Decatur Lake
Macon County, WisconsinHypereutrophic
Decatur Lake is a 3,335-acre highly nutrient-loaded hypereutrophic lake in Macon County, IL. Water quality is graded F based on 2 sampling years through 2022.
Source: EPA Water Quality Portal sampling records, local state DNR records, last sampled 2022-10-13. Grade methodology: LakeGrade methodology.
Swimming Safety
Avoid swimming, very poor water quality, potential algae toxins
Water Quality Grade: F, Very Poor
Very murky, less than 1.3 ft of visibility. Phosphorus level: 90 µg/L. Trophic State Index: 71.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.3 ft | F |
| Phosphorus | 90 µg/L | F |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 71 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 3.3K acres |
Location
County Ranking
Ranked #1 of 1 lakes in Macon County
Nearby Lakes in Macon County
More Wisconsin Lake Rankings
Other Grade F Lakes in Wisconsin
Hypereutrophic Lakes in Wisconsin
Reservoir Info (USACE NID)
Decatur Lake is a man-made reservoir impounded by the Lake Decatur Dam (completed 1922), built primarily for recreation on the SANGAMON RIVER; earth-type dam, 33 ft tall and 1,728 ft long.
- Surface area
- 3,335 ac
- Normal storage
- 18,420 ac-ft
- Max storage
- 57,520 ac-ft
- Drainage area
- 906 sq mi
- Hazard class
- High
- Owner
- CITY OF DECATUR
All listed purposes: Recreation;Water Supply.
Source: USACE National Inventory of Dams, NID ID IL00146 · Operator website
EPA Impairment Status
Decatur 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_REA · Official waterbody report
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: 2022-10-13
Monitoring stations: 3