Decatur Lake
Macon County, Illinois3,335 acresView mapDirections
Illinois EPA publishes the water-quality measurements for Decatur Lake. Here is what has been measured.
- Eutrophic
- Public access
- Limited data
Decatur Lake in Macon County, Illinois has water-quality measurements published by Illinois EPA, shown here as reported. Water clarity: 1.3 ft visibility. Phosphorus: 103 µg/L. Chlorophyll-a (algae): 25 µg/L. Surface area: 3335 acres.
Decatur Lake is a 3,335-acre productive, nutrient-rich eutrophic lake in Macon County, Illinois. 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. Methodology: how these readings are compared.
Decatur Lake covers about 3,300 acres in Macon County. It has one public boat landing. If you are planning to stay, you can search lake houses and cabins around Decatur Lake on Vrbo, or look for a hotel room near the lake on Booking.com or Expedia.
What Decatur Lake is good for
From access and monitoring records, not editorial picks.
- Boat launch1 documented ramp
- Big water3,335 acres
Swimming conditions
Readings onlyLimited data
Based on clarity and algae readings from past sampling — there is no live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
Whether Decatur Lake is safe to swim in today is a call for the Illinois Environmental Protection Agency and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Decatur Lake
These are the measurements Illinois EPA publishes for Decatur Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Very murky, less than 1.3 ft of visibility. Phosphorus 103 µg/L. Chlorophyll-a 25 µg/L. TSI 69.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 1.3ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 103µg/L | Poor GoodFairPoor |
| Chlorophyll-a (algae) | 25µg/L | Poor GoodFairPoor |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 69 | Eutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
High nutrients, frequent algae, reduced clarity
Ask about Decatur Lake
Answers come straight from the public measurements and survey records on this page.
Lake Details
Surface area
3,335 acres
Public access
Yes
Listed by Illinois EPA
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
Stays via Stay22
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 readings cover nutrients and algae, not lodging. Cabins and rentals around Decatur Lake are listed below.
Top-rated places to stay near Decatur Lake
Prices for Oct 23–25, 4 adults. Change dates on the map below.
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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
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.
Nearby Lakes in Macon County
Frequently asked questions about Decatur Lake
Where is Decatur Lake?
- Decatur Lake is a lake in Macon County, Illinois. Its water-quality measurements are published by Illinois EPA and shown on this page as reported.
What is the water quality of Decatur Lake?
- Decatur Lake in Macon County, Illinois has water-quality measurements published by Illinois EPA, shown here as reported. 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?
- Nutrient and algae readings are not a swimming clearance. 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.
Can I fish at Decatur Lake?
- Decatur Lake has 1 documented boat landing in the USGS National Boat Ramps inventory. Seasons, size and bag limits, and whether you need a license are set by Illinois's fish and wildlife agency, so check its current regulations before you go.
Explore Decatur Lake across LakeQuality
Depth, room, public access, and the state wake-sport rules
Species activity outlook by water temperature and season
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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)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from and data sources.
Other records on this page
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Named boat-access points from the USGS National Boat Ramps inventory