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Lake Simcoe Temperatures January 4, 2018

Posted by Chris Sullivan in CCR.
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As is it the closest body of water to my house I dive in Lake Simcoe relatively often. The drive takes about an hour, compared to 3 to 4 hours to the admittedly much more interesting diving in places like Brockville and Tobermory. While I’ve managed to do both of the latter sites as day trips, it is an arduous proposition compared to going somewhere from which I can get home by lunch time.

Simcoe is relatively shallow, with a maximum depth of about 42 metres and an average depth of 15m over its 725 square kilometre surface area. The deepest spot is the middle of Kempenfelt Bay, quite close to the hamlet of Shanty Bay where we shore dive, so we can get some depth there if we’re prepared for the cold water. As there are few fish, no wrecks, no reefs and nothing much but weeds and silty bottom, aside from looking at what was put there intentionally, garbage thrown overboard by passing boats, flotsam like deck chairs blown in from shore and the occasional golf ball, one can pass the time by doing drills and studying the marine environment.

This latter pastime caused me to stumble of the World Lake Database. The Lake Simcoe information dates from the mid 1980’s, and contains the following table of temperatures by month.

Station K45, 1984 
Depth May Jun Jul Aug Sep Oct
0 4.8 14.8 17.2 22.2 13.7 12.0
5 4.8 14.5 17.3 22.2 14.5 12.0
10 4.8 12.1 15.6 22.2 15.1 12.0
15 4.8 9.3 13.9 15.1 15.2 12.0
20 4.8 7.7 8.5 9.9 10.4 12.0
30 4.7 7.6 8.0 8.1 6.7 12.0

Depth is in metres, temperature in degrees Celsius. Station K45 is in the open water of Lake Simcoe, not in Kempenfelt Bay. There is a station in Kempenfelt – K42 – but so far I’ve not found any similar data from it. The October data looks suspicious with completely uniform temperature down to 30 metres, but I noticed from my own experience that the temperature becomes more uniform in the late summer so perhaps it is the case. 12C is quite a reasonable temperature so there is no reason to hang up the fins just because summer is over.

The chart below summarizes the water temperatures during my dives this year. In July my dry suit failed on a dive in Tobermory, Ontario and so everything after that is in a relatively thin wet suit, with nothing deeper than 20 metres or colder than 12 Celsius (and not for very long at that temperature I can assure you). Even the dry suit dives were curtailed somewhat because my hands were too cold.

You have to look at the chart’s legend to figure out when the dives took place. The ones that start above 20C are in July and August, the and the ones below 15C are from May and June. By the 12th of September, there was little difference in temperature between the surface and 15 metres.

The data were collected on my AP Inspiration EVP rebreather, which records depth, temperature and many other data points every 10 seconds to a precision, although not necessarily an accuracy, of 10 centimetres and 1/10th of a degree Celsius. For each day’s diving all of the data were summarized to the nearest metre by averaging the temperature readings. The result is not a dive profile, but a thermal profile by depth.

Simcoe Dives 2017

So the temperature difference by depth is greatest in the mid-summer months. By September, the temperature at depth is higher than in mid-summer. It is considerably warmer than the September average in 1984, but the dive was only 1 day in September and in a different part of the lake, so it would be hard to draw a conclusion from that difference, although the City of Barrie has published a Climate Change Strategy which indicates that there is warming trend in Kempenfelt Bay based on K42 data.

I’m looking forward to gathering a lot more data in 2018 after the ice melts. The broken dry suit has been replaced with a better one, including a thicker hood and for the first time ever, dry gloves! So next time there should be a lot more data from the depths.


1. Lake Simcoe Temperatures | Chronicle of an older diver - July 29, 2018

[…] relatively shallow so unfortunately I can’t show the effect all the way to the bottom, but from what I’ve read the relatively warmer water extends to the maximum depth of the lake. Also of note is the slightly […]


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