Otter Lake
Macoupin County, Illinois760 acresView mapDirections
Nutrients and algae run well above the regional average.
↓ down from D at the last regrade
- Hypereutrophic
- Impaired · Mercury
- #2 of 3 in the county
- 48 ft max depth
- Public access
- Limited data
Otter Lake in Macoupin County, Illinois has a water quality grade of F (Algae-prone). Water clarity: 1.8 ft visibility. Phosphorus: 81 µg/L. Chlorophyll-a (algae): 33.1 µg/L. Surface area: 760 acres.
- Clarity
- 1.8 ft
- Typical in August
- 75°F
- Max depth
- ~48 ft est.
- Area
- 760 acres
- Type
- Hypereutrophic
- Phosphorus
- 81 µg/L
- Chlorophyll-a
- 33.1 µg/L
- Public access
- Yes
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Stays near Otter Lake
Compare lakeside cabins, hotels, and vacation rentals near Otter Lake in Macoupin 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 Otter Lake are listed below.
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Otter Lake is a 760-acre highly nutrient-loaded hypereutrophic lake in Macoupin County, Illinois with an estimated average depth of about 48 feet. Water quality is graded F 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 EPA, last sampled 2022-11-02. Grade methodology: LakeQuality methodology.
What Otter Lake is good for
From access and monitoring records, not editorial picks.
- Boat launch1 documented ramp
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 F is nutrients and algae, not a swimming clearance. Otter Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, nutrients (phosphorus, nitrogen). Check local advisories before you swim.
Water Quality Grade: F (Algae-prone)
Very murky, less than 1.8 ft of visibility. Phosphorus 81 µg/L. Chlorophyll-a 33.1 µg/L. TSI 67. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.8 ft | Not graded |
| Phosphorus | 81 µg/L | Poor |
| Chlorophyll-a (Algae) | 33.1 µg/L | Poor |
| Trophic State Index (TSI) | 67 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Water Temperature
Otter Lake typically sits around 75°F at the surface in August. Summer average is about 75°F. 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 →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~48 ft est. |
| Average Depth | ~37 ft est. |
| Surface Area | 760 acres |
| Public Access | Yes |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Illinois has not published bathymetry for Otter Lake.
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth here is modelled, not surveyed — HydroLAKES v1.0 estimates lake depth from surrounding terrain rather than from soundings. Treat it as an approximation, and prefer a state bathymetric survey where one exists.
Recreation & Access
- Otter Lake ParkPublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Fishing Otter Lake
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
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.
County Ranking
Ranked #2 of 3 lakes in Macoupin County
Nearby Lakes in Macoupin County
Frequently asked questions about Otter Lake
Where is Otter Lake?
- Otter Lake is a lake in Macoupin County, Illinois. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Otter Lake?
- Otter Lake in Macoupin County, Illinois has a water quality grade of F (Algae-prone). Water clarity: 1.8 ft visibility. Phosphorus: 81 µg/L. Chlorophyll-a (algae): 33.1 µg/L. Surface area: 760 acres.
Is Otter Lake safe to swim in?
- The F is nutrients and algae, not a swimming clearance. Otter Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, nutrients (phosphorus, nitrogen). Check local advisories before you swim.
What is the water temperature of Otter Lake?
- Otter Lake typically sits around 75°F at the surface in August, from 235 EPA Water Quality Portal field readings (2011–2022). That is a typical value for the month, not a live reading.
How deep is Otter Lake?
- No published bathymetric survey covers Otter Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 48 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate. The lake covers 760 acres.
Are there boat ramps on Otter Lake?
- Otter Lake has 1 documented boat landing in the USGS National Boat Ramps inventory. Named access points include Otter Lake Park. Public access is listed as yes.
Explore Otter 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
Very high nutrients, dense algae, poor clarity
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Where this data comes from
60 measurements from 4 monitoring stations, 2022. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- illinois epa
IL_EPA_WQX60 samples · 2022
Analytical methods
- MEASURED IN FIELD
- Phosphorus by Colorimetry
- 10200 H 2 ~ Chlorophyll a-b-c Determination by spectrophotometer
Monitoring station identifiers (4)
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)
Lake size & watershed from HydroLAKES (HydroSHEDS)
Named boat-access points from the USGS National Boat Ramps inventory