Forbes Lake
Marion County, WisconsinEutrophic
Forbes Lake is a productive, nutrient-rich eutrophic lake in Marion County, IL. Water quality is graded D based on 2 sampling years through 2021. Compared to the 5 other monitored lakes in Marion County, Forbes Lake ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, local state DNR records, last sampled 2021-10-21. Grade methodology: LakeGrade methodology.
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
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
More Wisconsin Lake Rankings
Other Grade D Lakes in Wisconsin
Eutrophic Lakes in Wisconsin
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)
Data Sources
Water quality data from the EPA Water Quality Portal
Impairment status from EPA ATTAINS 303(d) database
Grading methodology based on Metropolitan Council standards
Most recent sample: 2021-10-21
Monitoring stations: 3