Late afternoon sun over Slim Lake, scattering a bright path across wind-textured water

Cook County Lake Quality

Illinois, 68 lakes, average grade D (Nutrient-rich)

Cook County has 68 monitored lakes, enough that within-county variation matters as much as the county average. Some are deep glacial basins, others are shallow agricultural-watershed waters, and the measurements reflect that mix. 16 of those lakes carry an A–F grade. The county-level summary describes that subset, not every water body listed. The county-wide average grade is D — water quality issues are persistent across most of the monitored basins. 7 lakes fall into the F bracket outright.

The county's lakes lean shallow, which makes the grades sensitive to wind, sediment-resuspension, and any nutrient inputs from the watershed. The cleanest lake on the books is Lake Opeka (A); the most-stressed is Horsetail Lake (F).

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Quick Answers for Cook County

Cleanest lakes in Cook County →Cleanest lakes in Illinois →Safest swimming lakes in Illinois →

Planning a trip? Check special fishing regulations for Lake Opeka — or check the official Illinois fishing regulations.

All Lakes in Cook County

68 lakes · 16 graded, 52 not rated16 graded lakes · 52 not rated, hidden

Include not-rated lakes (52)

#	Lake	Grade	Clarity	Phosphorus	Algae	Trophic	Access
1	ALake OpekaCook	A	4 ft	22.5 µg/L	5.3 µg/L	No recent samples	Not recorded
2	ALake CalumetCook	A	4.1 ft	16.5 µg/L	5.4 µg/L	No recent samples	Not recorded
3	APowderhorn LakeCook	A	10.1 ft	15 µg/L	4.8 µg/L	No recent samples	✓ Public
4	BAxehead LakeCook	B	11.9 ft	17 µg/L	13.8 µg/L	No recent samples	Not recorded
5	BWampum LakeCook	B	6.9 ft	24 µg/L	10.7 µg/L	No recent samples	Not recorded
6	DFlatfoot LakeCook	D	6.2 ft	18 µg/L	21.3 µg/L	No recent samples	✓ Public
7	DTurtlehead LakeCook	D	2.8 ft	54 µg/L	24.4 µg/L	No recent samples	Not recorded
8	DTampier LakeCook	D	1.7 ft	83 µg/L	29.7 µg/L	No recent samples	✓ Public
9	DSag Quarry West LakeCook	D	4.2 ft	65.5 µg/L	22.1 µg/L	No recent samples	Not recorded
10	FHorsetail LakeCook	F	2.2 ft	45 µg/L	39.6 µg/L	No recent samples	Not recorded
11	FSaganashkee Slough LakeCook	F	0.8 ft	170 µg/L	108.5 µg/L	No recent samples	✓ Public
12	FArrowhead LakeCook	F	2.1 ft	38 µg/L	37.6 µg/L	No recent samples	✓ Public
13	FBullfrog LakeCook	F	1.4 ft	73 µg/L	124.5 µg/L	No recent samples	Not recorded
14	FSag Quarry East LakeCook	F	5.4 ft	49 µg/L	37.5 µg/L	No recent samples	Not recorded
15	FSkokie Lagoons LakeCook	F	1.6 ft	229 µg/L	74.7 µg/L	No recent samples	✓ Public
16	FMidlothian LakeCook	F	1.2 ft	121 µg/L	61.5 µg/L	No recent samples	Not recorded
	NRConsumers Quarry LakeCook	NR	6.5 ft	No recent samples	No recent samples	No recent samples	Not recorded
	NRLake CharlemagneCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRLake ArlingtonCook	NR	4.5 ft	No recent samples	No recent samples	No recent samples	Not recorded
	NRGreat Lakes Quarries EastCook	NR	4.5 ft	No recent samples	No recent samples	No recent samples	Not recorded
	NRBode Lake NorthCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRGreat Lakes Quarries WestCook	NR	2.5 ft	No recent samples	No recent samples	No recent samples	✓ Public
	NRBull Frog LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRBelly Deep Slough LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRPapoose LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRSauk LakeCook	NR	1 ft	No recent samples	No recent samples	No recent samples	Not recorded
	NRMaple LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	✓ Public
	NRSag Quarry-West LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRCrabtree LakeCook	NR	2 ft	No recent samples	No recent samples	No recent samples	Not recorded
	NRIcebox Quarry LakeCook	NR	9 ft	No recent samples	No recent samples	No recent samples	Not recorded
	NRForge Quarry LakeCook	NR	8.5 ft	No recent samples	No recent samples	No recent samples	Not recorded
	NRDoughnut LakeCook	NR	5 ft	No recent samples	No recent samples	No recent samples	Not recorded
	NRBusse Lake Main PoolCook	NR	3.5 ft	No recent samples	No recent samples	No recent samples	✓ Public
	NRLake GlenviewCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRPogie LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRPalatine Marsh South LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRColumbia Basin LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRNorth PondCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRWashington Park Lagoon LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	✓ Public
	NRJackson Park Inner Harbor LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	✓ Public
	NRJackson Park West Lagoon LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRSherman Park Lagoon LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRHumboldt Park Lagoon LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRJackson Park Outer Harbor LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRGarfield Park Lagoon LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRDouglass Park Lagoon LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRSouth PondCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRJackson Park East Lagoon LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRMckinley Park Lagoon LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	✓ Public
	NRColumbus Park Lagoon LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRMarquette Park Lagoon LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRDiversey Harbor LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	✓ Public
	NRBurnham Prairie Marsh West LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRBig Marsh LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRBig Bend LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	✓ Public
	NRBeck LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	✓ Public
	NRSpring LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRHawley LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRKeene LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRHawthorn LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRBaker LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRGoose LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRTampier SloughCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRLongjohn SloughCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NROrland LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRLake Cottage GroveCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRDead Stick PondCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	Not recorded
	NRWolf LakeCook	NR	No recent samples	No recent samples	No recent samples	No recent samples	✓ Public

NR = not rated: there is not enough recent sampling to support a letter. That is EPA Integrated Reporting Category 3 (insufficient information), not a failing grade. How grading works.

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Other Counties in Illinois

Winnebago County

7 lakes · Avg grade A

DeKalb County

1 lake · Avg grade B

Kane County

3 lakes · Avg grade B

Perry County

7 lakes · Avg grade C

Rock Island County

3 lakes · Avg grade C

Jackson County

14 lakes · Avg grade C

See how Cook County stacks up in the national county water-quality rankings, or browse the full directory of graded lakes across all United States (49 states), Canada (Ontario, Québec, Nova Scotia, British Columbia and Newfoundland and Labrador), and 38 other country hubs..

Source: EPA National Aquatic Resource Surveys, 2026.