Lake St. John

Where Lake St. John is

Lake St. John lies in the County of Simcoe, within the Township of Ramara.

How deep is Lake St. John?

The deepest mapped contour on Lake St. John is 6.1 meters, and the water within that boundary descends even farther. Ontario’s bathymetry record holds 11 depth contours for Lake St. John.

See also Ontario's Largest and Deepest Lakes

Surveyed Depths

Ontario has surveyed the depths of several thousand of its lakes over more than half a century. Later surveys were conducted by boat with satellite positioning and produced hundreds of thousands of soundings per lake. Earlier surveys were charted by running transects across the water and drawing depth contours by hand.

Water quality

Ontario’s Lake Partner Program is a volunteer monitoring network. Participants collect water samples for phosphorus analysis and measure water clarity through the summer with a Secchi disk. Most sites are sampled for phosphorus once a year, in spring, and some several times a year between spring and fall. Phosphorus in many lakes rises through the summer, so the two kinds of average are measured on different bases.

Phosphorus is the nutrient that controls how much algae a lake can grow. A lake with little phosphorus stays clear and supports cold-water fish such as lake trout. As phosphorus rises, algae increase, the water clouds, and oxygen in the deep water falls as the algae decay. The province classifies each monitoring site by its average phosphorus:

• Oligotrophic, below 10 micrograms per litre: clear water and low algae.
• Mesotrophic, from 10 to 20: moderately productive water, with more algae and weed growth than an oligotrophic lake but rarely a bloom.
• Eutrophic, above 20: nutrient-rich water, prone to algal blooms and low oxygen at depth.

Lake St. John has 4 Lake Partner monitoring sites. Four have reported phosphorus for the three or more years the province requires before a status is assigned. All are classified eutrophic.

The longest record is held by site 5141/1 at Main Basin-deep spot, with phosphorus sampled several times a year between spring and fall over 10 years to 2025. Its long-term mean phosphorus concentration is 38.66 micrograms per litre, within the eutrophic band, which begins at 20.

A Secchi disk is a white or black-and-white plate lowered on a marked line until it disappears from view. The depth at which it vanishes is the Secchi depth, a direct measure of water clarity. Clearer water gives a deeper reading.

The 78 clarity readings at site 5141/1 give a median Secchi depth of 2.5 meters.

Median Secchi depth by year at site 5141/1

See also Lake Water Quality and Blue-Green Algae in Ontario

Water temperature

Ontario’s aquatic resource records class Lake St. John as a cool-water lake, with a mean depth of 4.3 meters against its maximum of 6.1.

A lake separates into layers through the summer where it is deep enough. Sunlight and wind warm and mix the upper few meters, and beneath a band of rapid cooling, the thermocline, the water stays cold. A shallow lake mixes to the bottom instead and warms through, sooner in spring and later into autumn.

No provincial monitoring station records water temperature on Lake St. John. The province’s longest in-water record is kept on eight research lakes near Dorset, sampled since 1976. A reading is taken at the surface, at one meter, at each meter to seven and then every two meters to the bottom, about every two weeks through the open-water season and about monthly under ice. The record holds 5,240 readings at a depth of one meter.

At one meter the eight lakes track one another closely. On a day when several were sampled, the warmest and the coolest lake differed by about one degree in every month of the year, and a 12 meter deep lake and a 37 meter deep lake read within 0.2 degrees of each other from June to October. The weather of the year, and not the lake, sets most of the width of the band below.

Water temperature at a depth of one meter (eight lakes near Dorset, ON)

How depth changes the temperature

The chart below is the province’s record on Harp Lake, 37 meters deep, drawn as the median temperature at each depth for each month over 49 years. In July the water at one meter is 23 degrees, at five meters 15, at seven meters 9, and from 13 meters to the bottom under 6. By October the warm layer has deepened to seven meters and cooled to 11 degrees. In February the whole column sits between 1 and 4 degrees under the ice, coldest at the top.

A shallower lake in the same record warms sooner. Heney Lake, six meters deep, reads about 1.8 degrees warmer than Harp Lake at one meter from January to May, 0.8 degrees warmer from June to August, and the same or cooler from September.

See also Lake Water Temperature in Ontario

Fish in Lake St. John

Ontario’s aquatic resource records name 26 fish species in Lake St. John.

