Colby Lake
St. Louis County, Minnesota518 acresView mapDirections
Minnesota PCA publishes the water-quality measurements for Colby Lake. Here is what has been measured.
- Eutrophic
- Impaired · Mercury
- 30 ft max depth
- Public access
- Limited data
Colby Lake in St. Louis County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. 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.
Minnesota DNR's Standard Survey on July 14, 2025 caught 0.33 walleye per gill net at Colby Lake. DNR survey details →
Colby Lake is a 518-acre productive, nutrient-rich eutrophic lake in St. Louis County, Minnesota with a maximum depth of 30 feet. 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. Methodology: how these readings are compared.
What Colby Lake is good for
From access and monitoring records, not editorial picks.
- Boat launch1 documented ramp
- Fishing15 species recorded
- Ice seasonIce-out around Apr 21
- Deep water30 ft surveyed max depth
Swimming conditions
Readings onlyLimited data
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. Colby Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury. Check local advisories before you swim.
Whether Colby 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 Colby Lake
These are the measurements Minnesota PCA publishes for Colby Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Very murky, less than 3 ft of visibility. Phosphorus 22 µg/L. Chlorophyll-a 3.2 µg/L. TSI 51.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 3ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 22µg/L | Good GoodFairPoor |
| Chlorophyll-a (algae) | 3.2µg/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. | 51 | Eutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
High nutrients, frequent algae, reduced clarity
Ask about Colby Lake
Answers come straight from the public measurements and survey records on this page.
Water Temperature
Colby Lake typically sits around 53°F at the surface in October. 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.
Column height runs from this lake’s coldest month (53°F) to its warmest (71°F).
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 ↗
Minnesota DNR's Standard Survey on July 14, 2025 caught 0.33 walleye per gill net at Colby Lake.
A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Colby 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
12 surveys on file from MN DNR Fisheries. Most recent: 2025-07-14 (Standard Survey).
12 surveys on file · most recent 2025-07-14 (Standard Survey)
| Species | Catch rate (CPUE) | Size | Avg weight |
|---|---|---|---|
| Bluegill | 5.23 | 3–7 in 2025-07-14 | 0.23 lb |
| Yellow Perch | 4.54 | 5–8 in 2025-07-14 | 0.15 lb |
| Black Crappie | 4.25 | 1–9 in 2025-07-14 | 0.26 lb |
| Northern Pike | 1.59 | 12–34 in 2025-07-14 | 2.34 lb |
| Channel Catfish | 1.54 | 14–27 in 2025-07-14 | 3.04 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
Yellow Perch
Black Crappie
Northern Pike
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
Lake Details
Max depth
30 ft
Surface area
518 acres
Shoreline
12 mi
Littoral zone
73%
Share shallower than 15 ft
Public access
Yes
Listed by Minnesota PCA
Lake Size & Watershed
Watershed (drainage) area
135 sq mi
HydroLAKES / HydroSHEDS
Elevation
1,437 ft
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Recreation & Access
- Colby LakePublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Stays via Stay22
Stays 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.
Top-rated places to stay near Colby Lake
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Fish Species
Click a species to see every lake where it is documented.
Fishing Colby Lake
Water Quality Trend: Worsening
Based on 6 years of monitoring (2020–2025).
Water clarity
Worsening6 years
Ice season
Typical ice-out
Apr 21
Earliest Apr 2, 2000 · Latest May 6, 2022
Typical ice-in
Nov 13
Earliest Oct 28, 1992 · Latest Dec 3, 2001
Most recent ice-out
May 6
2022
14 observations · Minnesota PCA ice records
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
Designated uses not supporting: Fish consumption.
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.
Nearby Lakes in St. Louis County
Frequently asked questions about Colby Lake
Where is Colby Lake?
- Colby Lake is a lake in St. Louis County, Minnesota. Its water-quality measurements are published by Minnesota PCA and shown on this page as reported.
What is the water quality of Colby Lake?
- Colby Lake in St. Louis County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. 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?
- Nutrient and algae readings are not a swimming clearance. Colby Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury. Check local advisories before you swim.
What is the water temperature of Colby Lake?
- Colby Lake typically sits around 53°F at the surface in October, 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.
What did the DNR fisheries survey find at Colby Lake?
- Minnesota DNR's Standard Survey on July 14, 2025 caught 0.33 walleye per gill net at Colby Lake. A DNR fisheries survey reports fish abundance and catch rates, not water quality. LakeQuality shows Colby 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 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.
Can I fish at Colby Lake?
- Colby 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 Colby Lake across LakeQuality
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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)
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
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