across Minnesota
Fish species and best lakes in Cass County
Species activity outlook by water temperature and season
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Where to stay at Minnesota's top lakes, each with its readings
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Lakes in the Pine River Watersheds (Brainerd Lakes), ranked by water quality
Minnesota's cleanest lakes, ranked on agency measurements
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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)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from and data sources.
Other records on this page
Lake details from Minnesota DNR LakeFinder
Named boat-access points from the USGS National Boat Ramps inventory
Ice data from Minnesota DNR Climate