Lake Rosseau

Where Lake Rosseau is

Lake Rosseau lies in the District Municipality of Muskoka and the District of Parry Sound, with shoreline in the Township of Muskoka Lakes and the Township of Seguin.

How deep is Lake Rosseau?

Lake Rosseau reaches a maximum recorded depth of 90.1 meters, determined from 156,694 soundings taken during Ontario’s 2000 to 2003 survey.

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 Rosseau has 18 Lake Partner monitoring sites. Six have reported phosphorus for the three or more years the province requires before a status is assigned. All are classified oligotrophic.

The longest record is held by site 2476/1 at CentralW-WellesleyIs, with phosphorus sampled once a year over 14 years to 2021. Its long-term mean phosphorus concentration is 6.86 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 91 clarity readings at site 2476/1 give a median Secchi depth of 4.3 meters.

Median Secchi depth by year at site 2476/1

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

Lake Rosseau Weather

The weather page for Lake Rosseau carries the forecast over the water, wave heights across the lake, the modelled water temperature, and the water level.

Lake Rosseau weather

Water temperature

Ontario’s aquatic resource records class Lake Rosseau as a cold-water lake, with a mean depth of 24 meters against its maximum of 90.1.

A deep lake separates into layers through the summer. Sunlight and wind warm and mix the upper few meters, and beneath a band of rapid cooling, the thermocline, the water stays cold through the whole season. A mean depth of 24 meters places most of Lake Rosseau’s volume in that cold layer, which is the condition lake trout and lake whitefish require.

No provincial monitoring station records water temperature on Lake Rosseau. The weather page for Lake Rosseau carries the LastLake model’s surface water temperature for each day of the coming week. 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 Rosseau

Ontario’s aquatic resource records name 27 fish species in Lake Rosseau. The province’s other records for the lake name two further species, rainbow trout and brook trout. They appear in the records below.

Popular species in the province’s records

• Lake Trouttrout and whitefish
• Lake Whitefishtrout and whitefish
• Ciscotrout and whitefish
• Northern Pikepike
• Muskellungepike
• Walleyeperch
• Yellow Perchperch
• Smallmouth Basssunfish
• Largemouth Basssunfish
• Black Crappiesunfish
• Rock Basssunfish
• Pumpkinseedsunfish
• Burbotcod
• Brown Bullheadcatfish

Spawning areas

Ontario maps 442 fish spawning records within the Lake Rosseau outline, for five different species. 337 are individual nests of a few square meters each, and the other 105 are mapped areas. Every record is recorded as verified and presently suitable, and none is classed as sensitive.

By number of records

By area mapped

• Smallmouth Bass337 nests900 m²
• Lake Trout85 areas7.3 ha
• Muskellunge14 areas4.7 ha
• Walleye5 areas1.0 ha
• Rainbow Smelt1 area500 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 Rosseau run from 1946 to 1990 and hold 100 releases. Walleye account for most of the fish: 7,392,600 released in thirteen of the years between 1946 and 1989. The last recorded release was 6,400 lake trout yearlings in 1990.

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

Eating the fish

The 2026 Guide to Eating Ontario Fish carries consumption advice for nine species taken from Lake Rosseau, tested for contaminants at a sampling location in Cardwell Twp., Muskoka D. M. 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

Lake Rosseau 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 Lake Rosseau

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

Lake Rosseau Ice Fishing

An ice hut on Lake Rosseau 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). Lake Rosseau 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 Out Date

The ice has been recorded leaving Lake Rosseau in 95 springs between 1900 and 1995. The median date is April 21. The earliest recorded break-up is March 27 in 1946, and the latest is May 7 in 1926.

Across the 95 recorded springs the median date has moved by less than a day each decade.

The record was contributed by Peter J. Dillon, Ontario Ministry of the Environment to the Global Lake and River Ice Phenology Database. The record only holds the ice-out dates for Lake Rosseau, and no ice-in date.

Ice out dates for 95 recorded springs between 1900 and 1995

Ice conditions

No public record reports the thickness or the condition of the ice on Lake Rosseau 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 and When the Lakes Will Freeze This Winter

Getting on the water

Twelve access marks are mapped on Lake Rosseau.

