Mermet Lake
Massac County, Illinois1 acresView mapDirections
Nutrients and algae run well above the regional average.
↓ down from D at the last regrade
- Hypereutrophic
- Impaired · Nutrients (phosphorus, nitrogen)
- #1 of 1 in the county
- 29 ft max depth
- Public access
- Limited data
Mermet Lake in Massac County, Illinois has a water quality grade of F (Algae-prone). Water clarity: 1.6 ft visibility. Phosphorus: 131 µg/L. Chlorophyll-a (algae): 88.5 µg/L. Surface area: 0.7 acres.
- Clarity
- 1.6 ft
- Typical in October
- 69°F
- Max depth
- ~29 ft est.
- Area
- 1 acres
- Type
- Hypereutrophic
- Phosphorus
- 131 µg/L
- Chlorophyll-a
- 88.5 µg/L
- Public access
- Yes
Stays via Stay22
Stays near Mermet Lake
Compare lakeside cabins, hotels, and vacation rentals near Mermet Lake in Massac County, Illinois — 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 Mermet Lake are listed below.
Top-rated places to stay near Mermet Lake
Prices for Sep 11–13, 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 Illinois’s top lakes →
Watch Mermet Lake’s letter
Email me if this lake’s grade moves. Grades stay free.
Or email
Mermet Lake is a 1-acre highly nutrient-loaded hypereutrophic lake in Massac County, Illinois with an estimated average depth of about 29 feet. Water quality is graded F based on 2 sampling years through 2021. Limited fish-species data available. Public access is available.
Source: EPA Water Quality Portal sampling records, Illinois EPA, last sampled 2021-10-18. Grade methodology: LakeQuality methodology.
Swimming Safety
Grade onlyLimited data
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.
The F is nutrients and algae, not a swimming clearance. Mermet Lake is listed impaired under Clean Water Act §303(d) (IR 5) for nutrients (phosphorus, nitrogen). Check local advisories before you swim.
Water Quality Grade: F (Algae-prone)
Very murky, less than 1.6 ft of visibility. Phosphorus 131 µg/L. Chlorophyll-a 88.5 µg/L. TSI 73. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.6 ft | Not graded |
| Phosphorus | 131 µg/L | Poor |
| Chlorophyll-a (Algae) | 88.5 µg/L | Poor |
| Trophic State Index (TSI) | 73 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Water Temperature
Mermet Lake typically sits around 69°F at the surface in October. Summer average is about 83°F. Usually warmest in Jun (about 83°F) and coldest in Apr (about 62°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 101 field readings (2016–2021) via the EPA Water Quality Portal. See the full Mermet Lake water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~29 ft est. |
| Average Depth | ~6 ft est. |
| Surface Area | 1 acres |
| Public Access | Yes |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Illinois has not published bathymetry for Mermet 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 Mermet Lake
Reservoir Info (USACE NID)
Mermet Lake is a man-made reservoir impounded by the Mermet Dam (completed 1950), built primarily for recreation on the TRIB TUCKER DITCH; earth-type dam, 10 ft tall and 20,000 ft long.
- Surface area
- 0.7 ac
- Normal storage
- 904 ac-ft
- Max storage
- 2,800 ac-ft
- Drainage area
- 0.96 sq mi
- Hazard class
- Low
- Owner
- ILLINOIS DEPARTMENT OF NATURAL RESOURCES
Source: USACE National Inventory of Dams, NID ID IL50071 · Operator website
EPA Impairment Status
Mermet Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
Source: EPA ATTAINS assessment unit IL_RAB · Official waterbody report
Regulated Pollution Sources Nearby
5 EPA-permitted facilities operate within about 5 miles of Mermet Lake under Clean Water Act (NPDES) discharge permits, including 1 classified by the EPA as major discharger. 2 facilities have a reported Clean Water Act violation in the past three years.
- Electric Energy IncJoppa · 3.3 mi away · NPDES IL0004171MajorRecent violation
- Lafarge Midwest IncJoppa · 3.6 mi away · NPDES IL0004081Recent violation
- Trunkline Gas CompanyVarious · 2.6 mi away · NPDES IL0075876
- Met-South IncJoppa · 2.6 mi away · NPDES IL0078751
- 100-2 Conversion ProjectMassac, Johnson, And Wayne Counties · 3.4 mi away · NPDES IL0080089
Facilities permitted to discharge under the Clean Water Act within ~5 miles; proximity does not by itself mean a facility affects this lake. Source: EPA ECHO (ICIS-NPDES), as of 2026-07.
County Ranking
Ranked #1 of 1 lakes in Massac County
Nearby Lakes in Massac County
Frequently asked questions about Mermet Lake
Where is Mermet Lake?
- Mermet Lake is a lake in Massac County, Illinois. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Mermet Lake?
- Mermet Lake in Massac County, Illinois has a water quality grade of F (Algae-prone). Water clarity: 1.6 ft visibility. Phosphorus: 131 µg/L. Chlorophyll-a (algae): 88.5 µg/L. Surface area: 0.7 acres.
Is Mermet Lake safe to swim in?
- The F is nutrients and algae, not a swimming clearance. Mermet Lake is listed impaired under Clean Water Act §303(d) (IR 5) for nutrients (phosphorus, nitrogen). Check local advisories before you swim.
What is the water temperature of Mermet Lake?
- Mermet Lake typically sits around 69°F at the surface in October, from 101 EPA Water Quality Portal field readings (2016–2021). That is a typical value for the month, not a live reading.
How deep is Mermet Lake?
- No published bathymetric survey covers Mermet Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 29 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 0.7 acres.
Explore Mermet Lake across LakeQuality
Species activity outlook by water temperature and season
Where to stay at Illinois's top lakes, each with its grade
Very high nutrients, dense algae, poor clarity
Illinois's cleanest and most-polluted lakes, ranked
More Illinois Lake Rankings
Other Grade F Lakes in Illinois
Hypereutrophic Lakes in Illinois
Where this data comes from
52 measurements from 3 monitoring stations, 2021. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- illinois epa
IL_EPA_WQX52 samples · 2021
Analytical methods
- MEASURED IN FIELD
- Phosphorus by Colorimetry
- 10200 H 2 ~ Chlorophyll a-b-c Determination by spectrophotometer
Monitoring station identifiers (3)
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
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)