Lake Water Temperature in Ontario
How Ontario’s lakes warm, layer and turn over through the year, from the province’s water temperature records on eight research lakes near Dorset since 1976 and on Lake Simcoe since 1980. The thermal class of each lake, and where the day’s water temperature is published for Georgian Bay, Lake Simcoe and Lake Nipissing.
Last updated 2026-09-28
- Dorset lakes, readings
- 105,259eight lakes, every depth, 1976 to 2025
- Warmest near Dorset
- 23.5 °Cmedian at one meter, second half of July
- Deep water, Harp Lake
- 4.3 °Cmedian at 35 meters in July
- Lake Simcoe, readings
- 102,757twelve stations, 1980 to 2024
- Warmest on Lake Simcoe
- 22.1 °Cmedian at one meter, second half of July
- Deep water, Lake Simcoe K42
- 7.3 °Cmedian at 39 meters in July
Lake Water Temperature by Month
Ontario’s longest record of water temperature in its inland lakes is kept on eight research lakes near Dorset, sampled by the Ministry of the Environment, Conservation and Parks since 1976. At a depth of one meter the water on those lakes is warmest in the second half of July, at a median of 23.5 °C, and coldest under the winter ice, at a median of 1.7 °C.
The record is kept as profiles: a reading at the surface and at depths down to the bottom, taken about every fourteen days through the open-water season and on winter dates through the ice. It holds 105,259 readings from 1976 to 2025, 5,240 of them at one meter.
On the Dorset lakes the water at one meter passes a median of 10 °C in the first half of May and 20 °C in the second half of June. It falls back below 20 °C in the first half of September and below 10 °C in the first half of November. On a day when four or more of the lakes were sampled, the warmest and the coolest differed at one meter by a median of 0.7 to 2.0 °C, depending on the month, so the weather of the year sets most of the width of the band in the chart below.
Lake Simcoe, a lake of 723 square kilometers, warms later in spring and holds its heat later into autumn. At the province’s twelve stations on the lake the water at one meter reads a median of 11.4 °C in the second half of May, when the Dorset lakes read 16.0 °C, and 10.2 °C in the first half of November, when they read 7.2 °C.
| Month | Dorset lakes, 1st to 15th | Dorset lakes, 16th to end | Lake Simcoe, 1st to 15th | Lake Simcoe, 16th to end |
|---|---|---|---|---|
| January | 2.31.3 to 3.2 | 1.71.2 to 2.5 | – | 0.1−0.2 to 0.3 |
| February | 2.01.1 to 3.1 | 1.70.9 to 3.0 | 0.2−0.1 to 0.7 | 0.3−0.1 to 0.6 |
| March | 1.71.1 to 3.5 | 1.71.1 to 2.9 | 0.40.1 to 0.7 | 0.50.3 to 0.6 |
| April | 4.12.1 to 7.1 | 6.23.3 to 9.4 | 3.22.2 to 6.4 | 5.03.1 to 8.6 |
| May | 10.56.6 to 15.1 | 16.012.2 to 20.3 | 7.84.5 to 13.2 | 11.47.8 to 15.3 |
| June | 19.016.4 to 22.0 | 21.118.8 to 23.5 | 15.411.7 to 18.3 | 18.414.6 to 21.1 |
| July | 22.620.1 to 25.0 | 23.521.5 to 25.6 | 20.317.0 to 23.0 | 22.119.1 to 23.5 |
| August | 22.821.2 to 24.5 | 22.020.3 to 23.7 | 22.020.1 to 24.1 | 21.619.3 to 23.4 |
| September | 19.918.1 to 21.9 | 17.515.2 to 20.0 | 19.917.6 to 22.3 | 18.015.9 to 20.6 |
| October | 13.110.6 to 15.4 | 10.48.3 to 13.3 | 15.512.4 to 17.7 | 12.19.5 to 13.6 |
| November | 7.24.9 to 9.0 | 5.13.7 to 6.8 | 10.28.3 to 11.8 | 7.25.1 to 8.6 |
| December | 2.81.8 to 4.1 | 2.01.2 to 2.6 | – | 2.51.6 to 3.4 |
How a Lake Warms and Layers
A lake deep enough to layer passes through four stages each year: winter under ice, spring turnover, the layers of summer and autumn turnover. The stages follow from one property of fresh water: it is densest at about 4 °C, and the densest water settles to the bottom.
