Union Lake
Union Lake in Polk County, Minnesota has a water quality grade of A (Excellent). Water clarity: 15 ft visibility. Phosphorus: 14 µg/L. Chlorophyll-a (algae): 2.7 µg/L.
- Clarity
- 15 ft
- Typical in August
- 74°F
- Type
- Mesotrophic
- Trend
- ↑ Improving
- Public access
- Yes
Next lake in Polk County
Union Lake is a mesotrophic lake in Polk County, Minnesota. Water quality is graded A based on 2 sampling years through 2025. Overall water quality is improving over recent sampling years. Limited fish-species data available. Public access is available with 1 public boat landing. Compared to the 18 other monitored lakes in Polk County, Union Lake ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2025-09-25. 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 15 ft down. Phosphorus level: 14 µg/L. Chlorophyll-a: 2.7 µg/L. Trophic State Index: 40.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 15 ft | Not graded |
| Phosphorus | 14 µg/L | Good |
| Chlorophyll-a (Algae) | 2.7 µg/L | Good |
| Trophic State Index (TSI) | 40 | Mesotrophic |
Moderate nutrients, good water quality
Where to stay near Union Lake
Compare lakeside cabins, hotels, and vacation rentals near Union Lake in Polk 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 Union Lake
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Water Temperature
Union Lake typically sits around 74°F at the surface in August. Summer average is about 72°F. Usually warmest in Aug (about 74°F) and coldest in Jun (about 69°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 17 field readings (2025–2025) via the EPA Water Quality Portal. See the full Union Lake water temperature breakdown →
Blue-Green Algae (Cyanobacteria)
There’s no recent EPA CyAN cyanobacteria estimate for Union Lake — clouds, ice, or an off-season gap in coverage. The most recent reading was moderate (89,536 cells/mL) on April 25, 2026.
Blue-green algae can appear and clear within days, so check your state’s current advisory before swimming. EPA CyAN ↗
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS).
Recreation & Access
- Union LakePublic
Fishing Union Lake
Water Quality Trend: ↑ Improving
Based on 6 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.503 m/yr | 6 |
| Phosphorus | ↓ Declining | +0.53 µg/L/yr | 6 |
| Chlorophyll-a | ↑ Improving | -0.09 µg/L/yr | 6 |
EPA Impairment Status
Union 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 MN60-0217-00 · Official waterbody report
County Ranking
Ranked #1 of 19 lakes in Polk County
Nearby Lakes in Polk County
Frequently asked questions about Union Lake
What is the water quality grade for Union Lake?
- Union Lake in Polk County, Minnesota has a water quality grade of A (Excellent). Water clarity: 15 ft visibility. Phosphorus: 14 µg/L. Chlorophyll-a (algae): 2.7 µg/L.
Is Union Lake safe to swim in?
- Excellent for swimming, crystal clear water with minimal algae. Union Lake has a water quality grade of A (Excellent). Algae concentration (chlorophyll-a) is 2.7 µg/L. Always check local health department advisories before swimming.
What is the water temperature of Union Lake?
- Union Lake typically sits around 74°F at the surface in August, from 17 EPA Water Quality Portal field readings (2025–2025). That is a typical value for the month, not a live reading.
How deep is Union Lake?
- The available depth estimate for Union Lake is shallower than its measured water clarity, which cannot be right, so no depth is shown.
Does Union Lake have blue-green algae?
- There is no recent EPA CyAN cyanobacteria estimate for Union Lake (clouds, ice, or an off-season gap). The most recent reading was moderate (89,536 cells/mL). Blue-green algae can appear and clear within days, so check Minnesota's current advisory before swimming.
Are there boat ramps on Union Lake?
- Union Lake has 1 documented boat landing in the USGS National Boat Ramps inventory. Named access points include Union Lake. Public access is listed as yes.
Explore Union Lake across LakeGrade
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Where this data comes from
186 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
MNPCA186 samples · 2020–2025
Analytical methods
- Field measurement/observation, generic method
- Phosphorus by Two Reagent Colorimetry
- 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
Lake size & watershed from HydroLAKES (HydroSHEDS)
Named boat-access points from the USGS National Boat Ramps inventory