Three Mile Lake

Where Three Mile Lake is

Three Mile Lake lies in the District of Parry Sound, within the Township of Armour.

How deep is Three Mile Lake?

The deepest mapped contour on Three Mile Lake is 6.1 meters, and the water within that boundary descends even farther. Ontario’s bathymetry record holds 24 depth contours for Three Mile Lake.

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.

Three Mile Lake has 3 Lake Partner monitoring sites. Two have reported phosphorus for the three or more years the province requires before a status is assigned. One of those sites is classified oligotrophic and one mesotrophic.

The longest record is held by site 7129/2 at Mid lk-deepst Watson, with phosphorus sampled several times a year between spring and fall over 15 years to 2025. Its long-term mean phosphorus concentration is 10.63 micrograms per litre, within the mesotrophic band, which runs from 10 to 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 48 clarity readings at site 7129/2 give a median Secchi depth of 3.5 meters.

Median Secchi depth by year at site 7129/2

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

Water temperature

Ontario’s aquatic resource records class Three Mile Lake as a cool-water lake, with a mean depth of 4 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 Three Mile Lake. 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 Three Mile Lake

Ontario’s aquatic resource records name 11 fish species in Three Mile Lake. The province’s other records for the lake name two further species, brook trout and lake trout. They appear in the records below.

Popular species in the province’s records

• Ciscotrout and whitefish
• Northern Pikepike
• Walleyeperch
• Yellow Perchperch
• Smallmouth Basssunfish
• Largemouth Basssunfish
• Rock Basssunfish
• Burbotcod
• Brown Bullheadcatfish

Spawning areas

Ontario maps 10 fish spawning records within the Three Mile Lake outline, for two different species, and all of them are mapped below.

By number of records

By area mapped

• Walleye8 areas5.7 ha
• Smallmouth Bass2 areas0.4 ha

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 Three Mile Lake run from 1947 to 2013 and hold 28 releases. Brook Trout account for most of the fish: 24,900 released in nine of the years between 1947 and 1991. The last recorded release was 10,000 walleye fingerlings in 2013.

Eating the fish

The 2026 Guide to Eating Ontario Fish carries consumption advice for one species taken from Three Mile Lake, tested for contaminants at a sampling location in Armour Twp., Parry Sound Dist. 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 15

Three Mile Lake lies in Fisheries Management Zone 15. Ontario sets its fishing seasons and catch limits by zone. The province describes it as a zone of many natural lake trout and brook trout lakes, with a well-developed road network and moderate angling effort.

Fisheries management zones around Three Mile Lake

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

Three Mile Lake Ice Fishing

An ice hut on Three Mile Lake must be removed by March 31. 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). Three Mile Lake lies within FMZ 15.

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 15 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 Three Mile Lake. The province’s nearest long record covers eight lakes near Dorset, about 50 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 Three Mile Lake 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

Three Mile Lake contains eight islands in the Ontario Hydro Network’s outline, zero of them over 10 hectares and two under a tenth of a hectare.

Official names

Ontario’s geographic names register lists 5 official names on the water and its islands: 5 islands.

Nearby lakes

• Pickerel Lake10 km away
• Doe Lake10 km away
• Clear Lake11 km away
• Sand Lake12 km away

All lakes

Questions about Three Mile Lake

• How deep is Three Mile Lake?

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

• How big is Three Mile Lake?

  Its surface covers 3.6 square kilometers and it holds 8 islands. Of its 16 kilometers of shoreline, 13 kilometers is the mainland shore and 3 kilometers is the shore of the islands.

• What fish are in Three Mile Lake?

  Ontario’s aquatic resource records name eleven species, among them walleye, smallmouth bass, largemouth bass, northern pike and yellow perch. The lake’s spawning, consumption and stocking records also name brook trout and lake trout.