Spring Turnover
When the ice goes out, the lake sits near 4 °C from top to bottom, and the wind mixes it through its whole depth. This is spring turnover. On Harp Lake, the deepest of the Dorset lakes, sampled to 37 meters, the April median runs between 3.8 and 4.6 °C from the surface to the deepest reading.
The Summer Layers and the Thermocline
Through May and June the surface warms faster than the wind can mix the heat downward. Warm water is lighter than cold, so it floats, and the lake separates into three layers: a warm, mixed surface layer called the epilimnion; a band of rapid cooling called the thermocline; and the cold deep water, the hypolimnion. In July on Harp Lake the median reads 23.1 °C at one meter, 14.6 °C at five meters and 6.0 °C at ten meters, and the steepest fall lies between four and five meters.
The thermocline deepens as the summer goes on. On Harp Lake the steepest fall in the median profile lies between three and four meters in May, four and five meters in June, four and five meters in July, five and six meters in August, six and seven meters in September and eight and nine meters in October.
Autumn Turnover
Through September and October the surface cools, grows denser and sinks, and the warm layer thickens until it reaches the bottom. In November the Harp Lake median runs between 4.5 and 5.4 °C from the surface to the deepest reading, and the lake mixes top to bottom again until ice forms.
Under the Ice
Under the ice the order reverses. The coldest water lies directly beneath the ice and the densest, near 4 °C, at the bottom. In February the Harp Lake median runs from 0.5 °C at the top of the water to 3.6 °C at 31 meters.
Shallow Lakes
A shallow lake mixes to the bottom whenever the wind blows and warms through. Heney Lake, sampled to 6.4 meters, reads about 1.8 °C warmer than Harp Lake at one meter from January to May and 0.8 °C warmer from June to August. Dickie Lake, sampled to 11.5 meters, reads within 0.2 °C of Harp Lake at one meter from June to October.
Water Temperature at Depth
Through the summer the water below the thermocline stays far colder than the surface. In July the median on Harp Lake reads 23.1 °C at one meter and 4.3 °C at 35 meters. At station K42 on Lake Simcoe, sampled to 43 meters, it reads 21.2 °C at one meter, 17.3 °C at ten meters, 9.6 °C at twenty and 7.3 °C at 39 meters.
The deep water of Harp Lake holds close to its spring temperature all summer. It reads between 4.3 and 4.5 °C at the deepest reading in every month from May to October. The deep water at station K42 warms through the season, from 4.9 °C at 39 meters in May to 9.9 °C in October, and its warm layer reaches deeper: in July its steepest fall lies between ten and eleven meters, against four and five on Harp Lake, and by September between seventeen and eighteen meters.
| Depth | May | June | July | August | September | October |
|---|---|---|---|---|---|---|
| Surface | 14.2 | 20.1 | 23.0 | 22.3 | 18.4 | 11.1 |
| 1 m | 14.0 | 20.1 | 23.1 | 22.2 | 18.5 | 11.2 |
| 3 m | 12.2 | 18.9 | 22.4 | 21.9 | 18.4 | 11.2 |
| 5 m | 8.7 | 11.7 | 14.6 | 17.7 | 17.3 | 11.0 |
| 7 m | 6.7 | 7.8 | 8.7 | 9.7 | 10.9 | 10.4 |
| 10 m | 5.4 | 5.8 | 6.0 | 6.4 | 6.5 | 7.0 |
| 15 m | 4.8 | 4.9 | 5.0 | 5.0 | 5.1 | 5.1 |
| 20 m | 4.5 | 4.6 | 4.6 | 4.7 | 4.7 | 4.8 |
| 25 m | 4.4 | 4.5 | 4.5 | 4.5 | 4.6 | 4.6 |
| 30 m | 4.4 | 4.5 | 4.4 | 4.5 | 4.5 | 4.5 |
| 35 m | 4.3 | 4.4 | 4.3 | 4.4 | 4.4 | 4.5 |
Lake Simcoe Water Temperature
Lake Simcoe is one of the few lakes in Ontario with a long water temperature record of its own. The Ministry of the Environment, Conservation and Parks has sampled temperature and dissolved oxygen at fixed stations on the lake since 1980, at the surface and at every meter to the bottom, about every fifteen days from spring to autumn. The record holds 102,757 readings at twelve stations on the lake, eight of them still sampled in 2024.
