Forbes Lake
Marion County, IllinoisView mapDirections
Illinois EPA publishes the water-quality measurements for Forbes Lake. Here is what has been measured.
- Hypereutrophic
- 55 ft max depth
- Public access
- Limited data
Forbes Lake in Marion County, Illinois has water-quality measurements published by Illinois EPA, shown here as reported. Water clarity: 1.4 ft visibility. Phosphorus: 140.5 µg/L. Chlorophyll-a (algae): 75.9 µg/L.
Forbes Lake is a highly nutrient-loaded hypereutrophic lake in Marion County, Illinois with an estimated average depth of about 55 feet. 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. Methodology: how these readings are compared.
Forbes Lake has one public boat landing. Vrbo is where to look for cabins and lake homes around Forbes Lake, and Booking.com and Expedia search hotels near the lake for your dates.
What Forbes Lake is good for
From access and monitoring records, not editorial picks.
- Boat launch1 documented ramp
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. Check local advisories before you swim.
Whether Forbes 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 Forbes Lake
These are the measurements Illinois EPA publishes for Forbes 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 1.4 ft of visibility. Phosphorus 140.5 µg/L. Chlorophyll-a 75.9 µg/L. TSI 74.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 1.4ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 141µg/L | Poor GoodFairPoor |
| Chlorophyll-a (algae) | 75.9µg/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. | 74 | Hypereutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
Very high nutrients, dense algae, poor clarity
Ask about Forbes Lake
Answers come straight from the public measurements and survey records on this page.
Water Temperature
Forbes Lake typically sits around 64°F at the surface in October. 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.
Column height runs from this lake’s coldest month (62°F) to its warmest (82°F).
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
Max depth
~55 ft est.
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 Forbes Lake.
Lake Size & Watershed
Watershed (drainage) area
21 sq mi
HydroLAKES / HydroSHEDS
Surface area
383 acres
HydroLAKES / HydroSHEDS
Shoreline
7.9 mi
HydroLAKES / HydroSHEDS
Elevation
522 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.
Recreation & Access
- Forbes LakePublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Stays via Stay22
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 readings cover nutrients and algae, not lodging. Cabins and rentals around Forbes Lake are listed below.
Top-rated places to stay near Forbes Lake
Prices for Oct 23–25, 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 trip? Cabins and lodging at Forbes Lake → · more Illinois lake trips
Follow Forbes Lake’s readings
Email me when new measurements post for this lake. Updates stay free.
Or email
Fishing Forbes Lake
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.
Nearby Lakes in Marion County
Frequently asked questions about Forbes Lake
Where is Forbes Lake?
- Forbes Lake is a lake in Marion County, Illinois. Its water-quality measurements are published by Illinois EPA and shown on this page as reported.
What is the water quality of Forbes Lake?
- Forbes Lake in Marion County, Illinois has water-quality measurements published by Illinois EPA, shown here as reported. 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?
- Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
What is the water temperature of Forbes Lake?
- Forbes Lake typically sits around 64°F at the surface in October, 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.
Can I fish at Forbes Lake?
- Forbes Lake has 1 documented boat landing in the USGS National Boat Ramps inventory. 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.
Explore Forbes Lake across LakeQuality
More lakes in Illinois
Other 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)
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
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