Mauvaise Terre Lake
Morgan County, Illinois262 acresView mapDirections
Nutrients and algae run well above the regional average.
- Hypereutrophic
- Impaired · Mercury
- #2 of 2 in the county
- 29 ft max depth
- Limited data
Mauvaise Terre Lake in Morgan County, Illinois has a water quality grade of F (Algae-prone). Phosphorus: 163.5 µg/L. Chlorophyll-a (algae): 62 µg/L. Surface area: 262 acres.
- Typical in August
- 82°F
- Max depth
- ~29 ft est.
- Area
- 262 acres
- Type
- Hypereutrophic
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Stays near Mauvaise Terre Lake
Compare lakeside cabins, hotels, and vacation rentals near Mauvaise Terre Lake in Morgan 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 Mauvaise Terre Lake are listed below.
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Mauvaise Terre Lake is a 262-acre highly nutrient-loaded hypereutrophic lake in Morgan County, Illinois with an estimated average depth of about 29 feet. Water quality is graded F based on 2 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 EPA, last sampled 2021-10-04. Grade methodology: LakeQuality methodology.
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. Mauvaise Terre Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, metals (other than mercury), nutrients (phosphorus, nitrogen), turbidity. Check local advisories before you swim.
Water Quality Grade: F (Algae-prone)
No clarity data. Phosphorus 163.5 µg/L. Chlorophyll-a 62 µg/L. TSI 74. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | No recent samples | |
| Phosphorus | 163.5 µg/L | Poor |
| Chlorophyll-a (Algae) | 62 µg/L | Poor |
| Trophic State Index (TSI) | 74 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Water Temperature
Mauvaise Terre Lake typically sits around 82°F at the surface in August. Summer average is about 79°F. 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 →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~29 ft est. |
| Average Depth | ~9 ft est. |
| Surface Area | 262 acres |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Illinois has not published bathymetry for Mauvaise Terre 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.
Fishing Mauvaise Terre Lake
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
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.
County Ranking
Ranked #2 of 2 lakes in Morgan County
Nearby Lakes in Morgan County
Frequently asked questions about Mauvaise Terre Lake
Where is Mauvaise Terre Lake?
- Mauvaise Terre Lake is a lake in Morgan County, Illinois. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Mauvaise Terre Lake?
- Mauvaise Terre Lake in Morgan County, Illinois has a water quality grade of F (Algae-prone). Phosphorus: 163.5 µg/L. Chlorophyll-a (algae): 62 µg/L. Surface area: 262 acres.
Is Mauvaise Terre Lake safe to swim in?
- The F is nutrients and algae, not a swimming clearance. Mauvaise Terre Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, metals (other than mercury), nutrients (phosphorus, nitrogen), turbidity. Check local advisories before you swim.
What is the water temperature of Mauvaise Terre Lake?
- Mauvaise Terre Lake typically sits around 82°F at the surface in August, from 38 EPA Water Quality Portal field readings (2011–2021). That is a typical value for the month, not a live reading.
How deep is Mauvaise Terre Lake?
- No published bathymetric survey covers Mauvaise Terre Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 29 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 262 acres.
Explore Mauvaise Terre 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
20 measurements from 1 monitoring station, 2021. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- illinois epa
IL_EPA_WQX20 samples · 2021
Analytical methods
- Phosphorus by Colorimetry
- 10200 H 2 ~ Chlorophyll a-b-c Determination by spectrophotometer
Monitoring station identifiers (1)
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)