Randolph Lake
Randolph County, IllinoisView mapDirections
Nutrients and algae run well above the regional average.
↓ down from D at the last regrade
- Hypereutrophic
- Impaired · Mercury
- #1 of 3 in the county
- 34 ft max depth
- Public access
- Limited data
Randolph Lake in Randolph County, Illinois has a water quality grade of F (Algae-prone). Water clarity: 1.7 ft visibility. Phosphorus: 87.5 µg/L. Chlorophyll-a (algae): 44.9 µg/L.
- Clarity
- 1.7 ft
- Typical in October
- 66°F
- Max depth
- ~34 ft est.
- Type
- Hypereutrophic
- Phosphorus
- 87.5 µg/L
- Chlorophyll-a
- 44.9 µg/L
- Public access
- Yes
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Stays near Randolph Lake
Compare lakeside cabins, hotels, and vacation rentals near Randolph Lake in Randolph 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 Randolph Lake are listed below.
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Watch Randolph Lake’s letter
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Randolph Lake is a highly nutrient-loaded hypereutrophic lake in Randolph County, Illinois with an estimated average depth of about 34 feet. Water quality is graded F based on 2 sampling years through 2021. Limited fish-species data available. Public access is available. Compared to the 2 other monitored lakes in Randolph County, Randolph Lake ranks #1 for water quality.
Source: EPA Water Quality Portal sampling records, Illinois EPA, last sampled 2021-10-13. 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. Randolph Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, nutrients (phosphorus, nitrogen). Check local advisories before you swim.
Water Quality Grade: F (Algae-prone)
Very murky, less than 1.7 ft of visibility. Phosphorus 87.5 µg/L. Chlorophyll-a 44.9 µg/L. TSI 69. The letter is not a swimming clearance.
| Metric | Value | Grade |
|---|---|---|
| Water Clarity (Secchi Depth) | 1.7 ft | Not graded |
| Phosphorus | 87.5 µg/L | Poor |
| Chlorophyll-a (Algae) | 44.9 µg/L | Poor |
| Trophic State Index (TSI) | 69 | Hypereutrophic |
Very high nutrients, dense algae, poor clarity
Water Temperature
Randolph Lake typically sits around 66°F at the surface in October. Summer average is about 71°F. Usually warmest in Jul (about 73°F) and coldest in May (about 61°F). These are typical values by month — not a live reading.
Typical surface water temperature by month, from 257 field readings (2016–2021) via the EPA Water Quality Portal. See the full Randolph Lake water temperature breakdown →
Lake Details
| Characteristic | Value |
|---|---|
| Maximum Depth | ~34 ft est. |
| Average Depth | ~11 ft est. |
| Public Access | Yes |
Depth marked “est.” is a modeled maximum from GLOBathy (Khazaei et al. 2022, CC0), not a measured survey. Illinois has not published bathymetry for Randolph 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 Randolph Lake
Reservoir Info (USACE NID)
Randolph Lake is a man-made reservoir impounded by the Randolph County Lake Dam (completed 1960), built primarily for recreation on the MORRISON BRANCH; earth-type dam, 47 ft tall and 665 ft long.
- Normal storage
- 946 ac-ft
- Max storage
- 1,563 ac-ft
- Hazard class
- Significant
- Owner
- ILLINOIS DEPARTMENT OF NATURAL RESOURCES
Source: USACE National Inventory of Dams, NID ID IL00551 · Operator website · Matched by proximity (0.44 km)
EPA Impairment Status
Randolph 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_RIB · Official waterbody report
County Ranking
Ranked #1 of 3 lakes in Randolph County
Nearby Lakes With Higher Grades
Randolph Lake holds Grade F. One nearby lake has a higher nutrient-and-algae grade — useful for a swim day, not a reason to skip lodging here.
Nearby Lakes in Randolph County
Frequently asked questions about Randolph Lake
Where is Randolph Lake?
- Randolph Lake is a lake in Randolph County, Illinois. LakeQuality grades it F for nutrients and algae. The letter is not a swimming clearance.
What is the water quality grade for Randolph Lake?
- Randolph Lake in Randolph County, Illinois has a water quality grade of F (Algae-prone). Water clarity: 1.7 ft visibility. Phosphorus: 87.5 µg/L. Chlorophyll-a (algae): 44.9 µg/L.
Is Randolph Lake safe to swim in?
- The F is nutrients and algae, not a swimming clearance. Randolph Lake is listed impaired under Clean Water Act §303(d) (IR 5) for mercury, nutrients (phosphorus, nitrogen). Check local advisories before you swim.
What is the water temperature of Randolph Lake?
- Randolph Lake typically sits around 66°F at the surface in October, from 257 EPA Water Quality Portal field readings (2016–2021). That is a typical value for the month, not a live reading.
How deep is Randolph Lake?
- No published bathymetric survey covers Randolph Lake, so its depth has not been measured. The HydroLAKES global model estimates an average depth of about 34 feet, but that is a modelled estimate derived from surrounding terrain rather than a measurement on the water, so treat it as approximate.
Explore Randolph Lake across LakeQuality
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
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
Lake size & watershed from HydroLAKES (HydroSHEDS)