Blackwater Lake
Cass County, Minnesota771 acresView mapDirections
Minnesota PCA publishes the water-quality measurements for Blackwater Lake. Here is what has been measured.
- Mesotrophic
- Impaired · Mercury
- 67 ft max depth
- Public access
Blackwater Lake in Cass County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. Water clarity: 14.1 ft visibility. Phosphorus: 12 µg/L. Chlorophyll-a (algae): 4 µg/L. Maximum depth: 67 ft. Surface area: 771.45 acres.
Minnesota DNR's Standard Survey on July 17, 2023 caught 2.00 walleye per gill net at Blackwater Lake. DNR survey details →
Blackwater Lake is a 771-acre mesotrophic lake in Cass County, Minnesota with a maximum depth of 67 feet. Overall water quality is declining over recent sampling years. The lake supports 16 documented fish species including black bullhead, black crappie, and bluegill. Public access is available. Compared to the 103 other monitored lakes in Cass County, Blackwater Lake ranks #33 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2025-09-29. Methodology: how these readings are compared.
Blackwater Lake covers about 770 acres in Cass County. The median ice-out date is April 17, which is when the open-water season begins. For a place on the water, look through cabin and lake-house rentals near Blackwater Lake on Vrbo. For a hotel or motel room, Booking.com and Expedia both search around the lake.
What Blackwater Lake is good for
From access and monitoring records, not editorial picks.
- Clear water14.1 ft of visibility
- Fishing16 species recorded
- Ice seasonIce-out around Apr 17
- Deep water67 ft surveyed max depth
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. Blackwater Lake is listed impaired under Clean Water Act §303(d) (IR 4A) for mercury. Check local advisories before you swim.
Whether Blackwater 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 Blackwater Lake
These are the measurements Minnesota PCA publishes for Blackwater Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Good clarity, visible to about 14.1 ft. Phosphorus 12 µg/L. Chlorophyll-a 4 µg/L. TSI 41.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 14.1ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 12µg/L | Good GoodFairPoor |
| Chlorophyll-a (algae) | 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. | 41 | Mesotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Moderate nutrients, good water quality
Ask about Blackwater Lake
Answers come straight from the public measurements and survey records on this page.
Water Temperature
Blackwater Lake typically sits around 65°F at the surface in September. Summer average is about 68°F. Usually warmest in Aug (about 72°F) and coldest in Jul (about 62°F). These are typical values by month — not a live reading.
Column height runs from this lake’s coldest month (62°F) to its warmest (72°F).
Typical surface water temperature by month, from 37 field readings (2016–2023) via the EPA Water Quality Portal. See the full Blackwater Lake water temperature breakdown →
Minnesota DNR's Standard Survey on July 17, 2023 caught 2.00 walleye per gill net at Blackwater Lake.
A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Blackwater Lake’s water-quality measurements separately, and the water-quality trend over 6 years of monitoring is declining. Catch rate is not a swim clearance and is not a weekly bite report.
DNR Fisheries Survey Summary
13 surveys on file from MN DNR Fisheries. Most recent: 2023-07-17 (Standard Survey).
13 surveys on file · most recent 2023-07-17 (Standard Survey)
| Species | Catch rate (CPUE) | Size | Avg weight |
|---|---|---|---|
| Bluegill | 19.10 | 3–9 in 2023-07-17 | 0.15 lb |
| Largemouth Bass | 15.41 | 5–17 in 2023-07-17 | 0.98 lb |
| Bluntnose Minnow | 13.57 | ||
| Yellow Perch | 6.71 | 5–8 in 2023-07-17 | 0.15 lb |
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.
