The State of Illinois's Lakes 2026

Cleanest lakes in Illinois

Highest combined water-quality score, 2026.

1. 1.ALake Calumet — Cook County4.1 ft clarity · 17 µg/L P · 5 µg/L chl-a
2. 2.CKinkaid Lake — Jackson County24 µg/L P · 16 µg/L chl-a
3. 3.FLong Lake — Lake County3.4 ft clarity · 81 µg/L P · 57 µg/L chl-a
4. 4.FSalem Lake — Marion County1.0 ft clarity · 234 µg/L P · 78 µg/L chl-a
5. 5.FVandalia Lake — Fayette County1.3 ft clarity · 104 µg/L P · 84 µg/L chl-a
6. 6.FFairfield Lake — Wayne County1.3 ft clarity · 148 µg/L P · 35 µg/L chl-a

Most-polluted lakes

Lowest combined water-quality score among monitored lakes.

1. 1.FLong Lake — Lake County3.4 ft clarity · 81 µg/L P · 57 µg/L chl-a
2. 2.FSalem Lake — Marion County1.0 ft clarity · 234 µg/L P · 78 µg/L chl-a
3. 3.FVandalia Lake — Fayette County1.3 ft clarity · 104 µg/L P · 84 µg/L chl-a
4. 4.FFairfield Lake — Wayne County1.3 ft clarity · 148 µg/L P · 35 µg/L chl-a
5. 5.CKinkaid Lake — Jackson County24 µg/L P · 16 µg/L chl-a
6. 6.ALake Calumet — Cook County4.1 ft clarity · 17 µg/L P · 5 µg/L chl-a

Worst algae hotspots

Highest measured chlorophyll-a (algae) concentration.

1. 1.NRQuiver Lake — Mason County140 µg/L chlorophyll-a
2. 2.FIsland Lake — McHenry County137 µg/L chlorophyll-a
3. 3.NRBoujan Swale Lake — Cass County136 µg/L chlorophyll-a
4. 4.FBullfrog Lake — Cook County125 µg/L chlorophyll-a
5. 5.FKinmundy Old Lake — Marion County115 µg/L chlorophyll-a
6. 6.FSaganashkee Slough Lake — Cook County109 µg/L chlorophyll-a
7. 7.FSlm Side Channel Lake — St. Clair County96 µg/L chlorophyll-a
8. 8.FSam Dale Lake — Wayne County95 µg/L chlorophyll-a
9. 9.FGovernor Bond Lake — Bond County91 µg/L chlorophyll-a
10. 10.FSlocum Lake — Lake County90 µg/L chlorophyll-a

Trending lakes

Improving

1. 1.DTurtlehead Lake — Cook County
2. 2.FLong Lake — Lake County
3. 3.FSalem Lake — Marion County
4. 4.FArrowhead Lake — Cook County
5. 5.FFairfield Lake — Wayne County
6. 6.NRColumbia Basin Lake — Cook County
7. 7.NRJackson Park Inner Harbor Lake — Cook County
8. 8.NRJackson Park West Lagoon Lake — Cook County

Declining

1. 1.DFlatfoot Lake — Cook County
2. 2.ALake Opeka — Cook County
3. 3.ALake Calumet — Cook County
4. 4.ACatherine Lake — Lake County
5. 5.NRBig Marsh Lake — Cook County
6. 6.NRLake Springfield — Sangamon County
7. 7.NRHastings Lake — Lake County