Decatur Lake is a 3,335-acre highly nutrient-loaded hypereutrophic lake in Macon County, Illinois. Water quality is graded F based on 2 sampling years through 2022. Limited fish-species data available. Public access is available with 5 public boat landings.
Source: EPA Water Quality Portal sampling records, Illinois DNR, last sampled 2022-10-13. Grade methodology: LakeGrade methodology.
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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 | 3,335 acres |
| Public Access | Yes |
Where to stay near Decatur Lake
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Fishing Decatur Lake
Recreation & Access
Location
County Ranking
Ranked #1 of 1 lakes in Macon County
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Very high nutrients, dense algae, poor clarity
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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
Regulated Pollution Sources Nearby
9 EPA-permitted facilities operate within about 5 miles of Decatur Lake under Clean Water Act (NPDES) discharge permits. 5 facilities have a reported Clean Water Act violation in the past three years.
- Primary Products Ingredients Americas, LlcDecatur · 0.4 mi away · NPDES IL0002381Recent violation
- Archer Daniels Midland Co - West ComplexDecatur · 0.7 mi away · NPDES IL0038113Recent violation
- Samuel Johnson Private Sewage Disposal SystemDecatur · 2.2 mi away · NPDES IL0080263Recent violation
- North Water Treatment PlantDecatur · 2.9 mi away · NPDES IL0060755Recent violation
- Fuyao Glass Illinois IncMount Zion · 4.3 mi away · NPDES IL0001791Recent violation
- Archer Daniels Midland East ComplexDecatur · 2.1 mi away · NPDES IL0061425
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)
Grading methodology based on Metropolitan Council standards
Public boat-access points from the USGS National Boat Ramps inventory
Most recent sample: 2022-10-13
Monitoring stations: 3