Otter Lake is a 760-acre productive, nutrient-rich eutrophic lake in Macoupin 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. Compared to the 2 other monitored lakes in Macoupin County, Otter Lake ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, Illinois DNR, last sampled 2022-11-02. Grade methodology: LakeGrade methodology.
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Swimming Safety
Swimming not recommended, poor water quality with high algae risk
Water Quality Grade: D, Poor
Very murky, less than 1.8 ft of visibility. Phosphorus level: 37 µg/L. Trophic State Index: 62.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.8 ft | F |
| Phosphorus | 37 µg/L | C |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 62 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 760 acres |
| Public Access | Yes |
Where to stay near Otter Lake
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Fishing Otter Lake
Recreation & Access
Water Temperature
Otter Lake averages about 75°F at the surface in summer, usually warmest in Jul (about 76°F) and coldest in Nov (about 57°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 235 field readings (2011–2022) via the EPA Water Quality Portal. See the full Otter Lake water temperature breakdown →
Location
County Ranking
Ranked #2 of 3 lakes in Macoupin County
Nearby Lakes in Macoupin County
Explore Otter Lake across LakeGrade
More Illinois Lake Rankings
Other Grade D Lakes in Illinois
Eutrophic Lakes in Illinois
Compare Otter Lake With Nearby Lakes
More records, surveys & data sources
Reservoir Info (USACE NID)
Otter Lake is a man-made reservoir impounded by the Otter Lake Dam (completed 1969), built primarily for recreation on the WEST FORK OTTER CREEK; earth-type dam, 71 ft tall and 1,010 ft long.
- Surface area
- 760 ac
- Normal storage
- 15,563 ac-ft
- Max storage
- 24,708 ac-ft
- Drainage area
- 20.2 sq mi
- Hazard class
- Significant
- Owner
- OTTER LAKE WATER COMMISSION
All listed purposes: Recreation;Water Supply.
Source: USACE National Inventory of Dams, NID ID IL00133 · Operator website
EPA Impairment Status
Otter 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_RDF · Official waterbody report
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
Regulated Pollution Sources Nearby
2 EPA-permitted facilities operate within about 5 miles of Otter Lake under Clean Water Act (NPDES) discharge permits.
- Otter Lake Wtr Commission WtpGirard · 0.4 mi away · NPDES IL0042552
- Christopher Bruley Private Sewage Disposal SystemGirard · 2.7 mi away · NPDES IL0080167
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)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Grading methodology based on Metropolitan Council standards
Public boat-access points from the USGS National Boat Ramps inventory
Most recent sample: 2022-11-02
Monitoring stations: 4