Mauvaise Terre Lake is a 262-acre highly nutrient-loaded hypereutrophic lake in Morgan County, Illinois. Water quality is graded F based on 1 sampling years through 2021. Compared to the 1 other monitored lakes in Morgan County, Mauvaise Terre Lake ranks #2 for water quality.
Source: EPA Water Quality Portal sampling records, Illinois DNR, last sampled 2021-10-04. 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
No clarity data. Phosphorus level: 162 µg/L. Trophic State Index: 78.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No data | |
| Phosphorus | 162 µg/L | F |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 78 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Lake Details
| Characteristic | Value |
|---|---|
| Surface Area | 262 acres |
Where to stay near Mauvaise Terre Lake
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Fishing Mauvaise Terre Lake
Water Temperature
Mauvaise Terre Lake averages about 79°F at the surface in summer, usually warmest in Jul (about 84°F) and coldest in Apr (about 57°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 38 field readings (2011–2021) via the EPA Water Quality Portal. See the full Mauvaise Terre Lake water temperature breakdown →
Location
County Ranking
Ranked #2 of 2 lakes in Morgan County
Cleaner Lakes Within 30 Miles
Mauvaise Terre Lake holds Grade F. 2 nearby lakes hold higher grades.
Nearby Lakes in Morgan County
Explore Mauvaise Terre Lake across LakeGrade
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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Other Grade F Lakes in Illinois
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More records, surveys & data sources
Reservoir Info (USACE NID)
Mauvaise Terre Lake is a man-made reservoir impounded by the Mauvaise Terre Lake Dam (completed 1923), built primarily for recreation on the MAUVAISE TERRE CREEK; earth-type dam, 30 ft tall and 850 ft long.
- Surface area
- 262 ac
- Normal storage
- 628 ac-ft
- Max storage
- 5,624 ac-ft
- Drainage area
- 34.3 sq mi
- Hazard class
- High
- Owner
- CITY OF JACKSONVILLE
All listed purposes: Recreation;Water Supply.
Source: USACE National Inventory of Dams, NID ID IL00709 · Operator website
EPA Impairment Status
Mauvaise Terre 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_SDL · Official waterbody report
Lake Size & Watershed
Size, shoreline, and watershed area from HydroLAKES (HydroSHEDS). Mean depth is a modeled estimate.
Regulated Pollution Sources Nearby
1 EPA-permitted facility operate within about 5 miles of Mauvaise Terre Lake under Clean Water Act (NPDES) discharge permits, including 1 classified by the EPA as major discharger. 1 facility has a reported Clean Water Act violation in the past three years.
- Jacksonville Stp, City OfJacksonville · 2.4 mi away · NPDES IL0021661MajorRecent violation
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
Most recent sample: 2021-10-04
Monitoring stations: 1