The State of Ohio's Lakes 2026

Cleanest lakes in Ohio

Highest combined water-quality score, 2026.

1. 1.DCaesar Creek Lake — Warren County3.2 ft clarity · 25 µg/L P · 20 µg/L chl-a

Most-polluted lakes

Lowest combined water-quality score among monitored lakes.

1. 1.DCaesar Creek Lake — Warren County3.2 ft clarity · 25 µg/L P · 20 µg/L chl-a

Worst algae hotspots

Highest measured chlorophyll-a (algae) concentration.

1. 1.FGrand Lake St Marys @ Intake — Mercer County117 µg/L chlorophyll-a
2. 2.FSpring Valley Lake Lake — Columbiana County84 µg/L chlorophyll-a
3. 3.FAuglaize R Lake — Paulding County73 µg/L chlorophyll-a
4. 4.NRTappan Lake @ Cadiz Public Water Supply Intake — Harrison County72 µg/L chlorophyll-a
5. 5.FHoover Reservoir — Delaware County70 µg/L chlorophyll-a
6. 6.NRTapan Lake Adj To Marina — Harrison County65 µg/L chlorophyll-a
7. 7.FSomerset Reservoir — Perry County64 µg/L chlorophyll-a
8. 8.FSt Joseph Lake — Perry County59 µg/L chlorophyll-a
9. 9.FAlum Creek Lake — Delaware County54 µg/L chlorophyll-a
10. 10.DMaumee R Upst Independence Dam Lake — Defiance County51 µg/L chlorophyll-a

Trending lakes

Improving

1. 1.NRCharles Mill Reservoir — Ashland County
2. 2.NRSites Lake — Richland County
3. 3.NRMaumee Bay Off Maumee Bay State Park Lake — Lucas County

Declining

1. 1.DCaesar Creek Lake — Warren County
2. 2.NRMaumee Bay Off Woodtick Peninsula Lake — Lucas County
3. 3.NRTappan Lake @ Cadiz Public Water Supply Intake — Harrison County