Since 1980, Ontario has sent crews out on Lake Simcoe roughly sixteen times a year to lower probes from the surface to the bottom and write down two numbers every metre or so: water temperature and dissolved oxygen. Forty-five years of that adds up to about 105,000 readings from 15 stations, and the whole record is public.
We put the record into the charts below. Scroll, and the chart follows the lake through a year: from under the ice, through turnover, summer layering, and the late-summer oxygen squeeze, to fall turnover. Then we compare the deep and shallow parts of the lake, and look at how much clearer the water has become.
Under the ice, the bottom is the warm part. In January to March the surface sits near 0.4 °C while the deepest water holds around 2 °C. Water is heaviest at about 4 °C, so the slightly warmer water settles to the bottom and stays there until the ice goes out.
April: one temperature from top to bottom. After ice-out the whole column in the deep stations sits around 3.5 °C. Wind can mix it all the way down. Of the deep-basin profiles taken in April, 88% show less than 1 °C between surface and bottom.
June to September: a warm lid on cold water. By August the median surface reading is 21.8 °C, while at 34 m it is 8.3 °C. In the deep stations, not one profile from June through September in 45 years shows the column mixed. Once the top layer warms, the wind can no longer stir the cold water underneath.
The cold layer runs short of oxygen. Cut off from the surface, the deep water slowly uses up its oxygen. In August, 11% of deep-basin readings below 10 m are under 5 mg/L; in September it is 40%, and in October 30%. Anglers commonly describe late summer as the time when fish hold above the cold layer rather than in it, and this is the reason the cold water stops being a refuge.
October and November: turnover. As the surface cools, the layers lose their difference and the lake mixes again. By November, 86% of deep-basin profiles are within 1 °C top to bottom, and oxygen at depth is back above 9 mg/L.
But that is only half of Simcoe. The pattern above comes from three deep stations in Kempenfelt Bay and the main basin. The shallower stations in the east and north behave differently: they mix again and again through summer. In August, 57% of their profiles are near-mixed; in September, 80%. In the deep basin it is 0% for both months. For temperature and oxygen, Simcoe acts like two lakes sharing one shoreline.
The water got clearer. Secchi depth is how far down a white disc stays visible. Through the 1980s and early 1990s, the yearly median sat mostly between 3 and 4 m. Zebra mussels were established in Simcoe by 1995, according to the Lake Simcoe Region Conservation Authority, which credits the mussels (along with phosphorus reductions) for the clearer water. Between 1996 and 2017, 19 of 22 years read 5 to 6 m.
The last three years have been murkier again. The yearly median was 3.5 m in 2022 and 4.0 m in both 2023 and 2024, closer to the pre-mussel range. Three years is not enough to call a trend, and the monitoring record alone does not say why.
What this means if you fish Simcoe
None of this tells you where the fish are on a given day. It tells you which water is even available to them, and when.
- In the deep basin, depth is seasonal. From August into October, the deepest water in Kempenfelt Bay and the main basin is often short of oxygen. Depth that held fish in June may be empty in September.
- The shallow basins do not follow the same calendar. Because they keep mixing, the oxygen squeeze measured in the deep stations does not carry over to them. A depth rule learned in Kempenfelt Bay may not apply off the east shore.
- Clearer water moves light deeper. A brighter bottom is one reason anglers commonly say Simcoe fish sit deeper, or bite later in the day, than they did a generation ago. The data here show the clarity change; they do not measure fish behaviour.
- Fall turnover resets the lake. After the layers merge in October and November, temperature and oxygen are roughly even from top to bottom again.
How we made these charts
All numbers come from the Ontario Lake Simcoe Monitoring dataset, 1980–2024. A "profile" is every reading from one station on one day. The deep basin means stations K39, K42 and K45; the shallow basin means E50, E51 and N31. For the depth charts we averaged each profile in 2 m layers, then took the median across all profiles in that month; months with fewer than five deep-basin profiles (December) are shown hatched. "Near-mixed" means the surface (top 2 m) and bottom (deepest 2 m) differ by less than 1 °C, using profiles with at least four readings spanning 8 m or more. Clarity is the median of all Secchi readings in each year.