Blackwater Lake
Blackwater Lake in Cass County, Minnesota has a water quality grade of A (Excellent). 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.
- Clarity
- 14.1 ft
- Typical in August
- 72°F
- Max depth
- 67 ft
- Area
- 771 acres
- Type
- Mesotrophic
- Trend
- ↓ Declining
- Public access
- Yes
- Ice-out
- Apr 17
- Open water
- 225 days
Next lake in Cass County
Blackwater Lake is a 771-acre mesotrophic lake in Cass County, Minnesota with a maximum depth of 67 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 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. Grade methodology: LakeGrade methodology.
Watch Blackwater Lake water quality
Subscribe for LakeQuality updates by email. No spam, unsubscribe anytime.
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)
Good clarity, visible to about 14.1 ft. Phosphorus level: 12 µg/L. Chlorophyll-a: 4 µg/L. Trophic State Index: 41.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 14.1 ft | Not graded |
| Phosphorus | 12 µg/L | Good |
| Chlorophyll-a (Algae) | 4 µg/L | Good |
| Trophic State Index (TSI) | 41 | Mesotrophic |
Moderate nutrients, good water quality
Where to stay 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
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 →
Water Temperature
Blackwater Lake typically sits around 72°F at the surface in August. 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.
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 →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 67 ft |
| Surface Area | 771 acres |
| Shoreline Length | 7.7 mi |
| Littoral Zone | 44% |
| Public Access | Yes |
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 Blackwater Lake
DNR Fisheries Survey Summary
13 surveys on file from MN DNR Fisheries. Most recent: 2023-07-17 (Standard Survey).
Top Species by Catch Rate
| Species | Avg CPUE | Avg Weight |
|---|---|---|
| Bluegill | 19.10 | 0.15 lb |
| MMS | 15.56 | — |
| Largemouth Bass | 15.41 | 0.98 lb |
| Bluntnose Minnow | 13.57 | — |
| Yellow Perch | 6.71 | 0.15 lb |
| CNM | 6.36 | — |
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
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
Water Quality Trend: ↓ Declining
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | -0.043 m/yr | 6 |
| Phosphorus | ↓ Declining | +0.6 µg/L/yr | 6 |
| Chlorophyll-a | ↓ Declining | +0.37 µg/L/yr | 6 |
Ice Season
Historical ice data from Minnesota DNR (40 observations).
Estimated open water season: 225 days
| Earliest | Latest | |
|---|---|---|
| Ice-Out | 2012-03-25 (2012) | 2013-05-13 (2013) |
| Ice-In | 2019-11-12 (2019) | 2016-12-14 (2016) |
Most recent ice-out: 2022-05-07
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
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit MN11-0274-00 · Official waterbody report
County Ranking
Ranked #33 of 104 lakes in Cass County
Nearby Lakes in Cass County
Frequently asked questions about Blackwater Lake
What is the water quality grade for Blackwater Lake?
- Blackwater Lake in Cass County, Minnesota has a water quality grade of A (Excellent). 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?
- Excellent for swimming, crystal clear water with minimal algae. Blackwater Lake has a water quality grade of A (Excellent). Algae concentration (chlorophyll-a) is 4 µg/L. Always check local health department advisories before swimming.
What is the water temperature of Blackwater Lake?
- Blackwater Lake typically sits around 72°F at the surface in August, 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.
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.
Explore Blackwater Lake across LakeGrade
Depth, room, public access, and the state wake-sport rules
Fish species and top-graded lakes across Minnesota
Fish species and best lakes in Cass County
Species activity outlook by water temperature and season
Top lakes and where to stay in the Brainerd Lakes area
Where to stay at Minnesota's top lakes, each with its grade
Lakes in the Mississippi Headwaters, ranked by water quality
Lakes in the Pine River Watersheds (Brainerd Lakes), ranked by water quality
Moderate nutrients, good water quality
Minnesota's cleanest and most-polluted lakes, ranked
More Minnesota Lake Rankings
Other Grade A Lakes in Minnesota
Mesotrophic Lakes in Minnesota
Compare Blackwater Lake With Nearby Lakes
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)
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