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.

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.

Lake Rosseau Aerial Photography

Ontario photographs the province from the air on a rolling schedule.

The photograph below is from the SCOOP2023 collection, flown in 2023 and 2024.

Water quality

Ontario’s Lake Partner Program is a volunteer monitoring network. Participants collect a water sample each spring for phosphorus analysis and measure water clarity through the summer with a Secchi disk.

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 spring phosphorus. Below 10 micrograms per litre is oligotrophic: clear water and low algae. From 10 to 20 is mesotrophic: moderately productive water, with more algae and weed growth than an oligotrophic lake but rarely a bloom. Above 20 is eutrophic: nutrient-rich water, prone to algal blooms and low oxygen at depth.

A Secchi disk is a 20 centimeter 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.

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, sampled over 14 springs to 2021. Its long-term mean phosphorus concentration is 6.86 micrograms per litre, within the oligotrophic band, which extends to 10. The site’s 91 clarity readings give a median Secchi depth of 4.3 meters.

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 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.

Satellite thermal imagery gives a surface reading on clear days: the Landsat sensors pass over Lake Rosseau every eight days at a resolution of 100 meters. In the province’s record the surface reading and the one meter reading agree to within half a degree nine times in ten in summer.

The water a few meters down is colder than any figure on this chart, in every month of the year.

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.

Fish in Lake Rosseau

Ontario’s aquatic resource records name 27 fish species in Lake Rosseau. The province classes the lake as cold water. The record took effect on 21 December 2016, and its note records how the species present were confirmed: “Fish species that were only observed in angler caught lake trout stomach content samples (i.e. probably bait) and not indigenous to these lakes are excluded. BsM Cycle 2 confirmed fish species presence Aug 15, 2016.” The province’s other records for the lake name two further species, rainbow trout and brook trout. They appear in the records below.

Fourteen of the 29 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 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 areas9.8 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 current stocking programme, recorded from 2016, lists no release into Lake Rosseau.

Eating the fish

The 2025 Guide to Eating Ontario Fish carries consumption advice for eight 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. The advice 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. Zone 15 covers central Ontario from Georgian Bay to the Ottawa River, bounded to the north by the Pickerel River, Highway 522 and the northern edge of Algonquin Park. It takes in Parry Sound, Huntsville, Bracebridge, Bancroft, Pembroke and Renfrew, and the whole of Algonquin Provincial Park. 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.

The figures are the number of fish an angler may catch and keep in a day and have in possession, under a sport fishing licence and under a conservation licence. The rows are the whole zone’s, and they apply to every lake in it. No waterbody exception names Lake Rosseau.

Lake Rosseau Ice Fishing

An ice hut on Lake Rosseau must be removed by March 31. Lake Rosseau lies in fisheries management zone 15, one of the twenty zones Ontario sets its fishing rules by, and one removal date applies across the whole zone.

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 zone 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.

When the ice goes out

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. It holds the day the ice left and carries no date for the day it formed, so it says nothing about when Lake Rosseau freezes.

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.

Getting on the water

Twelve access marks are mapped on Lake Rosseau: two boat launches, four enhanced shoreline access points, four marinas, one beach and one dam. Seven are Ontario’s own records, four are from the Overture Places dataset and one is an operator entry, and the list under the map says which is which. Boat launch is the province’s class for a ramp; enhanced shoreline access is its class for a place the public can reach the water on foot.

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

• Lake Joseph8 km away
• Three Mile Lake10 km away
• Skeleton Lake13 km away
• Whitefish Lake19 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, four enhanced shoreline access points, four marinas, one beach and one dam on Lake Rosseau.