At one meter the water is warmest in the second half of July, at a median of 22.1 °C across the stations. In June the median is 15.4 °C in the first half of the month and 18.4 °C in the second. The middle 80% of readings runs from 11.7 to 18.3 °C in the first half and from 14.6 to 21.1 °C in the second.
The stations read differently on the same day. In April and May, when Cook’s Bay and the shallow stations warm first, the warmest and the coolest station sampled on one day differed at one meter by a median of up to 4.2 °C; from June to August, by up to 3.5 °C. Each row below is one station’s own record.
| Station | Where | Sampled to, m | Years | Profiles | July at 1 m, °C |
|---|---|---|---|---|---|
| C1 | Cooks Bay 1 | 3.2 | 1980 to 2024 | 493 | 22.1 |
| C6 | Cooks Bay 6 | 13.4 | 1980 to 2024 | 495 | 22.0 |
| C9 | Cooks Bay 9 | 20 | 1980 to 2024 | 529 | 21.6 |
| E50 | – | 13 | 1980 to 2013 | 263 | 21.6 |
| E51 | – | 13.5 | 1980 to 2024 | 504 | 21.6 |
| K38 | Kempenfelt Bay 38 | 19 | 1982 to 1999 | 200 | 18.1 |
| K39 | Kempenfelt Bay 39 | 36 | 1980 to 2024 | 522 | 20.6 |
| K42 | Kempenfelt Bay 42 | 43 | 1980 to 2024 | 554 | 21.2 |
| K45 | Kempenfelt Bay 45 | 40 | 1980 to 2024 | 537 | 21.5 |
| N31 | – | 12 | 1980 to 1999 | 194 | 20.6 |
| N32 | – | 7.3 | 1981 to 2013 | 257 | 20.9 |
| S15 | – | 21 | 1985 to 2024 | 484 | 21.7 |
The day’s water temperature on Lake Simcoe is reported every hour by Environment and Climate Change Canada’s buoy 45151, moored at 44.50° N, 79.37° W, and posted with the Lake Simcoe marine forecast.
Lake Muskoka Water Temperature
No provincial monitoring station records water temperature on Lake Muskoka, Lake Rosseau or Lake Joseph, and Environment Canada’s buoy network has no buoy on the Muskoka lakes. The nearest long record is the Dorset one, 50 kilometers from Lake Muskoka, 52 from Lake Rosseau, 60 from Lake Joseph and 5 from Lake of Bays.
The province’s aquatic resource records class Lake Muskoka, Lake Rosseau and Lake Joseph as cold-water lakes, with mean depths of 15, 24 and 26 meters. On the Dorset lakes the water at one meter is warmest in the second half of July, at a median of 23.5 °C. Lake Simcoe, a large lake with a long record of its own, reads 4.6 °C colder than the Dorset lakes at one meter in the second half of May and 3.0 °C warmer in the first half of November.
