Decatur Lake
Macon County, Illinois3,335 acresView mapDirections
Nutrients and algae run above the regional average.
↑ up from F at the last regrade
- Eutrophic
- Impaired · Mercury
- #1 of 1 in the county
- Public access
- Limited data
Decatur Lake in Macon County, Illinois has a water quality grade of D (Nutrient-rich). Water clarity: 1.3 ft visibility. Phosphorus: 103 µg/L. Chlorophyll-a (algae): 25 µg/L. Surface area: 3335 acres.
- Clarity
- 1.3 ft
- Area
- 3,335 acres
- Type
- Eutrophic
- Phosphorus
- 103 µg/L
- Chlorophyll-a
- 25 µg/L
- Public access
- Yes
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Stays near Decatur Lake
Compare lakeside cabins, hotels, and vacation rentals near Decatur Lake in Macon 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 grade measures nutrients and algae, not lodging — cabins and rentals around Decatur Lake are listed below.
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Decatur Lake is a 3,335-acre productive, nutrient-rich eutrophic lake in Macon County, Illinois. Water quality is graded D based on 2 sampling years through 2022. Limited fish-species data available. Public access is available with 1 public boat landing.
Source: EPA Water Quality Portal sampling records, Illinois EPA, last sampled 2022-10-13. Grade methodology: LakeQuality methodology.
What Decatur Lake is good for
From access and monitoring records, not editorial picks.
- Boat launch1 documented ramp
- Big water3,335 acres
Swimming Safety
Grade onlyLimited data
Based on clarity and algae grades from past sampling — we do not hold live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
The D is nutrients and algae, not a swimming clearance. Decatur Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, metals (other than mercury), nutrients (phosphorus, nitrogen), pesticides, pCBs. Check local advisories before you swim.
Water Quality Grade: D (Nutrient-rich)
Very murky, less than 1.3 ft of visibility. Phosphorus 103 µg/L. Chlorophyll-a 25 µg/L. TSI 69. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.3 ft | Not graded |
| Phosphorus | 103 µg/L | Poor |
| Chlorophyll-a (Algae) | 25 µg/L | Poor |
| Trophic State Index (TSI) | 69 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 3,335 acres |
| Public Access | Yes |
No bathymetric survey has been published for Decatur Lake, so its depth is not documented here.
Recreation & Access
- Lake DecaturPublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Fishing Decatur Lake
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.
County Ranking
Ranked #1 of 1 lakes in Macon County
Nearby Lakes in Macon County
Frequently asked questions about Decatur Lake
Where is Decatur Lake?
- Decatur Lake is a lake in Macon County, Illinois. LakeQuality grades it D for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Decatur Lake?
- Decatur Lake in Macon County, Illinois has a water quality grade of D (Nutrient-rich). Water clarity: 1.3 ft visibility. Phosphorus: 103 µg/L. Chlorophyll-a (algae): 25 µg/L. Surface area: 3335 acres.
Is Decatur Lake safe to swim in?
- The D is nutrients and algae, not a swimming clearance. Decatur Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, metals (other than mercury), nutrients (phosphorus, nitrogen), pesticides, pCBs. Check local advisories before you swim.
Are there boat ramps on Decatur Lake?
- Decatur Lake has 1 documented boat landing in the USGS National Boat Ramps inventory. Named access points include Lake Decatur. Public access is listed as yes.
Explore Decatur Lake across LakeQuality
Depth, room, public access, and the state wake-sport rules
Species activity outlook by water temperature and season
Where to stay at Illinois's top lakes, each with its grade
High nutrients, frequent algae, reduced clarity
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Where this data comes from
48 measurements from 3 monitoring stations, 2022. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- illinois epa
IL_EPA_WQX48 samples · 2022
Analytical methods
- MEASURED IN FIELD
- Phosphorus by Colorimetry
- 10200 H 2 ~ Chlorophyll a-b-c Determination by spectrophotometer
Monitoring station identifiers (3)
Grade thresholds from the EPA National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See how grades are calculated and data sources.
Other records on this page
Impairment status from EPA ATTAINS 303(d) database
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Named boat-access points from the USGS National Boat Ramps inventory