Updated Aug 2026
Ontario Lake Water Quality
Water quality grades for 2,822 monitored lakes across 40 counties.
Across Ontario, 2,822 lakes carry an active water-quality grade spread over 40 counties. The underlying measurements come from the Ontario Ministry of the Environment, Conservation and Parks, alongside federal programs and volunteer monitoring networks. Roughly 57% of the state's graded lakes carry an A — 1,602 lakes that earn the top bracket on the combined Secchi/phosphorus/chlorophyll-a rubric. That share is high by national standards and reflects the depth and natural clarity of glacial lakes in the region.
The state's dominant trophic class is oligotrophic: clear, nutrient-poor, slow-growing. These lakes are also the most sensitive to disturbance — the same volume that buffers them against nutrients also slows recovery if those nutrients arrive in quantity. Southwestern Ontario ranks highest on average county grade (A). Middlesex sits lowest. With only 40 counties in the state, the cross-county spread is narrower than the within-county spread for most lakes.
Grade Distribution
How Ontario’s 2,822 graded lakes break down from A (excellent) to F.
Top-Graded Ontario Lakes
The highest water-quality grades in Ontario — clarity, depth, and A-F grade at a glance.
Crosby Lake
Leeds and Grenville Counties County, Ontario
See grade for Crosby Lake →Aird Lake
Renfrew County, Ontario
See grade for Aird Lake →Allen Lake
Haliburton County, Ontario
See grade for Allen Lake →Animoosh Lake
Nipissing District County, Ontario
See grade for Animoosh Lake →Anstruther Lake
Peterborough County, Ontario
See grade for Anstruther Lake →Apsey Lake
Sudbury District County, Ontario
See grade for Apsey Lake →Ardoch Lake
Frontenac County, Ontario
See grade for Ardoch Lake →Armishaw Lake
Parry Sound District County, Ontario
See grade for Armishaw Lake →Armstrong Lake
Kenora District County, Ontario
See grade for Armstrong Lake →Lake Map
Explore Ontario Lakes
Ontario reports, fishing & travel
Ontario Fishing & Recreation
Licenses, regulations, records, and where to fish across Ontario.
Counties
All 40 Ontario counties, ranked by average lake water quality. Compare them against every county nationwide in the county rankings, browse the full directory of graded lakes, or pick a different state.
Top 25 Cleanest Lakes
| Rank | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | Crosby Lake | Leeds and Grenville Counties | A | 17.1 ft |
| 2 | Aird Lake | Renfrew | A | 16.4 ft |
| 3 | Allen Lake | Haliburton | A | 19.4 ft |
| 4 | Animoosh Lake | Nipissing District | A | 15.6 ft |
| 5 | Anstruther Lake | Peterborough | A | 16.4 ft |
| 6 | Apsey Lake | Sudbury District | A | - |
| 7 | Ardoch Lake | Frontenac | A | 19 ft |
| 8 | Armishaw Lake | Parry Sound District | A | - |
| 9 | Armstrong Lake | Kenora District | A | 16.1 ft |
| 10 | Armstrong Lake | Sudbury District | A | 15.1 ft |
| 11 | Arnott Lake | Algoma District | A | 24 ft |
| 12 | Arrow Lake | Thunder Bay District | A | 24 ft |
| 13 | Ashby Lake | Lennox and Addington | A | 18 ft |
| 14 | Athelstane Lake | Thunder Bay District | A | - |
| 15 | Aylen Lake | Nipissing District | A | 17.9 ft |
| 16 | Bagot Long Lake | Renfrew | A | 15.1 ft |
| 17 | Bass Lake | Leeds and Grenville Counties | A | 19.4 ft |
| 18 | Bass Lake | Leeds and Grenville Counties | A | 19 ft |
| 19 | Basshaunt Lake | Haliburton | A | 15.7 ft |
| 20 | Basswood Lake | Algoma District | A | 37.7 ft |
| 21 | Batwing Lake | Thunder Bay District | A | - |
| 22 | Bay Lake | Hastings | A | 21.2 ft |
| 23 | Bear Lake | Haliburton | A | 19.4 ft |
| 24 | Bearhead Lake | Algoma District | A | 17.1 ft |
| 25 | Beech Lake | Haliburton | A | 15.9 ft |
25 Lakes Needing Attention
| Rank | Lake | County | Grade | Clarity |
|---|---|---|---|---|
| 1 | Utim Lake | Nipissing District | F | 2.6 ft |
| 2 | Slim Lake | Kenora District | F | 2.6 ft |
| 3 | Ouellet Lake | Cochrane District | F | 2.6 ft |
| 4 | G Ross Lord Reservoir | Golden Horseshoe | F | 1.3 ft |
| 5 | Clay Lake | Kenora District | F | 2.6 ft |
| 6 | Claireville Reservoir | Peel Region | F | 1.5 ft |
| 7 | Alder Lake | Region of Waterloo | F | 1.6 ft |
| 8 | Grenadier Pond | Golden Horseshoe | D | 3.3 ft |
| 9 | Abitibi Lake | Cochrane District | D | 1 ft |
| 10 | Wegg Lake | Kenora District | D | 3.9 ft |
| 11 | Uchi Lake | Kenora District | D | 5.9 ft |
| 12 | Tide Lake | Kenora District | D | 3.6 ft |
| 13 | Sowden Lake | Kenora District | D | 5.2 ft |
| 14 | Smye Lake | Thunder Bay District | D | 4.9 ft |
| 15 | Singapore Lake | Kenora District | D | 4.6 ft |
| 16 | Shabu Lake | Kenora District | D | 6.2 ft |
| 17 | Selwyn Lake | Thunder Bay District | D | 4.3 ft |
| 18 | Sandy Lake | Kenora District | D | 1.6 ft |
| 19 | Press L. Lake | Kenora District | D | 5.6 ft |
| 20 | Pikangikum Lake | Kenora District | D | 4.6 ft |
| 21 | Peeagwon Lake | Kenora District | D | 2.3 ft |
| 22 | Pashkokogan Lake | Thunder Bay District | D | 6.2 ft |
| 23 | Nungesser Lake | Kenora District | D | 6.6 ft |
| 24 | Night Hawk Lake | Cochrane District | D | 1 ft |
| 25 | Miniss L. Lake | Thunder Bay District | D | 6.2 ft |
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Source: EPA National Aquatic Resource Surveys, 2026.