Lac des Mille Lacs

Where Lac des Mille Lacs is

Lac des Mille Lacs lies in the District of Thunder Bay.

How deep is Lac des Mille Lacs?

The deepest mapped contour on Lac des Mille Lacs is 27.4 meters, and the water within that boundary descends even farther. Ontario’s bathymetry record holds 943 depth contours for Lac des Mille Lacs.

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.

Lac des Mille Lacs has 6 Lake Partner monitoring sites. Four have reported phosphorus for the three or more years the province requires before a status is assigned. One of those sites is classified mesotrophic and three eutrophic.

The longest record is held by site 2391/2 at Sawbill bay, with phosphorus sampled once a year over 3 years to 2024. Its long-term mean phosphorus concentration is 23.7 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 3 clarity readings at site 2391/2 give a median Secchi depth of 0.6 meters.

Median Secchi depth by year at site 2391/2

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

Lac des Mille Lacs Weather

The weather page for Lac des Mille Lacs carries the forecast over the water, wave heights across the lake, and the modelled water temperature.

Lac des Mille Lacs weather

Water temperature

Ontario’s aquatic resource records class Lac des Mille Lacs as a cool-water lake, with a mean depth of 6.3 meters against its maximum of 27.4.

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 Lac des Mille Lacs. The LastLake model computes the temperature of its surface water for each day from the daily mean air temperature at Environment Canada’s Atikokan climate station, 83 kilometers from the lake, and from the lake’s area of 24,518 hectares and its mean depth of 6.3 meters.

Each day’s figure carries the model’s typical error for that month on lakes of the same size. The figure in the chart below is the temperature near the surface, away from the shoreline. Bays, shallows and water deeper below the surface differ in temperature. Temperature is not measured when ice covers the lake. The weather page for Lac des Mille Lacs carries the model’s figures for the past weeks and the coming week.

See also Lake Water Temperature in Ontario

Fish in Lac des Mille Lacs

Ontario’s aquatic resource records name 13 fish species in Lac des Mille Lacs.

Popular species in the province’s records

• Lake Whitefishtrout and whitefish
• Ciscotrout and whitefish
• Northern Pikepike
• Walleyeperch
• Yellow Perchperch
• Smallmouth Basssunfish
• Burbotcod

Spawning areas

Ontario maps 7 fish spawning records within the Lac des Mille Lacs outline: 7 walleye spawning areas covering 2512.63 hectares. Every record is recorded as verified and presently suitable, and none is classed as sensitive.

By number of records

By area mapped

• Walleye7 areas2512.6 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.

Eating the fish

The 2026 Guide to Eating Ontario Fish carries consumption advice for eight species taken from Lac des Mille Lacs, tested for contaminants at a sampling location in Thunder Bay 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 6

Lac des Mille Lacs lies in Fisheries Management Zone 6. Ontario sets its fishing seasons and catch limits by zone.

Fisheries management zones around Lac des Mille Lacs

Zone-wide, Smelt may not be used as bait or possessed for use as bait except where exceptions apply.

Where a row reads zone-wide, the zone’s own season or limit above applies.

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

Islands and names

Lac des Mille Lacs contains 574 islands in the Ontario Hydro Network’s outline, 32 of them over 10 hectares and 196 under a tenth of a hectare.

Official names

Ontario’s geographic names register lists 28 official names on the water and its islands: 9 islands, twelve bays, three narrows and one point on an island. A further 5 named points stand on the mainland shore.

Bays 12

• Baril Bay
• Blind Bay
• Bolton Bay
• Boot Bay
• Carlstead Bay
• Maze Bay
• Open Bay
• Portage Bay
• Reserve Bay
• Rock Bay
• Sawmill Bay
• Shelter Bay

Harbours, cuts and narrows 3

• Birch Narrows
• Honkonen Narrows
• Owl Narrows

Points on the shore 5

• Birch Point
• Long Point
• Pine Point
• Poplar Point
• Sand Point

Nearby lakes

• Upper Shebandowan Lake27 km away
• Middle Shebandowan Lake30 km away
• Lower Shebandowan Lake38 km away
• Eva Lake49 km away

All lakes

Questions about Lac des Mille Lacs

• How deep is Lac des Mille Lacs?

  Its deepest charted contour is 27.4 meters, and the water within it is deeper.

• How big is Lac des Mille Lacs?

  Its surface covers 245.2 square kilometers and it holds 574 islands. Of its 881 kilometers of shoreline, 517 kilometers is the mainland shore and 364 kilometers is the shore of the islands.

• What fish are in Lac des Mille Lacs?

  Ontario’s aquatic resource records name thirteen species, among them walleye, smallmouth bass, northern pike, lake whitefish and yellow perch.