| Lake | Thermal class | Mean depth, m | Deepest, m | From Dorset |
|---|---|---|---|---|
| Lake of Bays | Cold-water | 22 | 79.2 | 5 km |
| Peninsula Lake | Cold-water | 11 | 36.6 | 18 km |
| Bigwind Lake | Cold-water | 8 | 34.7 | 22 km |
| Fairy Lake | Cold-water | 22 | 70.7 | 22 km |
| Bella Lake | Cold-water | 16 | 40.7 | 24 km |
| Mary Lake | Cold-water | 25 | 58.8 | 25 km |
| Dotty Lake | Cold-water | 5.7 | 30.6 | 26 km |
| Wood Lake | Cool-water | 5 | 12.2deepest contour | 27 km |
| Lake Vernon | Cold-water | 14 | 42.9 | 30 km |
| Skeleton Lake | Cold-water | 32 | 79.9 | 41 km |
| Buck Lake | Cold-water | 9.9 | 24.9 | 42 km |
| Three Mile Lake (Muskoka) | Cool-water | 3.5 | 9.1deepest contour | 42 km |
| Riley Lake | Cool-water | 5.4 | 15.2deepest contour | 48 km |
| Kahshe Lake | Cool-water | 4.9 | 18.3deepest contour | 49 km |
| Lake Muskoka | Cold-water | 15 | 73.1 | 50 km |
| Lake Rosseau | Cold-water | 24 | 90.1 | 52 km |
| Butterfly Lake | Cool-water | 3.7 | 4.6deepest contour | 57 km |
| Morrison Lake | Cool-water | 4 | 18.3 | 58 km |
| Sparrow Lake | Cool-water | 4.3 | 12.2deepest contour | 59 km |
| Lake Joseph | Cold-water | 26 | 92.5 | 60 km |
| Bass Lake (Muskoka) | Cool-water | 4.3 | 11deepest contour | 61 km |
| Stewart Lake | Cold-water | 6 | 18.3deepest contour | 66 km |
| Gibson Lake | Cool-water | 8 | 12.2deepest contour | 70 km |
| Six Mile Lake | Cool-water | 8 | – | 72 km |
| Gloucester Pool | Cool-water | 6.9 | 27.4deepest contour | 75 km |
The Haliburton lakes lie 6 to 57 kilometers from Dorset. The largest of them by surface area are Kawagama Lake, Oxtongue Lake, Redstone Lake, Drag Lake, Kashagawigamog Lake and Halls Lake, all six classed cold-water in the same records.
Georgian Bay Water Temperature
Two Environment and Climate Change Canada buoys report the water temperature of Georgian Bay every hour: buoy 45137 in the open bay, at 45.55° N, 81.00° W, and buoy 45143 in the south of the bay, at 44.95° N, 80.63° W. Both are posted with the Georgian Bay marine forecast.
The United States National Data Buoy Center posts the same two stations, 45137 and 45143, with their recent readings. Past readings from the Canadian buoys are archived by Fisheries and Oceans Canada’s Marine Environmental Data Section.
Georgian Bay is part of Lake Huron, and the Great Lakes Environmental Research Laboratory of the United States National Oceanic and Atmospheric Administration publishes the Great Lakes Surface Environmental Analysis, a daily map of surface water temperature and ice cover across the Great Lakes. The map is built from satellite imagery, with cloud-covered water filled from the imagery of nearby days.
Surface Temperature and Temperature at Depth
A lake’s surface temperature describes its top layer. A satellite thermal sensor reads the skin of the water through clear sky, a buoy reads the water at its mooring, and a profile reads the water at set depths from the surface to the bottom.
In summer the top meter is evenly mixed. In the Harp Lake record the median at the surface and the median at one meter differ by no more than 0.2 °C in any month from May to October. Below the warm layer the difference grows to many degrees: in July the water at 35 meters is 18.8 °C colder than the water at one meter.
Wind moves the surface layer. A strong, steady wind pushes warm surface water downwind and can draw colder water up along the upwind shore, an event called upwelling, so two shores of one lake can read differently on the same day.
Cold, Cool and Warm-Water Lakes
Ontario’s aquatic resource records give each lake a thermal class, cold, cool or warm, for its typical summer water temperature. The class is set from measured water temperature, from the fish or invertebrate community, or from both.
For lakes the ministry’s description sorts the classes by fish community. Lake trout, brook trout and lake whitefish are the species it names as typical of cold-water lakes, walleye and northern pike of cool-water lakes, and largemouth bass, carp and bullheads of warm-water lakes. Where the signs overlap, the class of the most temperature-sensitive community applies, so a deep, nutrient-poor lake holding lake whitefish, walleye and northern pike is classed cold-water.
