O'Brien Lake
O'Brien Lake in Dakota County, Minnesota has a water quality grade of A (Excellent). Water clarity: 18 ft visibility. Phosphorus: 20 µg/L. Chlorophyll-a (algae): 4.9 µg/L.
- Clarity
- 18 ft
- Typical in August
- 74°F
- Type
- Mesotrophic
- Trend
- → Stable
- Public access
- No
Next lake in Dakota County
O'Brien Lake is a mesotrophic lake in Dakota County, Minnesota. Water quality is graded A based on 2 sampling years through 2025. 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. Grade methodology: LakeGrade methodology.
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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)
Crystal clear, you can see 18 ft down. Phosphorus level: 20 µg/L. Chlorophyll-a: 4.9 µg/L. Trophic State Index: 43.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 18 ft | Not graded |
| Phosphorus | 20 µg/L | Good |
| Chlorophyll-a (Algae) | 4.9 µg/L | Good |
| Trophic State Index (TSI) | 43 | Mesotrophic |
Moderate nutrients, good water quality
Where to stay near O'Brien Lake
Compare lakeside cabins, hotels, and vacation rentals near O'Brien Lake in Dakota 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 O'Brien Lake
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Water Temperature
O'Brien Lake typically sits around 74°F at the surface in August. 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.
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 →
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 data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | → Stable | -0.016 m/yr | 6 |
| Phosphorus | ↑ Improving | -0.52 µg/L/yr | 5 |
| Chlorophyll-a | ↓ Declining | +0.28 µg/L/yr | 5 |
EPA Impairment Status
O'Brien 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 MN19-0064-00 · Official waterbody report · Matched by proximity (2.29 km)
County Ranking
Ranked #4 of 45 lakes in Dakota County
Nearby Lakes in Dakota County
Frequently asked questions about O'Brien Lake
What is the water quality grade for O'Brien Lake?
- O'Brien Lake in Dakota County, Minnesota has a water quality grade of A (Excellent). Water clarity: 18 ft visibility. Phosphorus: 20 µg/L. Chlorophyll-a (algae): 4.9 µg/L.
Is O'Brien Lake safe to swim in?
- Excellent for swimming, crystal clear water with minimal algae. O'Brien Lake has a water quality grade of A (Excellent). Algae concentration (chlorophyll-a) is 4.9 µg/L. Always check local health department advisories before swimming.
What is the water temperature of O'Brien Lake?
- O'Brien Lake typically sits around 74°F at the surface in August, from 39 EPA Water Quality Portal field readings (2017–2024). That is a typical value for the month, not a live reading.
Explore O'Brien Lake across LakeGrade
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Moderate nutrients, good water quality
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State Parks Near O'Brien Lake
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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)
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