Forbes Lake is a productive, nutrient-rich eutrophic lake in Marion County, Illinois. Water quality is graded D based on 2 sampling years through 2021. Limited fish-species data available. Public access is available with 2 public boat landings. 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 DNR, last sampled 2021-10-21. Grade methodology: LakeGrade methodology.
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Swimming Safety
Swimming not recommended, poor water quality with high algae risk
Water Quality Grade: D, Poor
Very murky, less than 1.4 ft of visibility. Phosphorus level: 74 µg/L. Trophic State Index: 69.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.4 ft | F |
| Phosphorus | 74 µg/L | D |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 69 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Where to stay near Forbes Lake
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Fishing Forbes Lake
Recreation & Access
Water Temperature
Forbes Lake averages about 80°F at the surface in summer, 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 →
Location
County Ranking
Ranked #2 of 6 lakes in Marion County
Cleaner Lakes Within 30 Miles
Forbes Lake holds Grade D. 2 nearby lakes hold higher grades.
- DKinmundy New LakeMarion County · 6.7 mi · Higher overall water quality score
- DWayne City LakeWayne County · 26.9 mi · Higher overall water quality score
Nearby Lakes in Marion County
Explore Forbes Lake across LakeGrade
More Illinois Lake Rankings
Other Grade D Lakes in Illinois
Eutrophic Lakes in Illinois
More records, surveys & data sources
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)
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
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.
Data Sources
Water quality data from the EPA Water Quality Portal
Impairment status from EPA ATTAINS 303(d) database
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grading methodology based on Metropolitan Council standards
Public boat-access points from the USGS National Boat Ramps inventory
Most recent sample: 2021-10-21
Monitoring stations: 3