Of the 246 lakes listed below, the records class 138 as cold-water, 62 as cool-water and 46 as warm-water. The class follows depth: the median of the lakes’ mean depths is 12.1 meters for the cold-water lakes, 4.3 for the cool-water lakes and 3.6 for the warm-water lakes. A deep lake keeps a cold layer below its thermocline through the summer, and a shallow lake warms to the bottom.
| Class | Lakes | Median of mean depths, m | Largest by surface area |
|---|---|---|---|
| Cold-water | 138 | 12.187 with a mean depth | Sturgeon Lake (Thunder Bay District), Lake Temagami, Wanapitei Lake, Lake Muskoka, Kakagi Lake, Dryberry Lake, Lake Panache, Lake of Bays |
| Cool-water | 62 | 4.349 with a mean depth | Lake Simcoe, White Lake (Thunder Bay District), Dinorwic Lake, Lake Couchiching, Kenogamisis Lake, Whitefish Lake (Thunder Bay District), Remi Lake, Red Lake |
| Warm-water | 46 | 3.631 with a mean depth | Lake Nipissing, Rice Lake, Lake Scugog, Lady Evelyn Lake, Pigeon Lake, Balsam Lake, Sturgeon Lake (Kawartha Lakes), Buckhorn Lake |
Cold-Water Lakes
- Abram Lake
- Ahmic Lake
- Anstruther Lake
- Aylen Lake
- Baptiste Lake
- Bark Lake
- Basswood Lake
- Bay Lake
- Beaver Lake
- Beech Lake
- Bella Lake
- Big Hawk Lake
- Big Rideau Lake
- Bigwind Lake
- Black Donald Lake
- Blackstone Lake
- Bob Lake
- Boshkung Lake
- Buck Lake
- Buckshot Lake
- Canning Lake
- Canoe Lake
- Cassels Lake
- Catchacoma Lake
- Centennial Lake
- Chandos Lake
- Charleston Lake
- Chiblow Lake
- Clear Lake (Parry Sound District)
- Clear Lake (Parry Sound District)
- Constant Lake
- Crane Lake
- Crow Lake
- Crystal Lake
- Davis Lake
- Desert Lake
- Doe Lake
- Dog Lake (Frontenac County)
- Dotty Lake
- Drag Lake
- Dryberry Lake
- Eagle Lake (Parry Sound District)
- Eagle Lake (Frontenac County)
- Eagle Lake (Haliburton County)
- Eagle Lake (Kenora District)
- Eels Lake
- Esson Lake
- Eva Lake
- Fairy Lake
- Golden Lake
- Gull Lake
- Haliburton Lake
- Halls Lake
- Hawkeye Lake
- Kakagi Lake
- Kamaniskeg Lake
- Kashagawigamog Lake
- Kawagama Lake
- Kennebec Lake
- Kennisis Lake
- Kukagami Lake
- Kushog Lake
- Lake Bernard
- Lake Cecebe
- Lake Clear
- Lake Doré
- Lake Joseph
- Lake Manitou
- Lake Muskoka
- Lake Nipigon
- Lake Opeongo
- Lake Panache
- Lake Rosseau
- Lake St. Peter
- Lake Talon
- Lake Temagami
- Lake Vernon
- Lake of Bays
- Larder Lake
- Lauzon Lake
- Little Kennisis Lake
- Lower Shebandowan Lake
- Manitouwabing Lake
- Maple Lake
- Mary Lake
- Matinenda Lake
- Middle Shebandowan Lake
- Mink Lake
- Minnitaki Lake
- Miskwabi Lake
- Missinaibi Lake
- Mississagua Lake
- Moore Lake
- Mountain Lake
- Muskrat Lake
- Nepahwin Lake
- Net Lake
- Obabika Lake
- Otter Lake (Parry Sound District)
- Oxtongue Lake
- Palmerston Lake
- Papineau Lake
- Paudash Lake
- Pelican Lake
- Peninsula Lake
- Percy Lake
- Rabbit Lake
- Ramsey Lake
- Ranger Lake
- Rankin Lake
- Raven Lake
- Redstone Lake
- Round Lake (Renfrew County)
