Boulder South Lake
Perry County, IllinoisView mapDirections
Clarity, phosphorus, and algae sit in the clean band for this region.
- Eutrophic
- Impaired · Mercury
- #1 of 2 in the county
- Public access
- Limited data
Boulder South Lake in Perry County, Illinois has a water quality grade of B (Good). Water clarity: 8.8 ft visibility. Phosphorus: 17.5 µg/L. Chlorophyll-a (algae): 13.5 µg/L.
- Clarity
- 8.8 ft
- Typical in October
- 67°F
- Type
- Eutrophic
- Public access
- Yes
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Watch Boulder South Lake’s letter
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Boulder South Lake is a productive, nutrient-rich eutrophic 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 1 public boat landing. 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 EPA, last sampled 2021-10-12. Grade methodology: LakeQuality methodology.
What Boulder South 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 B is nutrients and algae, not a swimming clearance. Boulder South Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury. Check local advisories before you swim.
Water Quality Grade: B (Good)
Moderate clarity, visible to about 8.8 ft. Phosphorus 17.5 µg/L. Chlorophyll-a 13.5 µg/L. TSI 49. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 8.8 ft | Not graded |
| Phosphorus | 17.5 µg/L | Good |
| Chlorophyll-a (Algae) | 13.5 µg/L | Good |
| Trophic State Index (TSI) | 49 | Eutrophic |
High nutrients, frequent algae, reduced clarity
Water Temperature
Boulder South Lake typically sits around 67°F at the surface in October. Summer average is about 67°F. 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 →
Recreation & Access
- Pyramid State ParkPublic
Named access points in the USGS National Boat Ramps inventory, within about 3 miles. More lakes with boat ramps
Fishing Boulder South Lake
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.
County Ranking
Ranked #1 of 2 lakes in Perry County
Nearby Lakes in Perry County
Frequently asked questions about Boulder South Lake
Where is Boulder South Lake?
- Boulder South Lake is a lake in Perry County, Illinois. LakeQuality grades it B for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Boulder South Lake?
- Boulder South Lake in Perry County, Illinois has a water quality grade of B (Good). Water clarity: 8.8 ft visibility. Phosphorus: 17.5 µg/L. Chlorophyll-a (algae): 13.5 µg/L.
Is Boulder South Lake safe to swim in?
- The B is nutrients and algae, not a swimming clearance. Boulder South Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury. Check local advisories before you swim.
What is the water temperature of Boulder South Lake?
- Boulder South Lake typically sits around 67°F at the surface in October, from 347 EPA Water Quality Portal field readings (2016–2021). That is a typical value for the month, not a live reading.
Are there boat ramps on Boulder South Lake?
- Boulder South Lake has 1 documented boat landing in the USGS National Boat Ramps inventory. Named access points include Pyramid State Park. Public access is listed as yes.
Explore Boulder South Lake across LakeQuality
More Illinois Lake Rankings
Other Grade B Lakes in Illinois
Eutrophic Lakes in Illinois
Where this data comes from
52 measurements from 3 monitoring stations, 2021. Retrieved from the EPA Water Quality Portal on 2026-08-02.
Collected and reported by
- illinois epa
IL_EPA_WQX52 samples · 2021
Analytical methods
- MEASURED IN FIELD
- Phosphorus by Colorimetry
- 10200 H 2 ~ Chlorophyll a-b-c Determination by spectrophotometer
Monitoring station identifiers (3)
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)
Named boat-access points from the USGS National Boat Ramps inventory