Bluegill
Largemouth Bass
Yellow Perch
From the 2023-07-17 survey
Blackwater Lake is a 767-acre lake with 7.6 miles of shoreline and a maximum depth of 67 feet located southwest of Longville, MN. A DNR owned public access is located on the northeast shore. The DNR has classified Minnesota's lakes into 43 different classes based on physical, chemical, and other characteristics.…
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for Blackwater Lake. 4 reports on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- —Fisheries Lake Survey — BlackwaterFisheries Lake Survey · MN DNR LakeFinder
- —Aquatic Plant Report — Blackwater (1597)Aquatic Plant Report · MN DNR LakeFinder
- —Fish Stocking History — BlackwaterFish Stocking History · MN DNR LakeFinder
- —Public Water Access Sites — BlackwaterPublic Water Access Sites · MN DNR LakeFinder
Lake Details
Max depth
67 ft
Surface area
771 acres
Shoreline
7.7 mi
Littoral zone
44%
Share shallower than 15 ft
Public access
Yes
Listed by Minnesota PCA
Stays via Stay22
Stays near Blackwater Lake
Compare lakeside cabins, hotels, and vacation rentals near Blackwater 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 Blackwater Lake
Prices for Oct 9–11, 4 adults. Change dates on the map below.
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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 Blackwater Lake
Water Quality Trend: Worsening
Based on 6 years of monitoring (2020–2025).
Water clarity
Stable6 years
Phosphorus
Worsening+0.6 µg/L/yr6 years
Chlorophyll-a
Worsening+0.37 µg/L/yr6 years
Ice season
Typical ice-out
Apr 17
Earliest Mar 25, 2012 · Latest May 13, 2013
Typical ice-in
Nov 28
Earliest Nov 12, 2019 · Latest Dec 14, 2016
Most recent ice-out
May 7
2022
40 observations · Minnesota PCA ice records
EPA Impairment Status
Blackwater Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 4A).
Causes of impairment
Designated uses not supporting: Fish consumption.
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit MN11-0274-00 · Official waterbody report
Nearby Lakes in Cass County
Frequently asked questions about Blackwater Lake
Where is Blackwater Lake?
- Blackwater 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 Blackwater Lake?
- Blackwater Lake in Cass County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. Water clarity: 14.1 ft visibility. Phosphorus: 12 µg/L. Chlorophyll-a (algae): 4 µg/L. Maximum depth: 67 ft. Surface area: 771.45 acres.
Is Blackwater Lake safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Blackwater Lake is listed impaired under Clean Water Act §303(d) (IR 4A) for mercury. Check local advisories before you swim.
What is the water temperature of Blackwater Lake?
- Blackwater Lake typically sits around 65°F at the surface in September, from 37 EPA Water Quality Portal field readings (2016–2023). That is a typical value for the month, not a live reading.
How deep is Blackwater Lake?
- Blackwater Lake has a maximum depth of 67 feet. The lake covers 771.45 acres.
What fish are in Blackwater Lake?
- Fish species found in Blackwater Lake include black bullhead, black crappie, bluegill, bowfin (dogfish), brown bullhead, hybrid sunfish, largemouth bass, northern pike, pumpkinseed, rock bass, smallmouth bass, tullibee (cisco), walleye, white sucker, yellow bullhead, yellow perch. Blackwater Lake is located in Cass County, Minnesota.
What did the DNR fisheries survey find at Blackwater Lake?
- Minnesota DNR's Standard Survey on July 17, 2023 caught 2.00 walleye per gill net at Blackwater Lake. A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Blackwater Lake’s water-quality measurements separately, and the water-quality trend over 6 years of monitoring is declining. Catch rate is not a swim clearance and is not a weekly bite report.
When does Blackwater Lake thaw?
- Blackwater Lake typically has ice-out around Apr 17. Ice typically forms around Nov 28. The open water season is approximately 225 days.
Can I fish at Blackwater Lake?
- Public access to Blackwater Lake is listed as yes on the lake record. 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 Blackwater Lake across LakeQuality
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Where this data comes from
142 measurements from 2 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
MNPCA142 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 (2)
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
Impairment status from EPA ATTAINS 303(d) database
Lake details from Minnesota DNR LakeFinder
Ice data from Minnesota DNR Climate