- Sand Lake (Parry Sound District)
- Sharbot Lake
- Sherborne Lake
- Silver Lake
- Skeleton Lake
- Soyers Lake
- St. Nora Lake
- Star Lake
- Stewart Lake
- Sturgeon Lake (Thunder Bay District)
- Thunder Lake
- Trout Lake (Nipissing District)
- Trout Lake (Sudbury District)
- Turtle Lake
- Twelve Mile Lake
- Upper Shebandowan Lake
- Wabaskang Lake
- Wahwashkesh Lake
- Wanapitei Lake
- Watabeag Lake
- Weslemkoon Lake
- White Otter Lake
- Whitefish Lake (Parry Sound District)
- Windy Lake
- Wollaston Lake
Cool-Water Lakes
- Bass Lake (Simcoe County)
- Bass Lake (Muskoka)
- Bear Lake
- Big Gull Lake
- Black Sturgeon Lakes
- Butterfly Lake
- Calabogie Lake
- Chesley Lake
- Conestogo Lake
- Crotch Lake
- Dinorwic Lake
- Dog Lake (Thunder Bay District)
- Farlain Lake
- Four Mile Lake
- Frederick House Lake
- Gibson Lake
- Gloucester Pool
- Gooderham Lake
- Harris Lake
- Head Lake (Kawartha Lakes)
- Head Lake (Haliburton County)
- Healey Lake
- Horseshoe Lake (Parry Sound District)
- Horseshoe Lake (Haliburton County)
- Kahshe Lake
- Kashwakamak Lake
- Kenogami Lake
- Kenogamisis Lake
- Lac des Mille Lacs
- Lake Belwood
- Lake Couchiching
- Lake Eugenia
- Lake Lulu
- Lake Simcoe
- Lake St. John
- Lake of the Woods
- Long Lake (Timiskaming District)
- Longpine Lake
- MacLean Lake
- Malachi Lake
- Marmion Lake
- Miller Lake
- Mistinikon Lake
- Morrison Lake
- Pickerel Lake
- Porcupine Lake
- Red Lake
- Remi Lake
- Riley Lake
- Sand Lake (Leeds and Grenville)
- Sesekinika Lake
- Six Mile Lake
- Skootamatta Lake
- Sparrow Lake
- Tea Lake
- Three Mile Lake (Muskoka)
- Three Mile Lake (Parry Sound District)
- Wabigoon Lake
- White Lake (Thunder Bay District)
- Whitefish Lake (Thunder Bay District)
- Wildgoose Lake
- Wood Lake
Warm-Water Lakes
- Agnew Lake
- Balsam Lake
- Belmont Lake
- Big Bald Lake
- Big Cedar Lake
- Buckhorn Lake
- Cameron Lake
- Canal Lake
- Chemong Lake
- Clear Lake (Peterborough County)
- Crowe Lake
- Dalrymple Lake
- Jack Lake
- Kasshabog Lake
- Lady Evelyn Lake
- Lake Nipissing
- Lake Nosbonsing
- Lake Scugog
- Little Bald Lake
- Long Lake (Sudbury District)
- Loughborough Lake
- Lovesick Lake
- Lower Buckhorn Lake
- Mississippi Lake
- Mitchell Lake
- Moira Lake
- Nepewassi Lake
- Onaping Lake
- Pigeon Lake
- Puslinch Lake
- Red Cedar Lake
- Restoule Lake
- Rice Lake
- Richard Lake
- Round Lake (Peterborough County)
- Salerno Lake
- Sandy Lake
- Shadow Lake
- South Lake
- Stony Lake
- Sturgeon Lake (Kawartha Lakes)
- Varty Lake
- Vermilion Lake
- White Lake (Lanark County)
- White Lake (Peterborough County)
- Whitefish Lake (Leeds and Grenville)
Water Temperature Today
Environment and Climate Change Canada reports the water temperature every hour from six marine buoys on Georgian Bay, the North Channel and three inland lakes: Lake Nipissing, Lake Simcoe and Lake of the Woods. Each reading is posted on the buoy’s page with the marine forecast for its area.
