Flat Lake
Becker County, Minnesota1,754 acresView mapDirections
Minnesota PCA publishes the water-quality measurements for Flat Lake. Here is what has been measured.
- Mesotrophic
- 48 ft max depth
Flat Lake in Becker County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. Water clarity: 11.2 ft visibility. Phosphorus: 13 µg/L. Chlorophyll-a (algae): 3.7 µg/L. Surface area: 1754 acres.
Flat Lake is a 1,754-acre mesotrophic lake in Becker County, Minnesota with an estimated average depth of about 48 feet. 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. Methodology: how these readings are compared.
What Flat Lake is good for
From access and monitoring records, not editorial picks.
- Clear water11.2 ft of visibility
- Big water1,754 acres
Swimming conditions
Readings onlyLow 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. Check local advisories before you swim.
Whether Flat 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.
Water quality measurements for Flat Lake
These are the measurements Minnesota PCA publishes for Flat Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Good clarity, visible to about 11.2 ft. Phosphorus 13 µg/L. Chlorophyll-a 3.7 µg/L. TSI 42.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 11.2ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 13µg/L | Good GoodFairPoor |
| Chlorophyll-a (algae) | 3.7µg/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. | 42 | Mesotrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Moderate nutrients, good water quality
Ask about Flat Lake
Answers come straight from the public measurements and survey records on this page.
Water Temperature
Flat Lake typically sits around 65°F at the surface in September. 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.
Column height runs from this lake’s coldest month (65°F) to its warmest (78°F).
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
Max depth
~48 ft est.
Surface area
1,754 acres
Public access
No
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 Flat Lake.
Lake Size & Watershed
Watershed (drainage) area
17 sq mi
HydroLAKES / HydroSHEDS
Shoreline
7.2 mi
HydroLAKES / HydroSHEDS
Elevation
1,460 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.
Stays via Stay22
Stays 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.
Top-rated places to stay near Flat 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 Flat Lake’s readings
Email me when new measurements post for this lake. Updates stay free.
Or email
Fishing Flat Lake
Water quality readings over time
2 seasons of samples on record (2024–2025). Calling a direction takes at least 4 years of the same measurement.
Water clarity
Improving2 years
Phosphorus
Improving-0.5 µg/L/yr2 years
Chlorophyll-a
Worsening+0.1 µg/L/yr2 years
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
Nearby Lakes in Becker County
Frequently asked questions about Flat Lake
Where is Flat Lake?
- Flat Lake is a lake in Becker County, Minnesota. Its water-quality measurements are published by Minnesota PCA and shown on this page as reported.
What is the water quality of Flat Lake?
- Flat Lake in Becker County, Minnesota has water-quality measurements published by Minnesota PCA, shown here as reported. 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?
- Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
What is the water temperature of Flat Lake?
- Flat Lake typically sits around 65°F at the surface in September, 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.
Can I fish at Flat Lake?
- Public access to Flat Lake is listed as no on the lake record, so confirm before you make the trip. 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 Flat Lake across LakeQuality
Fish species and the cleanest lakes across Minnesota
Fish species and best lakes in Becker County
Species activity outlook by water temperature and season
Top lakes and where to stay in the Detroit Lakes Area area
Top lakes and where to stay in the Park Rapids Area area
Where to stay at Minnesota's top lakes, each with its readings
Lakes in the Red River of the North Watershed, ranked by water quality
Minnesota's cleanest lakes, ranked on agency measurements
More lakes in Minnesota
Other Lakes in Minnesota
Mesotrophic Lakes in Minnesota
Compare Flat Lake With Nearby Lakes
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)
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.
Other records on this page
Lake size & watershed from HydroLAKES (HydroSHEDS)
Lake details from Minnesota DNR LakeFinder