Boulder South Lake
Perry County, IllinoisView mapDirections
Illinois EPA publishes the water-quality measurements for Boulder South Lake. Here is what has been measured.
- Eutrophic
- Public access
- Limited data
Boulder South Lake in Perry County, Illinois has water-quality measurements published by Illinois EPA, shown here as reported. Water clarity: 8.8 ft visibility. Phosphorus: 17.5 µg/L. Chlorophyll-a (algae): 13.5 µg/L.
Boulder South Lake is a productive, nutrient-rich eutrophic lake in Perry County, Illinois. 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. Methodology: how these readings are compared.
What Boulder South Lake is good for
From access and monitoring records, not editorial picks.
- Boat launch1 documented ramp
Swimming conditions
Readings onlyLimited data
Based on clarity and algae readings from past sampling — there is no live water temperature and a current cyanobacteria reading for this lake. Check your state or county health department before you swim.
Nutrient and algae readings are not a swimming clearance. Check local advisories before you swim.
Whether Boulder South Lake is safe to swim in today is a call for the Illinois Environmental Protection Agency and the county health department, who post current advisories. LakeQuality does not issue swimming clearances.
Water quality measurements for Boulder South Lake
These are the measurements Illinois EPA publishes for Boulder South Lake, shown as reported. The condition class beside each reading is the EPA National Lakes Assessment class for this lake’s ecoregion.
Moderate clarity, visible to about 8.8 ft. Phosphorus 17.5 µg/L. Chlorophyll-a 13.5 µg/L. TSI 49.
| Measure | Reading | Where it sits |
|---|---|---|
| Water clarity (Secchi depth) | 8.8ft | Measured, not scored How deep a black-and-white disk stays visible from the surface. Higher is clearer. |
| Total phosphorus | 17.5µg/L | Good GoodFairPoor |
| Chlorophyll-a (algae) | 13.5µg/L | Good GoodFairPoor |
| Trophic state (TSI)Carlson's 0–100 index of how much algae a lake can support, from clarity, phosphorus and chlorophyll-a. | 49 | Eutrophic OligotrophicMesotrophicEutrophicHypereutrophic |
High nutrients, frequent algae, reduced clarity
Ask about Boulder South Lake
Answers come straight from the public measurements and survey records on this page.
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.
Column height runs from this lake’s coldest month (56°F) to its warmest (69°F).
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
Stays via Stay22
Stays near Boulder South Lake
Compare lakeside cabins, hotels, and vacation rentals near Boulder South Lake in Perry 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.
Top-rated places to stay near Boulder South Lake
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Fishing Boulder South Lake
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.
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. Its water-quality measurements are published by Illinois EPA and shown on this page as reported.
What is the water quality of Boulder South Lake?
- Boulder South Lake in Perry County, Illinois has water-quality measurements published by Illinois EPA, shown here as reported. 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?
- Nutrient and algae readings are not a swimming clearance. 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.
Can I fish at Boulder South Lake?
- Boulder South Lake has 1 documented boat landing in the USGS National Boat Ramps inventory. Seasons, size and bag limits, and whether you need a license are set by Illinois's fish and wildlife agency, so check its current regulations before you go.
Explore Boulder South Lake across LakeQuality
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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)
EPA condition classes from the National Lakes Assessment (EPA 841-R-24-006, Table 6-3), applied by ecoregion. See where the data comes from and data sources.
Other records on this page
Regulated discharger data from EPA ECHO (ICIS-NPDES)
Named boat-access points from the USGS National Boat Ramps inventory