Stewart Lake

Where Stewart Lake is

Stewart Lake lies in the District Municipality of Muskoka, with shoreline in the Township of Muskoka Lakes and the Township of Georgian Bay.

How deep is Stewart Lake?

The deepest mapped contour on Stewart Lake is 18.3 meters, and the water within that boundary descends even farther. Ontario’s bathymetry record holds 17 depth contours for Stewart 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.

Stewart Lake has 2 Lake Partner monitoring sites. One of them has reported phosphorus for the three or more years the province requires before a status is assigned, and is classified oligotrophic.

The longest record is held by site 5166/2 at mid lake, deep spot, with phosphorus sampled once a year over 8 years to 2017. Its long-term mean phosphorus concentration is 6.65 micrograms per litre, within the oligotrophic band, which extends to 10.

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 13 clarity readings at site 5166/2 give a median Secchi depth of 3.0 meters.

Median Secchi depth by year at site 5166/2

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

Water temperature

Ontario’s aquatic resource records class Stewart Lake as a cold-water lake, with a mean depth of 6 meters against its maximum of 18.3.

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 Stewart 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 Stewart Lake

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

Popular species in the province’s records

• Northern Pikepike
• Yellow Perchperch
• Smallmouth Basssunfish
• Largemouth Basssunfish
• Black Crappiesunfish
• Rock Basssunfish
• Pumpkinseedsunfish

Stocking history

Ontario’s stocking records for Stewart Lake run from 1946 to 2015 and hold 22 releases. Splake account for most of the fish: 21,804 released in nine of the years between 2002 and 2015. The last recorded release was 1,500 splake yearlings in 2015.

Eating the fish

The 2026 Guide to Eating Ontario Fish carries consumption advice for two species taken from Stewart Lake, tested for contaminants at a sampling location in Freeman & Medora Twps., Muskoka 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. 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

Stewart 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 Stewart Lake

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

Stewart Lake Ice Fishing

An ice hut on Stewart 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). Stewart 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 Stewart Lake. The province’s nearest long record covers eight lakes near Dorset, about 66 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 Stewart 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

Stewart Lake contains two islands in the Ontario Hydro Network’s outline, of 0.9 and 0.2 hectares.

Official names

Ontario’s geographic names register lists 1 official name on the water and its islands: 1 island.

Nearby lakes

• Bass Lake6 km away
• Lake Joseph7 km away
• Little Lake Joseph9 km away
• Butterfly Lake12 km away

All lakes

Questions about Stewart Lake

• How deep is Stewart Lake?

  Its deepest charted contour is 18.3 meters, and the water within it is deeper.

• How big is Stewart Lake?

  Its surface covers 1.5 square kilometers and it holds 2 islands. Of its 12 kilometers of shoreline, 11 kilometers is the mainland shore and 1 kilometer is the shore of the islands.

• What fish are in Stewart Lake?

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

• Is the water clear?

  The one monitoring site with a phosphorus status is classified oligotrophic, the clearest of the province’s three bands.