Forbes Lake
Marion County, IllinoisView mapDirections
Nutrients and algae run well above the regional average.
↓ down from D at the last regrade
- Hypereutrophic
- Impaired · Mercury
- #2 of 6 in the county
- 55 ft max depth
- Public access
- Limited data
Forbes Lake in Marion County, Illinois has a water quality grade of F (Algae-prone). Water clarity: 1.4 ft visibility. Phosphorus: 140.5 µg/L. Chlorophyll-a (algae): 75.9 µg/L.
- Clarity
- 1.4 ft
- Typical in August
- 82°F
- Max depth
- ~55 ft est.
- Type
- Hypereutrophic
- Public access
- Yes
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Stays near Forbes Lake
Compare lakeside cabins, hotels, and vacation rentals near Forbes Lake in Marion 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 Forbes Lake are listed below.
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Forbes Lake is a highly nutrient-loaded hypereutrophic lake in Marion County, Illinois with an estimated average depth of about 55 feet. Water quality is graded F based on 2 sampling years through 2021. Limited fish-species data available. Public access is available with 1 public boat landing. Compared to the 5 other monitored lakes in Marion County, Forbes Lake ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, Illinois EPA, last sampled 2021-10-21. Grade methodology: LakeQuality methodology.
What Forbes Lake is good for
From access and monitoring records, not editorial picks.
- Boat launch1 documented ramp
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. Forbes Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, nutrients (phosphorus, nitrogen). Check local advisories before you swim.
Water Quality Grade: F (Algae-prone)
Very murky, less than 1.4 ft of visibility. Phosphorus 140.5 µg/L. Chlorophyll-a 75.9 µg/L. TSI 74. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.4 ft | Not graded |
| Phosphorus | 140.5 µg/L | Poor |
| Chlorophyll-a (Algae) | 75.9 µg/L | Poor |
| Trophic State Index (TSI) | 74 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Water Temperature
Forbes Lake typically sits around 82°F at the surface in August. Summer average is about 80°F. Usually warmest in Aug (about 82°F) and coldest in May (about 62°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 106 field readings (2011–2021) via the EPA Water Quality Portal. See the full Forbes Lake water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~55 ft est. |
| Average Depth | ~41 ft est. |
| 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 Forbes 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.
Recreation & Access
- Forbes LakePublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Fishing Forbes Lake
EPA Impairment Status
Forbes Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
A Total Maximum Daily Load (TMDL) plan has been approved.
Source: EPA ATTAINS assessment unit IL_RCD · Official waterbody report · Matched by proximity (0.53 km)
Regulated Pollution Sources Nearby
2 EPA-permitted facilities operate within about 5 miles of Forbes Lake under Clean Water Act (NPDES) discharge permits. 2 facilities have a reported Clean Water Act violation in the past three years.
- Stephen Forbes State Park - Shower Bldg StpKinmundy · 1.3 mi away · NPDES IL0068977Recent violation
- Stephen Forbes State Park - Concession BldgKinmundy · 1.3 mi away · NPDES IL0073903Recent violation
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 #2 of 6 lakes in Marion County
Nearby Lakes in Marion County
Frequently asked questions about Forbes Lake
Where is Forbes Lake?
- Forbes Lake is a lake in Marion County, Illinois. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Forbes Lake?
- Forbes Lake in Marion County, Illinois has a water quality grade of F (Algae-prone). Water clarity: 1.4 ft visibility. Phosphorus: 140.5 µg/L. Chlorophyll-a (algae): 75.9 µg/L.
Is Forbes Lake safe to swim in?
- The F is nutrients and algae, not a swimming clearance. Forbes Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, nutrients (phosphorus, nitrogen). Check local advisories before you swim.
What is the water temperature of Forbes Lake?
- Forbes Lake typically sits around 82°F at the surface in August, from 106 EPA Water Quality Portal field readings (2011–2021). That is a typical value for the month, not a live reading.
How deep is Forbes Lake?
- No published bathymetric survey covers Forbes Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 55 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate.
Are there boat ramps on Forbes Lake?
- Forbes Lake has 1 documented boat landing in the USGS National Boat Ramps inventory. Named access points include Forbes Lake. Public access is listed as yes.
Explore Forbes 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)
Named boat-access points from the USGS National Boat Ramps inventory