See also Boat Launches on Ontario Lakes

Islands and names

Lake Rosseau contains 96 islands in the Ontario Hydro Network’s outline, eight of them over 10 hectares and 24 under a tenth of a hectare. Tobin Island, at 445 hectares, is larger than the next ten together, which cover 223.

Official names

Ontario’s geographic names register lists 140 official names on the water and its islands: 80 islands, 36 bays, five shoals, four rocks, two harbours, one cut, ten points on islands and two rivers. A further 48 named points stand on the mainland shore. The register records the origin of one name: Bay of the Singing Sand, “Wind blows through pine trees across sand in bay.”

Bays 36

• Adams Bay
• Arthurlie Bay
• Barn Bay
• Bass Bay
• Bay of the Singing Sand
• Beechwood Bay
• Beley Bay
• Bellevue Bay
• Brackenrig Bay
• Bullfrog Bay
• Cameron Bay
• Coate Bay
• East Portage Bay
• Ferndale Bay
• Forges Bay
• Garden Bay
• Johns Bay
• Lawrason Bay
• Lily Bay
• Long Bay
• Maple Leaf Bay
• Marys Bay
• Morgan Bay
• Morinus Bay
• Mutchinbacker Bay
• Paton Bay
• Rosseau Bay
• Sandy Bay
• Sicamous Bay
• Skeleton Bay
• Snowshoe Bay
• Sucker Bay
• Tennessee Bay
• The Bluff Bay
• Wallace Bay
• Wileys Bay

Shoals and rocks 9

• Baker Shoal
• Bells Rock
• Country Club Shoal
• Duck Rock
• Gull Rock
• Joseph River Shoal
• Kenozja Shoal
• Lighthouse Shoal
• Tommy Rock

Harbours, cuts and narrows 3

• Cayleys Cut
• Rest Harbour
• Snug Harbour

Points on the shore 48

• Almeda Point
• Armstrong Point
• Bear Point
• Beley Point
• Black Rock Point
• Clause Point
• Clovelly Point
• Craig Point
• Crossley Point
• Eagles Nest Point
• Echo Point
• Elarton Point
• Ferndale Point
• Flamarion Point
• Four Mile Point
• Glenavy Point
• Greenridge Point
• Gregory Point
• Harraby Point
• Hemlock Point
• Idyl-Wild Point
• Jenckes Point
• Kelley Point
• Kingsett Point
• Lash Point
• Llanllar Point
• Llanrwst Point
• Lord Aylmer Point
• Macassa Point
• Maplehurst Point
• Moss Rock Point
• Oak Bank Point
• Onnalinda Point
• Orgills Point
• Paton Point
• Pickerel Point
• Point Charles
• Point Frederick
• Point Iolene
• Point Stenning
• Proctors Point
• Ravenscrag Point
• Rossmoyne Point
• Rothesay Point
• Stormy Point
• Sunset Point
• Wildcat Point
• Worthington Point

Communities on the lake 5

• Rosseau
• Windermere
• Minett
• Port Sandfield
• Port Carling

Nearby lakes

• Little Lake Joseph7 km away
• Lake Joseph8 km away
• Three Mile Lake10 km away
• Butterfly Lake11 km away

All lakes

Questions about Lake Rosseau

• How deep is Lake Rosseau?

  Its maximum recorded depth is 90.1 meters, determined during Ontario’s 2000 to 2003 survey.

• How big is Lake Rosseau?

  Its surface covers 62.9 square kilometers and it holds 96 islands. Of its 217 kilometers of shoreline, 139 kilometers is the mainland shore and 78 kilometers is the shore of the islands.

• What fish are in Lake Rosseau?

  Ontario’s aquatic resource records name 27 species, among them lake trout, walleye, smallmouth bass, largemouth bass, northern pike and muskellunge. The lake’s spawning, consumption and stocking records also name rainbow trout and brook trout.

• Is the water clear?

  All six monitoring sites with a phosphorus status are classified oligotrophic, the clearest of the province’s three bands.

• Is there a public boat launch?

  Records mark two boat launches.