O'Brien Lake
Dakota County, MinnesotaView mapDirections
Minnesota PCA publishes the water-quality measurements for O'Brien Lake. Here is what has been measured.
- Mesotrophic
- Eurasian watermilfoil
O'Brien Lake in Dakota County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. Water clarity: 18 ft visibility. Phosphorus: 20 µg/L. Chlorophyll-a (algae): 4.9 µg/L.
O'Brien Lake is a mesotrophic lake in Dakota County, Minnesota. Phosphorus level is improving over recent sampling years. Limited fish-species data available. No public access — the lake is private or restricted. Compared to the 44 other monitored lakes in Dakota County, O'Brien Lake ranks #4 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2025-10-05. 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 20 µg/L. Chlorophyll-a 4.9 µg/L. TSI 43.
| 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 | 20µg/L | Good GoodFairPoor |
| Chlorophyll-a (algae) | 4.9µg/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. | 43 | Mesotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Moderate nutrients, good water quality
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 70°F at the surface in September. Summer average is about 75°F. Usually warmest in Jul (about 78°F) and coldest in May (about 65°F). These are typical values by month — not a live reading.
Column height runs from this lake’s coldest month (65°F) to its warmest (78°F).
Typical surface water temperature by month, from 39 field readings (2017–2024) via the EPA Water Quality Portal. See the full O'Brien Lake water temperature breakdown →
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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
Invasive & Introduced Species
Red chips are ecologically harmful invasive species. Gray chips are stocked or introduced fish that aren't a current ecological concern.
Water Quality Trend: Stable
Based on 6 years of monitoring (2020–2025).
Water clarity
Stable6 years
Phosphorus
Improving-0.52 µg/L/yr5 years
Chlorophyll-a
Worsening+0.28 µg/L/yr5 years
Nearby Lakes in Dakota County
Frequently asked questions about O'Brien Lake
Where is O'Brien Lake?
- O'Brien Lake is a lake in Dakota 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 Dakota County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. Water clarity: 18 ft visibility. Phosphorus: 20 µg/L. Chlorophyll-a (algae): 4.9 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 70°F at the surface in September, from 39 EPA Water Quality Portal field readings (2017–2024). That is a typical value for the month, not a live reading.
Can I fish at O'Brien Lake?
- Public access to O'Brien Lake is listed as no on the lake record, so confirm before you make the trip. 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
Fish species and the cleanest lakes across Minnesota
Fish species and best lakes in Dakota County
Species activity outlook by water temperature and season
Where to stay at Minnesota's top lakes, each with its readings
Lakes in the Minnesota River Watershed, ranked by water quality
Minnesota's cleanest lakes, ranked on agency measurements
More lakes in Minnesota
State Parks Near O'Brien Lake
Other Lakes in Minnesota
Mesotrophic Lakes in Minnesota
Where this data comes from
169 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
MNPCA169 samples · 2020–2025
Analytical methods
- Field measurement/observation, generic method
- Total Phosphorus After Block Digestion
- Phosphorus by Colorimetry
- Analysis of Chlorophyll in Periphyton and Plankton
- 10200 H ~ Chlorophyll a-b-c Determination
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.