Union Lake is a lake in Polk County, Minnesota. Its water-quality measurements are published by Minnesota PCA and shown on this page as reported.

What is the water temperature of Union Lake?

Union Lake typically sits around 70°F at the surface in September, 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?

No published bathymetric survey covers Union Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 95 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate.

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.

Can I fish at Union Lake?

Union 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.

Union Lake

Polk County, MinnesotaView map Directions

Minnesota PCA publishes the water-quality measurements for Union Lake. Here is what has been measured.

Clarity

15 ft

Clearer than most

Secchi · sampled 2025

Phosphorus

14 µg/L

Typical for Minnesota

Measured by Minnesota PCA

Chlorophyll-a

2.7 µg/L

Less algae than most

Measured by Minnesota PCA

Water temp

~70°F

Typical for Sep · EPA WQP

Max depth

~95 ft est.

GLOBathy model

Type

Mesotrophic

Trophic class

Mesotrophic
Impaired · Mercury
95 ft max depth
Public access

See the measurements Directions Where to stay

Union Lake in Polk County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. Water clarity: 15 ft visibility. Phosphorus: 14 µg/L. Chlorophyll-a (algae): 2.7 µg/L.

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Union Lake is a mesotrophic lake in Polk County, Minnesota with an estimated average depth of about 95 feet. 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. Methodology: how these readings are compared.

Union Lake has one public boat landing. For a place on the water, look through cabin and lake-house rentals near Union Lake on Vrbo. For a hotel or motel room, Booking.com and Expedia both search around the lake.

What Union Lake is good for

From access and monitoring records, not editorial picks.

Clear water15 ft of visibility
Boat launch1 documented ramp

Swimming conditions

Readings only

Low 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. Union Lake is listed impaired under Clean Water Act §303(d) (IR 4A) for mercury. Check local advisories before you swim.

Whether Union 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.

Compare swimming lakes near Detroit Lakes →

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Water quality measurements for Union Lake

These are the measurements Minnesota PCA publishes for Union 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 15 ft down. Phosphorus 14 µg/L. Chlorophyll-a 2.7 µg/L. TSI 40.

Water quality readings
Measure	Reading	Where it sits
Water clarity (Secchi depth)	15ft	Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer.
Total phosphorus	14µg/L	Good GoodFairPoor
Chlorophyll-a (algae)	2.7µg/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.	40	Mesotrophic OligotrophicMesotrophicEutrophicHypereutrophic

Moderate nutrients, good water quality

Ask about Union Lake

Answers come straight from the public measurements and survey records on this page.

Water Temperature

Union Lake typically sits around 70°F at the surface in September. 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.

Jan

Feb

Mar

Apr

May

69°

Jun

73°

Jul

74°

Aug

70°

Sep

Oct

Nov

Dec

Column height runs from this lake’s coldest month (69°F) to its warmest (74°F).

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)

No current satellite reading

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 Details

Max depth

~95 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 Union Lake.

Lake Size & Watershed

Watershed (drainage) area

21 sq mi

HydroLAKES / HydroSHEDS

Surface area

776 acres

HydroLAKES / HydroSHEDS

Shoreline

11.1 mi

HydroLAKES / HydroSHEDS

Elevation

1,214 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.

Recreation & Access

Boat Landings

Yes

Public Access

Union 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 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

Prices for Oct 9–11, 4 adults. Change dates on the map below.

Availability and prices from Stay22 partners (Booking.com, Vrbo, Expedia). We may earn a commission from bookings, at no extra cost to you.

Planning a whole trip? See where to stay at Minnesota’s top lakes →

Follow Union Lake’s readings

Email me when new measurements post for this lake. Updates stay free.

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Fishing Union Lake

→ Union Lake fishing regulations (limits, seasons, special rules)

Water Quality Trend: Improving

Based on 6 years of monitoring (2020–2025).

Water clarity
Improving
6 years
Phosphorus
Worsening
+0.53 µg/L/yr6 years
Chlorophyll-a
Improving
-0.09 µg/L/yr6 years

See year-by-year chart →

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

Mercury

Designated uses not supporting: Fish consumption.

A Total Maximum Daily Load (TMDL) plan has been approved.

Source: EPA ATTAINS assessment unit MN60-0217-00 · Official waterbody report

Nearby Lakes in Polk County

Crooked Lake—View →Crystal Lake—View →Unnamed Lake2 ft clear · Eutrophic—View →Leo Lake—View →Maltrud Lake—View →Eighteen Lake—View →

Frequently asked questions about Union Lake

Where is Union Lake?: Union Lake is a lake in Polk County, Minnesota. Its water-quality measurements are published by Minnesota PCA and shown on this page as reported.
What is the water quality of Union Lake?: Union Lake in Polk County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. Water clarity: 15 ft visibility. Phosphorus: 14 µg/L. Chlorophyll-a (algae): 2.7 µg/L.
Is Union Lake safe to swim in?: Nutrient and algae readings are not a swimming clearance. Union Lake is listed impaired under Clean Water Act §303(d) (IR 4A) for mercury. Check local advisories before you swim.
What is the water temperature of Union Lake?: Union Lake typically sits around 70°F at the surface in September, 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?: No published bathymetric survey covers Union Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 95 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate.
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.
Can I fish at Union Lake?: Union 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 Union Lake across LakeQuality

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Official ratings and named-lake measurements

Other Lakes in Minnesota

Sargent Lake

Mahnomen County · 8.9 ft

Hospital Lake

Becker County · 18 ft

North Twin Lake

Mahnomen County · 8.5 ft

Fish Hook Lake

Becker County · 9.5 ft

White Earth Lake

Becker County · 15 ft

South Twin Lake

Mahnomen County · 10.2 ft

Mesotrophic Lakes in Minnesota

Sargent Lake North Twin Lake Fish Hook Lake South Twin Lake

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 WaterMNPCA186 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)

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.