Popular species in the province’s records

• Northern Pikepike
• Muskellungepike
• Walleyeperch
• Yellow Perchperch
• Smallmouth Basssunfish
• Largemouth Basssunfish
• Black Crappiesunfish
• Rock Basssunfish
• Pumpkinseedsunfish
• Bluegillsunfish
• Brown Bullheadcatfish
• Channel Catfishcatfish
• Common Carpminnow

Spawning areas

Ontario maps 2 fish spawning records within the Lake St. John outline, for two different species, all of them nests or records of under 100 square meters each. Every record is recorded as verified and presently suitable, and none is classed as sensitive.

By number of records

By area mapped

• Northern Pike1 records0 m²
• Walleye1 records0 m²

The federal Fisheries Act prohibits the harmful alteration, disruption or destruction of fish habitat without an authorization, so a dock, dredging, infill or shoreline work at a mapped spawning area is reviewed on that basis.

Stocking history

Ontario’s stocking records for Lake St. John run from 1946 to 1967 and hold 32 releases. Walleye account for most of the fish: 3,385,000 released in ten of the years between 1946 and 1956. The last recorded release was 40,000 muskellunge frys in 1967.

The province’s record of stocking for recreational fishing, kept from 2016, lists no release into Lake St. John.

Eating the fish

The 2026 Guide to Eating Ontario Fish carries consumption advice for eleven species taken from Lake St. John, tested for contaminants at a sampling location in Rama Twp., Simcoe Co. Each figure below is the most meals a month the guide advises for a fish of that length. Where mercury sets the advice, the figure falls as the fish gets longer, because mercury accumulates over a fish’s life. Where the guide has no result yet for certain contaminants at a location, it sets a precautionary limit of 16 meals a month. A zero means the guide advises no meals of that fish at that size.

General population: adultsSensitive population: children under fifteen, women who are or may become pregnant

Fisheries Management Zone 16

Lake St. John lies in Fisheries Management Zone 16. Ontario sets its fishing seasons and catch limits by zone. The province describes it as a zone of large river systems, with Lake Simcoe and Lake Couchiching its large lakes, carrying important recreational and tourism fisheries under high fishing effort.

Fisheries management zones around Lake St. John

See also Fishing Seasons in Ontario, Zone by Zone and Fish Stocking in Ontario Lakes

Lake St. John Ice Fishing

An ice hut on Lake St. John must be removed by March 15. The province sets dates for ice hut removal as a safety measure to ensure huts are removed before thawing occurs. One date applies to an entire fisheries management zone (FMZ). Lake St. John lies within FMZ 16.

An ice hut must also be removed before the ice breaks up, whether or not that date has passed. The deadline clears huts from the ice ahead of the thaw and does not close the fishing season.

Registration is required in FMZ 16 for an ice hut built of anything other than fabric, and for a fabric tent covering seven square meters or more. Registration is free, and the number is displayed on the outside of the hut.

Ice In / Out Date

No ice record is kept on Lake St. John. The province’s nearest long record covers eight lakes near Dorset, about 68 kilometers away. It holds a date for every winter since 1975, observed by eye and from cameras set on the shore.

On those eight lakes the median date of complete ice cover is December 7 and the median date of complete ice loss is April 20, a median season of 134 days.

The date the ice forms has moved later through the record: a November 26 average across 1971 to 1980, and December 11 across 2011 to 2020. The date it leaves has moved by less than a day each decade over the same period. The two charts below are drawn to one scale of 26 days, so they can be read against each other.

Ice in date (eight lakes near Dorset, ON)

Ice out date (eight lakes near Dorset, ON)

Ice conditions

No public record reports the thickness or the condition of the ice on Lake St. John on any given day. Ontario publishes ice safety guidance for anyone going out on the ice, at ontario.ca.

See also Lake Ice in Ontario

Islands and names

Lake St. John contains ten islands in the Ontario Hydro Network’s outline, zero of them over 10 hectares and seven under a tenth of a hectare.

Nearby lakes

• Lake Couchiching5 km away
• Sparrow Lake16 km away
• Dalrymple Lake16 km away
• Bass Lake17 km away

All lakes

Questions about Lake St. John

• How deep is Lake St. John?

  Its deepest charted contour is 6.1 meters, and the water within it is deeper.

• How big is Lake St. John?

  Its surface covers 6.5 square kilometers and it holds 10 islands. Of its 19 kilometers of shoreline, 17 kilometers is the mainland shore and 2 kilometers is the shore of the islands.

• What fish are in Lake St. John?

  Ontario’s aquatic resource records name 26 species, among them walleye, smallmouth bass, largemouth bass, northern pike, muskellunge and yellow perch.

• Is the water clear?

  All four monitoring sites with a phosphorus status are classified eutrophic, the most nutrient-rich of the province’s three bands.