| Buoy | Station | Position |
|---|---|---|
| Georgian Bay | 45137 | 45.55° N, 81.00° W |
| South Georgian BayGeorgian Bay | 45143 | 44.95° N, 80.63° W |
| North ChannelNorth Channel, Lake Huron | 45154 | 46.05° N, 82.63° W |
| Lake Simcoe | 45151 | 44.50° N, 79.37° W |
| Lake Nipissing | 45152 | 46.23° N, 79.72° W |
| Lake of the Woods | 45148 | 49.63° N, 94.57° W |
The same pages carry the open-water buoys of Lake Superior, Lake Huron, Lake Erie and Lake Ontario. None of Environment Canada’s buoys is moored on the Muskoka, Haliburton or Kawartha lakes.
For the Great Lakes as a whole, including Georgian Bay, the Great Lakes Surface Environmental Analysis maps the surface temperature each day from satellite imagery.
Cold Water Guidance
Guidance on cold water, cold water shock and swimming or paddling in cold water is published by the Lifesaving Society and the Canadian Safe Boating Council.
| Source | What it covers |
|---|---|
| Lifesaving Society, Cold Water and Ice | Cold water immersion, ice safety and self-rescue after a fall through ice, from the society’s Ontario branch. |
| Canadian Safe Boating Council, Cold Water | The stages of cold water immersion and the part a lifejacket plays in each. |
A surface reading describes the top layer of a lake. From late spring to early autumn the water below the warm surface layer is colder than the surface, and in a layered lake the deep water is colder by many degrees.
Where the Records Come From
- The Dorset lakes. The Ministry of the Environment, Conservation and Parks has sampled temperature and dissolved oxygen profiles on Blue Chalk Lake, Chub Lake, Crosson Lake, Dickie Lake, Harp Lake, Heney Lake, Plastic Lake and Red Chalk Lake from the Dorset Environmental Science Centre since 1976. The record is published as Inland Lakes and Streams, Physical Conditions.
- Lake Simcoe Monitoring. The same ministry’s stations on Lake Simcoe, sampled since 1980 for temperature, dissolved oxygen, nutrients, clarity and plankton.
- Aquatic resource area records. The Ministry of Natural Resources’ summary for each waterbody, holding its thermal class, mean and maximum depth, surface area and fish species.
- Broad-scale Monitoring. The province samples about 650 inland lakes, each once in a five-year cycle, since 2008, and takes measurements from the surface to the bottom to build habitat profiles. Its open data release carries 21 measures of water chemistry and clarity and no water temperature.
- Environment and Climate Change Canada’s marine buoys, which report hourly from their moorings.
Common Questions
When is Lake Muskoka warmest?
No provincial station records water temperature on Lake Muskoka. On the eight research lakes near Dorset, 50 kilometers away, the water at one meter is warmest in the second half of July, at a median of 23.5 °C, and holds a median of 20 °C or more from the second half of June to the second half of August.
How cold is Lake Simcoe in June?
At the province’s twelve stations on Lake Simcoe the water at one meter reads a median of 15.4 °C in the first half of June and 18.4 °C in the second half. Cook’s Bay and the shallow stations warm first, and the deep water stays far colder.
When is lake water warmest in Ontario?
In the second half of July on both provincial records. The Dorset lakes read a median of 23.5 °C at one meter then, and the Lake Simcoe stations 22.1 °C.
What is the water temperature in Georgian Bay today?
Environment and Climate Change Canada’s buoys 45137, in the open bay, and 45143, in the south of the bay, report it every hour on the Georgian Bay marine forecast page. The Great Lakes Surface Environmental Analysis maps it daily from satellite imagery.
What is a thermocline?
The thermocline is the band of water in a layered lake where the temperature falls fastest with depth, between the warm surface layer and the cold deep water. On Harp Lake in July it lies between four and five meters, and it deepens through the summer.
When do Ontario lakes turn over?
In spring, soon after the ice goes out, and in autumn, before freeze-up. On Harp Lake the median profile is nearly even from top to bottom in April and in November, the two months of turnover in the record.
How cold is the bottom of a lake in summer?
On Harp Lake the water at 35 meters reads between 4.3 and 4.5 °C from May to October. At station K42 on Lake Simcoe the water at 39 meters reads 7.3 °C in July and warms to 9.9 °C by October.
