The State of Illinois's Lakes 2026

Cleanest lakes in Illinois

Cleanest combined nutrient and algae readings, 2026.

1. 1.Lake Calumet — Cook County4.1 ft clarity · 17 µg/L P · 5 µg/L chl-a
2. 2.Kinkaid Lake — Jackson County24 µg/L P · 16 µg/L chl-a
3. 3.Long Lake — Lake County3.4 ft clarity · 81 µg/L P · 57 µg/L chl-a
4. 4.Salem Lake — Marion County1.0 ft clarity · 234 µg/L P · 78 µg/L chl-a
5. 5.Vandalia Lake — Fayette County1.3 ft clarity · 104 µg/L P · 84 µg/L chl-a
6. 6.Fairfield Lake — Wayne County1.3 ft clarity · 148 µg/L P · 35 µg/L chl-a

Highest measured algae

Highest measured chlorophyll-a (algae) concentration.

1. 1.Quiver Lake — Mason County140 µg/L chlorophyll-a
2. 2.Island Lake — McHenry County137 µg/L chlorophyll-a
3. 3.Boujan Swale Lake — Cass County136 µg/L chlorophyll-a
4. 4.Bullfrog Lake — Cook County125 µg/L chlorophyll-a
5. 5.Kinmundy Old Lake — Marion County115 µg/L chlorophyll-a
6. 6.Saganashkee Slough Lake — Cook County109 µg/L chlorophyll-a
7. 7.Slm Side Channel Lake — St. Clair County96 µg/L chlorophyll-a
8. 8.Sam Dale Lake — Wayne County95 µg/L chlorophyll-a
9. 9.Governor Bond Lake — Bond County91 µg/L chlorophyll-a
10. 10.Slocum Lake — Lake County90 µg/L chlorophyll-a

Trending lakes

Improving

1. 1.Turtlehead Lake — Cook County
2. 2.Long Lake — Lake County
3. 3.Salem Lake — Marion County
4. 4.Arrowhead Lake — Cook County
5. 5.Fairfield Lake — Wayne County
6. 6.Columbia Basin Lake — Cook County
7. 7.Jackson Park Inner Harbor Lake — Cook County
8. 8.Jackson Park West Lagoon Lake — Cook County

Declining

1. 1.Flatfoot Lake — Cook County
2. 2.Lake Opeka — Cook County
3. 3.Lake Calumet — Cook County
4. 4.Catherine Lake — Lake County
5. 5.Big Marsh Lake — Cook County
6. 6.Lake Springfield — Sangamon County
7. 7.Hastings Lake — Lake County