Nepewassi Lake

Where Nepewassi Lake is

Nepewassi Lake lies in the District of Sudbury, with shoreline in the Municipality of Markstay-Warren and the Municipality of St.-Charles.

How deep is Nepewassi Lake?

The deepest mapped contour on Nepewassi Lake is 12.2 meters, and the water within that boundary descends even farther. Ontario’s bathymetry record holds 38 depth contours for Nepewassi 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.

Nepewassi Lake has 5 Lake Partner monitoring sites. Four have reported phosphorus for the three or more years the province requires before a status is assigned. All are classified mesotrophic.

The longest record is held by site 3449/3 at Mine Bay, deep spot, with phosphorus sampled several times a year between spring and fall over 10 years to 2025. Its long-term mean phosphorus concentration is 13.21 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 50 clarity readings at site 3449/3 give a median Secchi depth of 2.0 meters.

Median Secchi depth by year at site 3449/3

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

Nepewassi Lake Weather

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

Nepewassi Lake weather

Water temperature

Ontario’s aquatic resource records class Nepewassi Lake as a warm-water lake.

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 Nepewassi Lake. The LastLake model computes the temperature of its surface water for each day from the daily mean air temperature at Environment Canada’s Sudbury A climate station, 32 kilometers from the lake, or the next nearest station when its record is incomplete, and from the lake’s area of 1,158 hectares and its mean depth. The province’s records give no mean depth for Nepewassi Lake; the model uses 4.2 meters, estimated from the lake’s depth contours.

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 Nepewassi Lake carries the model’s figures for the past weeks and the coming week.

See also Lake Water Temperature in Ontario

Fish in Nepewassi Lake

Ontario’s aquatic resource records name 9 fish species in Nepewassi Lake.

Popular species in the province’s records

• Northern Pikepike
• Muskellungepike
• Walleyeperch
• Yellow Perchperch
• Smallmouth Basssunfish
• Rock Basssunfish
• Pumpkinseedsunfish
• Brown Bullheadcatfish

Spawning areas

Ontario maps 2 fish spawning records within the Nepewassi Lake outline, for two different species, and all of them are mapped below. Every record is recorded as verified and presently suitable, and none is classed as sensitive.

By number of records

By area mapped

• Muskellunge1 area6.5 ha
• Northern Pike1 area11.5 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 Nepewassi Lake run from 1948 to 1972 and hold 23 releases. Walleye account for most of the fish: 2,000,000 released in seven of the years between 1948 and 1960. The last recorded release was 20,000 muskellunge frys in 1972.

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

Eating the fish

The 2026 Guide to Eating Ontario Fish carries consumption advice for four species taken from Nepewassi Lake, tested for contaminants at a sampling location in Burwash, Hendrie & Hawley Twps., Sudbury 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 10

Nepewassi Lake lies in Fisheries Management Zone 10. Ontario sets its fishing seasons and catch limits by zone.

Fisheries management zones around Nepewassi Lake

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

Nepewassi Lake Ice Fishing

An ice hut on Nepewassi 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). Nepewassi Lake lies within FMZ 10.

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 10 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 conditions

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

Islands and names

Nepewassi Lake contains 30 islands in the Ontario Hydro Network’s outline, two of them over 10 hectares and sixteen under a tenth of a hectare.

Official names

Ontario’s geographic names register lists 7 official names on the water and its islands: 4 islands and three bays.

Bays 3

• Barlow Bay
• Mine Bay
• Morel's Bay

Nearby lakes

• Trout Lake17 km away
• Richard Lake22 km away
• Ramsey Lake27 km away
• Nepahwin Lake28 km away

All lakes

Questions about Nepewassi Lake

• How deep is Nepewassi Lake?

  Its deepest charted contour is 12.2 meters, and the water within it is deeper.

• How big is Nepewassi Lake?

  Its surface covers 11.6 square kilometers and it holds 30 islands. Of its 97 kilometers of shoreline, 81 kilometers is the mainland shore and 16 kilometers is the shore of the islands.

• What fish are in Nepewassi Lake?

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

• Is the water clear?

  All four monitoring sites with a phosphorus status are classified mesotrophic, the middle of the province’s three bands.