Question 1

What is the best lake to swim in Illinois?

Accepted Answer

Axehead Lake in Cook County ranks #1 for swimming-safety in our 2026 Illinois dataset, combining grade-A water clarity with low algae and public access. The full top 25 above ranks the swim-safest lakes statewide.

Question 2

How do you know if a lake is safe to swim in Illinois?

Accepted Answer

Three signals: (1) water clarity — Secchi depth above 6 feet is a strong sign of low suspended sediment and pathogens; (2) chlorophyll-a below 10 µg/L means low risk of harmful algae bloom; (3) a current grade of A or B from year-round sampling. Illinois DNR and county health departments post seasonal advisories — always check before you swim, especially in late summer when blue-green algae bloom risk peaks.

Question 3

Are lakes in Illinois safe to swim in?

Accepted Answer

Most monitored Illinois lakes are safe to swim in under normal conditions, but water quality varies widely. The lakes ranked above are filtered to grade A and B with chlorophyll-a below 10 µg/L — meaning they consistently test cleaner and have lower algae bloom risk than the state average. Always check posted advisories before swimming and avoid water after heavy rain (bacterial contamination spikes from runoff).

Question 4

What lakes have algae blooms in Illinois?

Accepted Answer

Algae blooms (especially blue-green / cyanobacteria) are most common in shallow, warm, nutrient-rich (eutrophic) lakes during late summer. Lakes with high phosphorus and chlorophyll-a measurements are at higher risk. The lakes on this page are filtered to exclude high-algae lakes. For the opposite list, see our most-polluted-lakes trend page.

Question 5

When is it safe to swim in lakes in Illinois?

Accepted Answer

Illinois lakes are typically safest to swim in late spring through early summer (May–July), when water is cool and algae bloom risk is lowest. Risk rises in late summer and early fall (August–September) when warm water and accumulated nutrients drive cyanobacteria blooms. Always avoid swimming after heavy rainfall (bacterial spikes from runoff) and check current state DNR advisories the day you go.

Question 6

What's the cleanest lake in Illinois?

Accepted Answer

The single cleanest lake by combined water-quality score is featured on the Illinois cleanest-lakes page. The lakes here are filtered specifically for swimming safety — same A/B grade requirement, plus low chlorophyll-a (active algae filter). For the broader cleanliness ranking, see /best/cleanest/il.

Rank	Lake	County	Grade	Swim Safety	Clarity	Algae	Phosphorus	Area
1	Axehead Lake	Cook	A	Low algae	11.9 ft	—	10 µg/L	-
2	Powderhorn Lake	Cook	A	Low algae	10.1 ft	—	13 µg/L	-
3	Baumann Park Lake	Winnebago	A	Low algae	10.2 ft	—	11.5 µg/L	-
4	Silver Lake	McHenry	B	Low algae	7 ft	—	11 µg/L	-
5	Boulder South Lake	Perry	B	Low algae	8.8 ft	—	14.5 µg/L	-
6	Wampum Lake	Cook	B	Low algae	6.9 ft	—	16 µg/L	-
7	Round Lake	Lake	B	Low algae	7.4 ft	—	19.5 µg/L	242 acres
8	Big Rock Lake	Kane	B	Low algae	8.1 ft	—	10 µg/L	-
9	Flatfoot Lake	Cook	B	Low algae	6.3 ft	—	10.5 µg/L	-
10	Opeka Lake	Cook	B	Low algae	3.9 ft	—	15.5 µg/L	-
11	One Horse Gap Lake	Pope	B	Low algae	4 ft	—	17 µg/L	29 acres
12	Glendale Lake	Pope	B	Low algae	4.8 ft	—	16 µg/L	84 acres
13	Tecumseh Lake	Hardin	B	Low algae	3.6 ft	—	18 µg/L	13 acres
14	Grays Lake	Lake	B	Low algae	3.5 ft	—	18 µg/L	-
15	Calumet Lake	Cook	B	Low algae	4.3 ft	—	10.5 µg/L	0 acres
16	Lily Lake	McHenry	B	Low algae	3.3 ft	—	16 µg/L	-
17	Catherine Lake	Lake	B	Low algae	7.4 ft	—	27.5 µg/L	-

Safest Lakes to Swim in Illinois

Frequently Asked Questions

Our swimming-safety filter

Where to swim near a Illinois city

Where to stay near Illinois swimming lakes