O'Brien Lake
Crow Wing County, MinnesotaView mapDirections
Minnesota PCA publishes the water-quality measurements for O'Brien Lake. Here is what has been measured.
- Oligotrophic
- 56 ft max depth
- Public access
O'Brien Lake in Crow Wing County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. Water clarity: 18 ft visibility. Phosphorus: 7.5 µg/L. Chlorophyll-a (algae): 2.1 µg/L.
O'Brien Lake is a cold, clear oligotrophic lake in Crow Wing County, Minnesota with an estimated average depth of about 56 feet. Overall water quality is declining over recent sampling years. Limited fish-species data available. Public access is available. Compared to the 84 other monitored lakes in Crow Wing County, O'Brien Lake ranks #48 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2025-09-15. Methodology: how these readings are compared.
For a place on the water, look through cabin and lake-house rentals near O'Brien Lake on Vrbo. For a hotel or motel room, Booking.com and Expedia both search around the lake.
What O'Brien Lake is good for
From access and monitoring records, not editorial picks.
- Clear water18 ft of visibility
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. Check local advisories before you swim.
Whether O'Brien 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 O'Brien Lake
These are the measurements Minnesota PCA publishes for O'Brien Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Crystal clear, you can see 18 ft down. Phosphorus 7.5 µg/L. Chlorophyll-a 2.1 µg/L. TSI 36.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 18ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 7.5µg/L | Good GoodFairPoor |
| Chlorophyll-a (algae) | 2.1µg/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. | 36 | Oligotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Low nutrients, clear water. No reading is a swimming clearance.
Ask about O'Brien Lake
Answers come straight from the public measurements and survey records on this page.
Water Temperature
O'Brien Lake typically sits around 65°F at the surface in September. Summer average is about 71°F. Usually warmest in Jul (about 74°F) and coldest in May (about 57°F). These are typical values by month — not a live reading.
Column height runs from this lake’s coldest month (57°F) to its warmest (74°F).
Typical surface water temperature by month, from 34 field readings (2017–2025) via the EPA Water Quality Portal. See the full O'Brien Lake water temperature breakdown →
DNR Reports & Resources
Minnesota DNR LakeFinder publishes lake survey, fish stocking, water access, and aquatic plant data for O'Brien Lake. 1 report on file.
These links open on the official state DNR websites (Wisconsin DNR survey PDFs · Minnesota DNR LakeFinder).
- —Public Water Access Sites — O'Brien (Northeast Bay)Public Water Access Sites · MN DNR LakeFinder
Lake Details
Max depth
~56 ft est.
Public access
Yes
Listed by Minnesota PCA
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Minnesota has not published bathymetry for O'Brien Lake.
Lake Size & Watershed
Watershed (drainage) area
13 sq mi
HydroLAKES / HydroSHEDS
Surface area
158 acres
HydroLAKES / HydroSHEDS
Shoreline
4.2 mi
HydroLAKES / HydroSHEDS
Elevation
1,234 ft
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth here is modelled, not surveyed — HydroLAKES v1.0 estimates lake depth from surrounding terrain rather than from soundings. Treat it as an approximation, and prefer a state bathymetric survey where one exists.
Stays via Stay22
Stays near O'Brien Lake
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Top-rated places to stay near O'Brien Lake
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Fishing O'Brien Lake
Water Quality Trend: Worsening
Based on 6 years of monitoring (2020–2025).
Water clarity
Worsening6 years
Phosphorus
Improving-0.33 µg/L/yr6 years
Chlorophyll-a
Worsening+0.25 µg/L/yr6 years
Nearby Lakes in Crow Wing County
Frequently asked questions about O'Brien Lake
Where is O'Brien Lake?
- O'Brien Lake is a lake in Crow Wing County, Minnesota. Its water-quality measurements are published by Minnesota PCA and shown on this page as reported.
What is the water quality of O'Brien Lake?
- O'Brien Lake in Crow Wing County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. Water clarity: 18 ft visibility. Phosphorus: 7.5 µg/L. Chlorophyll-a (algae): 2.1 µg/L.
Is O'Brien Lake safe to swim in?
- Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
What is the water temperature of O'Brien Lake?
- O'Brien Lake typically sits around 65°F at the surface in September, from 34 EPA Water Quality Portal field readings (2017–2025). That is a typical value for the month, not a live reading.
How deep is O'Brien Lake?
- No published bathymetric survey covers O'Brien Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 56 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate.
Can I fish at O'Brien Lake?
- Public access to O'Brien 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 O'Brien Lake across LakeQuality
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Where this data comes from
76 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
MNPCA76 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)
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
Lake size & watershed from HydroLAKES (HydroSHEDS)
Lake details from Minnesota DNR LakeFinder