Bertha Lake
Bertha Lake in Crow Wing County, Minnesota has a water quality grade of A (Excellent). Water clarity: 17.1 ft visibility. Phosphorus: 13 µg/L. Chlorophyll-a (algae): 2.4 µg/L. Maximum depth: 64 ft. Surface area: 336.87 acres.
- Clarity
- 17.1 ft
- Typical in August
- 64°F
- Max depth
- 64 ft
- Area
- 337 acres
- Type
- Oligotrophic
- Trend
- ↓ Declining
- Public access
- No
- Ice-out
- Apr 18
Next lake in Crow Wing County
Bertha Lake is a 337-acre cold, clear oligotrophic lake in Crow Wing County, Minnesota with a maximum depth of 64 feet. Water quality is graded A based on 2 sampling years through 2025. Overall water quality is declining over recent sampling years. The lake supports 22 documented fish species including black bullhead, black crappie, and bluegill. No public access — the lake is private or restricted. Compared to the 84 other monitored lakes in Crow Wing County, Bertha Lake ranks #53 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2025-09-14. 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 17.1 ft down. Phosphorus level: 13 µg/L. Chlorophyll-a: 2.4 µg/L. Trophic State Index: 39.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 17.1 ft | Not graded |
| Phosphorus | 13 µg/L | Good |
| Chlorophyll-a (Algae) | 2.4 µg/L | Good |
| Trophic State Index (TSI) | 39 | Oligotrophic |
Low nutrients, clear water, excellent for swimming
Where to stay near Bertha Lake
Compare lakeside cabins, hotels, and vacation rentals near Bertha Lake in Crow Wing 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
Bertha Lake typically sits around 64°F at the surface in August. Summer average is about 63°F. Usually warmest in Aug (about 64°F) and coldest in May (about 55°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 189 field readings (2017–2025) via the EPA Water Quality Portal. See the full Bertha Lake water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 64 ft |
| Surface Area | 337 acres |
| Shoreline Length | 3.9 mi |
| Littoral Zone | 42% |
| Public Access | No |
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Fish Species
Click a species to see every graded lake where it is documented.
Fishing Bertha 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
11 surveys on file from MN DNR Fisheries. Most recent: 2024-08-14 (Targeted Survey).
Top Species by Catch Rate
| Species | Avg CPUE | Avg Weight |
|---|---|---|
| Bluegill | 31.89 | 0.13 lb |
| Bluntnose Minnow | 18.42 | — |
| Tullibee (Cisco) | 15.55 | 0.37 lb |
| BKF | 10.07 | — |
| JND | 9.17 | — |
| Yellow Perch | 8.35 | 0.15 lb |
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.
Bluegill
Tullibee (Cisco)
Yellow Perch
From the 2024-08-14 survey
Temperature and dissolved oxygen profiles were collected in the deepest basin in Bertha Lake on August 14, 2024, to evaluate the quantity and quality of cold, oxygenated water (i.e., oxythermal habitat) available to Lake Whitefish, the most sensitive coldwater species present in this lake (Cisco are also present).…
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for Bertha Lake. 1 report on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- —Fisheries Lake Survey — BerthaFisheries Lake Survey · MN DNR LakeFinder
Water Quality Trend: ↓ Declining
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | -0.044 m/yr | 6 |
| Phosphorus | ↓ Declining | +1.54 µg/L/yr | 6 |
| Chlorophyll-a | → Stable | -0.02 µg/L/yr | 6 |
Ice Season
Historical ice data from Minnesota DNR (11 observations).
| Earliest | Latest | |
|---|---|---|
| Ice-Out | 2024-03-17 (2024) | 2022-05-06 (2022) |
| Ice-In | 2021-12-07 (2021) | 2021-12-07 (2021) |
Most recent ice-out: 2025-04-15
EPA Impairment Status
Bertha Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
Source: EPA ATTAINS assessment unit MN18-0378-00 · Official waterbody report · Matched by proximity (2.58 km)
County Ranking
Ranked #53 of 85 lakes in Crow Wing County
Nearby Lakes in Crow Wing County
Frequently asked questions about Bertha Lake
What is the water quality grade for Bertha Lake?
- Bertha Lake in Crow Wing County, Minnesota has a water quality grade of A (Excellent). Water clarity: 17.1 ft visibility. Phosphorus: 13 µg/L. Chlorophyll-a (algae): 2.4 µg/L. Maximum depth: 64 ft. Surface area: 336.87 acres.
Is Bertha Lake safe to swim in?
- Excellent for swimming, crystal clear water with minimal algae. Bertha 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 Bertha Lake?
- Bertha Lake typically sits around 64°F at the surface in August, from 189 EPA Water Quality Portal field readings (2017–2025). That is a typical value for the month, not a live reading.
How deep is Bertha Lake?
- Bertha Lake has a maximum depth of 64 feet. The lake covers 336.87 acres.
What fish are in Bertha Lake?
- Fish species found in Bertha Lake include black bullhead, black crappie, bluegill, bowfin (dogfish), brown bullhead, burbot, greater redhorse, green sunfish, hybrid sunfish, lake whitefish, largemouth bass, northern pike, northern sunfish, pumpkinseed, rock bass, shorthead redhorse, smallmouth bass, tullibee (cisco), walleye, white sucker, yellow bullhead, yellow perch. Bertha Lake is located in Crow Wing County, Minnesota.
When does Bertha Lake thaw?
- Bertha Lake typically has ice-out around Apr 18.
Explore Bertha Lake across LakeGrade
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Where this data comes from
106 measurements from 1 monitoring station, 2020–2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Minnesota Pollution Control Agency - Ambient Surface Water
MNPCA106 samples · 2020–2025
Analytical methods
- Field measurement/observation, generic method
- Phosphorus by Two Reagent Colorimetry
- 10200 H ~ Chlorophyll a-b-c Determination
- 10200 H 2 ~ Chlorophyll a-b-c Determination by spectrophotometer
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
Impairment status from EPA ATTAINS 303(d) database
Lake size & watershed from HydroLAKES (HydroSHEDS)
Lake details from Minnesota DNR LakeFinder
Ice data from Minnesota DNR Climate