Boulder South Lake is a mesotrophic lake in Perry County, Illinois. Water quality is graded B based on 2 sampling years through 2021. Limited fish-species data available. Public access is available with 5 public boat landings. Compared to the 1 other monitored lakes in Perry County, Boulder South Lake ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Illinois DNR, last sampled 2021-10-12. Grade methodology: LakeGrade methodology.
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Swimming Safety
Good for swimming, clear water with low algae levels
Water Quality Grade: B, Good
Moderate clarity, visible to about 8.8 ft. Phosphorus level: 14.5 µg/L. Trophic State Index: 44.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 8.8 ft | C |
| Phosphorus | 14.5 µg/L | A |
| Chlorophyll-a (Algae) | No data | |
| Trophic State Index (TSI) | 44 | Mesotrophic |
Moderate nutrients, good water quality
Where to stay near Boulder South Lake
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Fishing Boulder South Lake
Recreation & Access
Water Temperature
Boulder South Lake averages about 67°F at the surface in summer, usually warmest in Jul (about 69°F) and coldest in Apr (about 56°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 347 field readings (2016–2021) via the EPA Water Quality Portal. See the full Boulder South Lake water temperature breakdown →
Location
County Ranking
Ranked #1 of 2 lakes in Perry County
Nearby Lakes in Perry County
Explore Boulder South Lake across LakeGrade
More Illinois Lake Rankings
Other Grade B Lakes in Illinois
Mesotrophic Lakes in Illinois
More records, surveys & data sources
EPA Impairment Status
Boulder South Lake is officially listed as impaired under Clean Water Act §303(d) in the 2024 EPA reporting cycle (IR category 5).
Causes of impairment
Source: EPA ATTAINS assessment unit IL_RNZK · Official waterbody report
Regulated Pollution Sources Nearby
1 EPA-permitted facility operate within about 5 miles of Boulder South Lake under Clean Water Act (NPDES) discharge permits. 1 facility has a reported Clean Water Act violation in the past three years.
- Knight Hawk Coal LlcPercy · 1 mi away · NPDES IL0080064Recent 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)
Grading methodology based on Metropolitan Council standards
Public boat-access points from the USGS National Boat Ramps inventory
Most recent sample: 2021-10-12
Monitoring stations: 3