Flat Lake
Flat Lake in Becker County, Minnesota has a water quality grade of A (Excellent). Water clarity: 11.2 ft visibility. Phosphorus: 13 µg/L. Chlorophyll-a (algae): 3.7 µg/L. Surface area: 1754 acres.
- Clarity
- 11.2 ft
- Typical in August
- 69°F
- Max depth
- ~48 ft est.
- Area
- 1,754 acres
- Type
- Mesotrophic
- Trend
- ↑ Improving
- Public access
- No
Next lake in Becker County
Flat Lake is a 1,754-acre mesotrophic lake in Becker County, Minnesota with an estimated average depth of about 48 feet. 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. No public access — the lake is private or restricted. Compared to the 83 other monitored lakes in Becker County, Flat Lake ranks #53 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)
Good clarity, visible to about 11.2 ft. Phosphorus level: 13 µg/L. Chlorophyll-a: 3.7 µg/L. Trophic State Index: 42.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 11.2 ft | Not graded |
| Phosphorus | 13 µg/L | Good |
| Chlorophyll-a (Algae) | 3.7 µg/L | Good |
| Trophic State Index (TSI) | 42 | Mesotrophic |
Moderate nutrients, good water quality
Where to stay near Flat Lake
Compare lakeside cabins, hotels, and vacation rentals near Flat Lake in Becker 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.
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Water Temperature
Flat Lake typically sits around 69°F at the surface in August. Summer average is about 71°F. Usually warmest in Jul (about 78°F) and coldest in Jun (about 65°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 38 field readings (2010–2025) via the EPA Water Quality Portal. See the full Flat Lake water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~48 ft est. |
| Average Depth | ~14 ft est. |
| Surface Area | 1,754 acres |
| Public Access | No |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Minnesota has not published bathymetry for Flat Lake.
Lake Size & Watershed
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.
Fishing Flat Lake
Water Quality Trend: ↑ Improving
Based on 2 years of monitoring data (2020-2025).
| Metric | Trend | Change/Year | Years |
|---|---|---|---|
| Water Clarity | ↑ Improving | +0.9 m/yr | 2 |
| Phosphorus | ↑ Improving | -0.5 µg/L/yr | 2 |
| Chlorophyll-a | ↓ Declining | +0.1 µg/L/yr | 2 |
Reservoir Info (USACE NID)
Flat Lake is a man-made reservoir impounded by the Flat Lake (completed 1941), built primarily for fish and wildlife pond on the Egg River; earth-type dam, 8 ft tall and 330 ft long.
- Surface area
- 1,754 ac
- Normal storage
- 5,020 ac-ft
- Max storage
- 12,100 ac-ft
- Drainage area
- 33.5 sq mi
- Hazard class
- Low
- Owner
- USFWS
Source: USACE National Inventory of Dams, NID ID MN00616 · Operator website
County Ranking
Ranked #53 of 84 lakes in Becker County
Nearby Lakes in Becker County
Frequently asked questions about Flat Lake
What is the water quality grade for Flat Lake?
- Flat Lake in Becker County, Minnesota has a water quality grade of A (Excellent). Water clarity: 11.2 ft visibility. Phosphorus: 13 µg/L. Chlorophyll-a (algae): 3.7 µg/L. Surface area: 1754 acres.
Is Flat Lake safe to swim in?
- Excellent for swimming, crystal clear water with minimal algae. Flat Lake has a water quality grade of A (Excellent). Algae concentration (chlorophyll-a) is 3.7 µg/L. Always check local health department advisories before swimming.
What is the water temperature of Flat Lake?
- Flat Lake typically sits around 69°F at the surface in August, from 38 EPA Water Quality Portal field readings (2010–2025). That is a typical value for the month, not a live reading.
How deep is Flat Lake?
- No published bathymetric survey covers Flat Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 48 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate. The lake covers 1754 acres.
Explore Flat Lake across LakeGrade
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Where this data comes from
32 measurements from 1 monitoring station, 2024–2025. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- Minnesota Pollution Control Agency - Ambient Surface Water
MNPCA32 samples · 2024–2025
Analytical methods
- Field measurement/observation, generic method
- Phosphorus by Two Reagent Colorimetry
- 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
Lake size & watershed from HydroLAKES (HydroSHEDS)
Lake details from Minnesota DNR LakeFinder