Clearest Lakes in Illinois — Water Quality Rankings (2026)

The single clearest lake in Illinois is Loch Lomond Lake in Lake County, with a Secchi depth of 16.5 feet — meaning a Secchi disk lowered from the surface disappears at that depth. Loch Lomond Lake holds an overall water quality grade of NR. The 25 clearest lakes in Illinois are ranked below by 2026 measurements.

How clear are Illinois's clearest lakes?

Across the 97 Illinois lakes with Secchi clarity readings in our 2026 dataset, the 25 clearest range from 4.8 to 16.5 feet of visibility. Loch Lomond Lake sits at the top, 4.6 ft clearer than the next lake on the list, well above the 8-foot median of this leading group — a spread that shows how quickly clarity falls off once you move past a state's handful of truly pristine waters.

The clearest water is not spread evenly. Cook County alone accounts for 9 of the 25 top-ranked lakes, with the rest drawn from 11 other counties. That clustering follows the terrain: deep, glacially carved basins ringed by forest take in little sediment or fertilizer runoff, so phosphorus stays low and the water stays transparent.

Clarity and overall health usually align here: 10 of these 25 lakes also earn an A- or B-range overall grade. Where the two diverge — a clear lake carrying a weaker grade — the cause is typically elevated phosphorus that transparency alone can't reveal, which is why our grade combines phosphorus and chlorophyll-a with Secchi depth rather than relying on clarity by itself.

Rank	Lake	County	Secchi Depth	Grade	Max Depth	Area
1	Loch Lomond Lake	Lake	16.5 ft	NR	-	72 acres
2	Axehead Lake	Cook	11.9 ft	B	-	-
3	Timber Lake	Carroll	11.5 ft	NR	-	-
4	Timber Lake	Lake	10.6 ft	NR	-	-
5	Baumann Park Lake	Winnebago	10.2 ft	A	-	-
6	Powderhorn Lake	Cook	10.1 ft	A	-	-
7	Silver Lake	McHenry	10 ft	NR	-	-
8	Icebox Quarry Lake	Cook	9 ft	NR	-	-
9	Boulder South Lake	Perry	8.8 ft	B	-	-
10	Forge Quarry Lake	Cook	8.5 ft	NR	-	-
11	Bass Lake	Lee	8.1 ft	NR	-	-
12	Big Rock Lake	Kane	8.1 ft	B	-	-
13	Channel Lake	Lake	8 ft	B	-	-
14	Round Lake	Lake	7.4 ft	B	-	242 acres
15	Catherine Lake	Lake	7.4 ft	A	-	-
16	Wampum Lake	Cook	6.9 ft	B	-	-
17	Woodhaven Lake	Lee	6.7 ft	NR	-	-
18	Consumers Quarry Lake	Cook	6.5 ft	NR	-	-
19	Flatfoot Lake	Cook	6.2 ft	D	-	-
20	Sag Quarry East Lake	Cook	5.4 ft	F	-	-
21	Galena Lake	Jo Daviess	5.2 ft	NR	-	-
22	Doughnut Lake	Cook	5 ft	NR	-	-
23	Shabbona Lake	DeKalb	4.9 ft	B	-	-
24	Monee Lake	Will	4.9 ft	D	-	-
25	Glendale Lake	Pope	4.8 ft	F	-	84 acres

Where to stay near Illinois's clearest lakes

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Clearest lakes across all states →Cleanest lakes in Illinois (combined score) →All Illinois lakes →

Frequently Asked Questions

What is the clearest lake in Illinois?

Loch Lomond Lake is the clearest lake in Illinois by Secchi depth, with water transparency of 16.5 feet. Secchi depth is measured by lowering a black-and-white disk until it disappears from view — deeper readings mean clearer water. Located in Lake County, Loch Lomond Lake is not rated for water quality — its monitoring record does not meet the evidence bar.

How is water clarity measured?

Water clarity is measured by Secchi depth, named after the 19th-century Italian astronomer Pietro Angelo Secchi who developed the method. A standardized black-and-white disk is lowered into the water on a rope; the depth (in feet or meters) at which the disk disappears from view is recorded. Greater Secchi depth means clearer water — usually a sign of low sediment, low phosphorus, and limited algae growth.

What makes a lake clear?

Lake clarity is driven by three factors: low suspended sediment (clay, silt), low nutrient load (especially phosphorus, which fuels algae), and low chlorophyll-a (active algae biomass). Deep glacial lakes with rocky basins and forested watersheds — common in northern Illinois — tend to be clearest because they have little soil runoff and limited nutrient input. Shallow agricultural-watershed lakes are typically less clear because they receive more sediment and fertilizer runoff.

Is the clearest lake the cleanest?

Not always. Clarity (Secchi depth) is one of three indicators we use; the others are total phosphorus and chlorophyll-a. A lake can be very clear but still have elevated phosphorus that just hasn't bloomed yet, or chlorophyll-a from a recent bloom that's still settling. The most reliable measure of overall lake health is the combined grade, which weights all three signals. See our companion cleanest-lakes ranking for Illinois for the combined-score list.

How does Illinois compare to other states?

Illinois's clearest waters compete with the most pristine lakes in our 38-state dataset, where top-tier lakes typically register Secchi depths of 15–25 feet. Nationally, the gold standard is Crater Lake in Oregon (~100 ft) and Lake Tahoe (~65 ft).

Source: EPA National Aquatic Resource Surveys + state DNR Secchi sampling, 2026.