Grass Lake
Grass Lake in Hennepin County, Minnesota has a water quality grade of D (Nutrient-rich). Phosphorus: 112 µg/L. Chlorophyll-a (algae): 20.3 µg/L.
- Typical in August
- 71°F
- Type
- Eutrophic
- Trend
- ↑ Improving
- Public access
- No
- Ice-out
- Mar 30
Grade D measures nutrients and algae, not whether this is a good place for a cabin trip. Cabins and rentals near Grass Lake →
Next lake in Hennepin County
Grass Lake is a productive, nutrient-rich eutrophic lake in Hennepin County, Minnesota. Water quality is graded D based on 2 sampling years through 2024. Overall water quality is improving over recent sampling years. Limited fish-species data available. No public access — the lake is private or restricted. Compared to the 80 other monitored lakes in Hennepin County, Grass Lake ranks #50 for water quality.
Source: EPA Water Quality Portal sampling records, Minnesota DNR LakeFinder, last sampled 2024-10-22. Grade methodology: LakeGrade methodology.
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Swimming Safety
Grade onlyAlgae risk
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.
Nutrients and algae run higher than average — check local advisories before swimming, and skip the water if you see scum
Water Quality Grade: D (Nutrient-rich)
No clarity data. Phosphorus level: 112 µg/L. Chlorophyll-a: 20.3 µg/L. Trophic State Index: 66.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | 112 µg/L | Poor |
| Chlorophyll-a (Algae) | 20.3 µg/L | Poor |
| Trophic State Index (TSI) | 66 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Where to stay near Grass Lake
Compare lakeside cabins, hotels, and vacation rentals near Grass Lake in Hennepin 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. The water-quality grade measures nutrients and algae, not lodging — cabins and rentals around Grass Lake are listed below.
Top-rated places to stay near Grass Lake
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 →
Water Temperature
Grass Lake typically sits around 71°F at the surface in August. Summer average is about 72°F. Usually warmest in Jul (about 77°F) and coldest in Apr (about 44°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 18 field readings (2010–2024) via the EPA Water Quality Portal. See the full Grass Lake water temperature breakdown →
Fishing Grass Lake
Water Quality Trend: ↑ Improving
Based on 3 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Phosphorus | ↑ Improving | -9.37 µg/L/yr | 3 |
| Chlorophyll-a | ↑ Improving | -6.45 µg/L/yr | 3 |
Ice Season
Historical ice data from Minnesota DNR (9 observations).
| Earliest | Latest | |
|---|---|---|
| Ice-Out | 2024-03-04 (2024) | 2023-04-11 (2023) |
| Ice-In | 2017-12-08 (2017) | 2024-01-03 (2024) |
Most recent ice-out: 2024-03-04
EPA Impairment Status
Grass 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 MN27-0022-00 · Official waterbody report · Matched by proximity (2.38 km)
County Ranking
Ranked #50 of 81 lakes in Hennepin County
Nearby Lakes With Higher Grades
Grass Lake holds Grade D. 2 nearby lakes hold higher nutrient-and-algae grades — useful for a swim day, not a reason to skip lodging here.
See full comparison →Nearby Lakes in Hennepin County
Frequently asked questions about Grass Lake
What is the water quality grade for Grass Lake?
- Grass Lake in Hennepin County, Minnesota has a water quality grade of D (Nutrient-rich). Phosphorus: 112 µg/L. Chlorophyll-a (algae): 20.3 µg/L.
Is Grass Lake safe to swim in?
- Nutrients and algae run higher than average — check local advisories before swimming, and skip the water if you see scum. Grass Lake has a water quality grade of D (Nutrient-rich). Algae concentration (chlorophyll-a) is 20.3 µg/L. Always check local health department advisories before swimming.
What is the water temperature of Grass Lake?
- Grass Lake typically sits around 71°F at the surface in August, from 18 EPA Water Quality Portal field readings (2010–2024). That is a typical value for the month, not a live reading.
When does Grass Lake thaw?
- Grass Lake typically has ice-out around Mar 30.
Explore Grass Lake across LakeGrade
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High nutrients, frequent algae, reduced clarity
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Where this data comes from
63 measurements from 1 monitoring station, 2020–2024. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Minnesota Pollution Control Agency - Ambient Surface Water
MNPCA63 samples · 2020–2024
Analytical methods
- 4500 P E ~ Phosphorus in Water by Colorimetry- Ascorbic Acid Method
- 10200 H ~ Chlorophyll a-b-c Determination
- Turbidity, Nephelometric Method
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
Ice data from Minnesota DNR Climate