What is a cold-water lake?
A lake the province’s aquatic resource records class as cold-water, by its typical summer water temperature and its fish community. Lake trout, brook trout and lake whitefish are the species the ministry names as typical of such lakes.
Which fish are typical of cool-water lakes?
The ministry’s description names walleye and northern pike as typical of cool-water lakes, and largemouth bass, carp and bullheads of warm-water lakes.
Is there a buoy on Lake Nipissing?
Environment and Climate Change Canada’s buoy 45152 reports the water temperature on Lake Nipissing every hour, moored at 46.23° N, 79.72° W.
Where is guidance on swimming in cold water published?
The Lifesaving Society and the Canadian Safe Boating Council each publish guidance on cold water and cold water shock.
Lake Maps
Every lake linked in this guide has a page of its own, with its depth survey and the province’s records for the lake. Printed depth maps of the drawn lakes are made to order from the province’s own depth soundings.
See the lake maps All lake pagesSmall print
- Readings at one meter
- Each one meter figure is the median of every reading taken at that depth in that half of the month, across every year of the record, and the range holds the middle eight readings in ten. A single year can fall outside it, and so can a single day.
- The Dorset lakes
- Every Dorset figure comes from readings on eight research lakes near Dorset, none sampled deeper than 37 meters, and describes those lakes alone. A larger and deeper lake can warm later in spring and cool later in autumn than the lakes in the record, as Lake Simcoe does.
- Lake Simcoe stations
- Each Lake Simcoe figure belongs to the province’s stations. The one meter figures pool the readings of the twelve stations on the lake, and the three Holland River stations are left out. A bay and the open lake read differently on the same day, so no single figure describes the whole lake.
- Profiles
- A profile figure is the median at one depth in one month across the years of the record. In a single year the thermocline can lie a few meters above or below the median, and turnover comes on a different date each year.
- Thermal class
- The thermal class is the province’s classification for fisheries management. It is set from measured temperature, from the fish or invertebrate community, or from both, and the record does not say which applied to a given lake. It describes a typical summer and records no temperature.
- Distances
- A distance from Dorset is the straight line from the village to the middle of the lake’s mapped outline.
- Buoys
- A buoy reports the water at its mooring. Environment and Climate Change Canada notes that a buoy’s observations might not represent conditions across the whole marine area it is named for. No buoy reading is reproduced here.
- Satellite temperature
- A satellite thermal sensor reads the surface skin of the water through clear sky, and a cloud-covered day gives no reading. The Great Lakes Surface Environmental Analysis fills cloud-covered water from the imagery of nearby days.
- Current conditions and safety
- Nothing here reports the water temperature of any lake on any day, and nothing here states a safe temperature for swimming, paddling or immersion. Cold water guidance belongs to the bodies that publish it.
- Water temperature near Dorset: Inland Lakes and Streams, Physical Conditions, Dorset Environmental Science Centre, Ministry of the Environment, Conservation and Parks, Open Government Licence – Ontario, 1976 to 2025.
- Lake Simcoe water temperature: Lake Simcoe Monitoring, Ministry of the Environment, Conservation and Parks, Open Government Licence – Ontario, 1980 to 2024.
- Thermal class, mean depth and surface area: Ontario Aquatic Resource Area records, Ministry of Natural Resources, Open Government Licence – Ontario. Class definitions restated from the ministry’s Aquatic Resource Area data description.
- Deepest figures: Ontario Ministry of Natural Resources bathymetry and the Ontario Hydro Network, Open Government Licence – Ontario.
- Buoy stations and positions: Environment and Climate Change Canada, Marine Weather, read 2026-09-28.
- Great Lakes surface temperature: Great Lakes Surface Environmental Analysis, NOAA Great Lakes Environmental Research Laboratory, CoastWatch Great Lakes Node.
- Broad-scale Monitoring: Government of Ontario, read 2026-09-28; Broadscale Monitoring Program Lake Water Chemistry, Open Government Licence – Ontario.
- Density of water: U.S. Geological Survey, Water Science School, Water Density.