Colby Lake
Colby Lake in St. Louis County, Minnesota has a water quality grade of A (Excellent). Water clarity: 3 ft visibility. Phosphorus: 22 µg/L. Chlorophyll-a (algae): 3.2 µg/L. Maximum depth: 30 ft. Surface area: 517.72 acres.
- Clarity
- 3 ft
- Typical in August
- 64°F
- Max depth
- 30 ft
- Area
- 518 acres
- Type
- Eutrophic
- Trend
- ↓ Declining
- Public access
- Yes
- Ice-out
- Apr 21
- Open water
- 206 days
Next lake in St. Louis County
Colby Lake is a 518-acre productive, nutrient-rich eutrophic lake in St. Louis County, Minnesota with a maximum depth of 30 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 15 documented fish species including black crappie, bluegill, and brown bullhead. Public access is available with 1 public boat landing. Compared to the 67 other monitored lakes in St. Louis County, Colby Lake ranks #3 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2025-10-08. Grade methodology: LakeGrade methodology.
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Swimming Safety
Grade onlyLimited data
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)
Very murky, less than 3 ft of visibility. Phosphorus level: 22 µg/L. Chlorophyll-a: 3.2 µg/L. Trophic State Index: 51.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 3 ft | Not graded |
| Phosphorus | 22 µg/L | Good |
| Chlorophyll-a (Algae) | 3.2 µg/L | Good |
| Trophic State Index (TSI) | 51 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Where to stay near Colby Lake
Compare lakeside cabins, hotels, and vacation rentals near Colby Lake in St. Louis 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
Colby Lake typically sits around 64°F at the surface in August. Summer average is about 66°F. Usually warmest in Jun (about 71°F) and coldest in May (about 53°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 67 field readings (2017–2025) via the EPA Water Quality Portal. See the full Colby Lake water temperature breakdown →
Blue-Green Algae (Cyanobacteria)
There’s no recent EPA CyAN cyanobacteria estimate for Colby Lake — clouds, ice, or an off-season gap in coverage. The most recent reading was high (328,095 cells/mL) on May 31, 2025.
Blue-green algae can appear and clear within days, so check your state’s current advisory before swimming. EPA CyAN ↗
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | 30 ft |
| Surface Area | 518 acres |
| Shoreline Length | 12 mi |
| Littoral Zone | 73% |
| Public Access | Yes |
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Recreation & Access
- Colby LakePublic
Fish Species
Click a species to see every graded lake where it is documented.
Fishing Colby Lake
DNR Fisheries Survey Summary
12 surveys on file from MN DNR Fisheries. Most recent: 2025-07-14 (Standard Survey).
Top Species by Catch Rate
| Species | Avg CPUE | Avg Weight |
|---|---|---|
| Bluegill | 5.23 | 0.23 lb |
| Yellow Perch | 4.54 | 0.15 lb |
| Black Crappie | 4.25 | 0.26 lb |
| RHS | 1.67 | 1.16 lb |
| Northern Pike | 1.59 | 2.34 lb |
| Channel Catfish | 1.54 | 3.04 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
Yellow Perch
Black Crappie
RHS
From the 2025-07-14 survey
Colby is a 518-acre lake located immediately north of Hoyt Lakes. It is comparable to North and South Twin by Aurora, and Bailey's in downtown Virginia. The Partridge River runs through the lake and connects to the upper St. Louis River shortly downstream. This river connection makes Colby's fish community an outlier…
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for Colby Lake. 3 reports on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- —Fisheries Lake Survey — ColbyFisheries Lake Survey · MN DNR LakeFinder
- —Aquatic Plant Report — Colby (1878)Aquatic Plant Report · MN DNR LakeFinder
- —Public Water Access Sites — ColbyPublic 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 | ↓ Declining | -0.135 m/yr | 6 |
Ice Season
Historical ice data from Minnesota DNR (14 observations).
Estimated open water season: 206 days
| Earliest | Latest | |
|---|---|---|
| Ice-Out | 2000-04-02 (2000) | 2022-05-06 (2022) |
| Ice-In | 1992-10-28 (1992) | 2001-12-03 (2001) |
Most recent ice-out: 2022-05-06
EPA Impairment Status
Colby 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 MN69-0249-00 · Official waterbody report
Regulated Pollution Sources Nearby
1 EPA-permitted facility operate within about 5 miles of Colby Lake under Clean Water Act (NPDES) discharge permits, including 1 classified by the EPA as major discharger. 1 facility has a reported Clean Water Act violation in the past three years.
- Minnesota Power - Laskin Energy CenterHoyt Lakes · 0.7 mi away · NPDES MN0000990MajorRecent violation
Facilities permitted to discharge under the Clean Water Act within ~5 miles; proximity does not by itself mean a facility affects this lake. Source: EPA ECHO (ICIS-NPDES), as of 2026-07.
County Ranking
Ranked #3 of 68 lakes in St. Louis County
Nearby Lakes in St. Louis County
Frequently asked questions about Colby Lake
What is the water quality grade for Colby Lake?
- Colby Lake in St. Louis County, Minnesota has a water quality grade of A (Excellent). Water clarity: 3 ft visibility. Phosphorus: 22 µg/L. Chlorophyll-a (algae): 3.2 µg/L. Maximum depth: 30 ft. Surface area: 517.72 acres.
Is Colby Lake safe to swim in?
- Excellent for swimming, crystal clear water with minimal algae. Colby Lake has a water quality grade of A (Excellent). Algae concentration (chlorophyll-a) is 3.2 µg/L. Always check local health department advisories before swimming.
What is the water temperature of Colby Lake?
- Colby Lake typically sits around 64°F at the surface in August, from 67 EPA Water Quality Portal field readings (2017–2025). That is a typical value for the month, not a live reading.
How deep is Colby Lake?
- Colby Lake has a maximum depth of 30 feet. The lake covers 517.72 acres.
What fish are in Colby Lake?
- Fish species found in Colby Lake include black crappie, bluegill, brown bullhead, burbot, channel catfish, hybrid sunfish, largemouth bass, northern pike, pumpkinseed, rock bass, shorthead redhorse, walleye, white sucker, yellow bullhead, yellow perch. Colby Lake is located in St. Louis County, Minnesota.
When does Colby Lake thaw?
- Colby Lake typically has ice-out around Apr 21. Ice typically forms around Nov 13. The open water season is approximately 206 days.
Does Colby Lake have blue-green algae?
- There is no recent EPA CyAN cyanobacteria estimate for Colby Lake (clouds, ice, or an off-season gap). The most recent reading was high (328,095 cells/mL). Blue-green algae can appear and clear within days, so check Minnesota's current advisory before swimming.
Are there boat ramps on Colby Lake?
- Colby Lake has 1 documented boat landing in the USGS National Boat Ramps inventory. Named access points include Colby Lake. Public access is listed as yes.
Explore Colby Lake across LakeGrade
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High nutrients, frequent algae, reduced clarity
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Where this data comes from
45 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
MNPCA45 samples · 2020–2025
Analytical methods
- Field measurement/observation, generic method
- 4500 P E ~ Phosphorus in Water by Colorimetry- Ascorbic Acid Method
- 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
Regulated discharger data from EPA ECHO (ICIS-NPDES)
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