Mermet Lake
Massac County, Illinois1 acresView mapDirections
Illinois EPA publishes the water-quality measurements for Mermet Lake. Here is what has been measured.
- Hypereutrophic
- Impaired · Nutrients (phosphorus, nitrogen)
- 29 ft max depth
- Public access
- Limited data
Mermet Lake in Massac County, Illinois has water-quality measurements published by Illinois EPA, shown here as reported. Water clarity: 1.6 ft visibility. Phosphorus: 131 µg/L. Chlorophyll-a (algae): 88.5 µg/L. Surface area: 0.7 acres.
Mermet Lake is a 1-acre highly nutrient-loaded hypereutrophic lake in Massac County, Illinois with an estimated average depth of about 29 feet. Limited fish-species data available. Public access is available.
Source: EPA Water Quality Portal sampling records, Illinois EPA, last sampled 2021-10-18. Methodology: how these readings are compared.
Swimming conditions
Readings onlyLimited data
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. Mermet Lake is listed impaired under Clean Water Act §303(d) (IR 5) for nutrients (phosphorus, nitrogen). Check local advisories before you swim.
Whether Mermet Lake is safe to swim in today is a call for the Illinois Environmental Protection Agency and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Mermet Lake
These are the measurements Illinois EPA publishes for Mermet Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Very murky, less than 1.6 ft of visibility. Phosphorus 131 µg/L. Chlorophyll-a 88.5 µg/L. TSI 73.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 1.6ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 131µg/L | Poor GoodFairPoor |
| Chlorophyll-a (algae) | 88.5µg/L | Poor GoodFairPoor |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 73 | Hypereutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Very high nutrients, dense algae, poor clarity
Ask about Mermet Lake
Answers come straight from the public measurements and survey records on this page.
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.
Column height runs from this lake’s coldest month (62°F) to its warmest (83°F).
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
Max depth
~29 ft est.
Surface area
1 acres
Public access
Yes
Listed by Illinois EPA
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
Watershed (drainage) area
9 sq mi
HydroLAKES / HydroSHEDS
Shoreline
5.2 mi
HydroLAKES / HydroSHEDS
Elevation
328 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 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 readings cover nutrients and algae, not lodging. Cabins and rentals around Mermet Lake are listed below.
Top-rated places to stay near Mermet Lake
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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
Designated uses not supporting: Other.
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.
Nearby Lakes in Massac County
Frequently asked questions about Mermet Lake
Where is Mermet Lake?
- Mermet Lake is a lake in Massac County, Illinois. Its water-quality measurements are published by Illinois EPA and shown on this page as reported.
What is the water quality of Mermet Lake?
- Mermet Lake in Massac County, Illinois has water-quality measurements published by Illinois EPA, shown here as reported. 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?
- Nutrient and algae readings are 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.
Can I fish at Mermet Lake?
- Public access to Mermet Lake is listed as yes on the lake record. Seasons, size and bag limits, and whether you need a license are set by Illinois's fish and wildlife agency, so check its current regulations before you go.
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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)
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
Impairment status from EPA ATTAINS 303(d) database
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)