The State of Illinois’s Lakes 2026

Cleanest lakes in Illinois

Highest combined water-quality score, 2026.

1. 1.AAxehead Lake — Cook County11.9 ft clarity · 10 µg/L P
2. 2.APowderhorn Lake — Cook County10.1 ft clarity · 13 µg/L P
3. 3.ABaumann Park Lake — Winnebago County10.2 ft clarity · 12 µg/L P
4. 4.BSilver Lake — McHenry County7.0 ft clarity · 11 µg/L P
5. 5.BBoulder South Lake — Perry County8.8 ft clarity · 15 µg/L P
6. 6.BWampum Lake — Cook County6.9 ft clarity · 16 µg/L P
7. 7.BRound Lake — Lake County7.4 ft clarity · 20 µg/L P
8. 8.BBig Rock Lake — Kane County8.1 ft clarity · 10 µg/L P
9. 9.BFlatfoot Lake — Cook County6.3 ft clarity · 11 µg/L P
10. 10.BOpeka Lake — Cook County3.9 ft clarity · 16 µg/L P

Most-polluted lakes

Lowest combined water-quality score among monitored lakes.

1. 1.FSalem Lake — Marion County1.0 ft clarity · 197 µg/L P
2. 2.FSaganashkee Slough Lake — Cook County0.8 ft clarity · 94 µg/L P
3. 3.FVandalia Lake — Fayette County1.3 ft clarity · 93 µg/L P
4. 4.FHoliday Shores Lake — Madison County1.7 ft clarity · 127 µg/L P
5. 5.FSkokie Lagoons Lake — Cook County1.6 ft clarity · 155 µg/L P
6. 6.FSam Dale Lake — Wayne County1.2 ft clarity · 137 µg/L P
7. 7.FRaccoon Lake — Marion County1.2 ft clarity · 203 µg/L P
8. 8.FCarlyle Lake — Clinton County1.8 ft clarity · 201 µg/L P
9. 9.FSlm Side Channel Lake — St. Clair County0.8 ft clarity · 276 µg/L P
10. 10.FFairfield Lake — Wayne County1.3 ft clarity · 114 µg/L P

Worst algae hotspots

Highest measured chlorophyll-a (algae) concentration.

1. 1.FQuiver Lake — Mason County140 µg/L chlorophyll-a
2. 2.FBoujan Swale Lake — Cass County136 µg/L chlorophyll-a
3. 3.DLong Lake — Lake County48 µg/L chlorophyll-a

Trending lakes

Improving

1. 1.CTurtlehead Lake — Cook County
2. 2.DColumbia Basin Lake — Cook County
3. 3.DTurtlehead Lake — Cook County
4. 4.DJackson Park East Lagoon Lake — Cook County
5. 5.DLong Lake — Lake County
6. 6.DArrowhead Lake — Cook County
7. 7.DLong Lake — Lake County
8. 8.FSalem Lake — Marion County

Declining

1. 1.FBig Marsh Lake — Cook County
2. 2.FLake Calumet — Cook County
3. 3.FLake Springfield — Sangamon County
4. 4.FVandalia Lake — Fayette County
5. 5.DLake Opeka — Cook County
6. 6.DFlatfoot Lake — Cook County
7. 7.DHastings Lake — Lake County
8. 8.BFlatfoot Lake — Cook County