Silt cloud in Great Slave Lake spells trouble for fishers and fish

A screenshot of Great Slave Lake on August 20, 2020 from Nasa's Worldview program, which provides near real-time satellite imagery
A screenshot of Great Slave Lake on August 20, 2020 from Nasa's Worldview program, which provides near real-time satellite imagery. Brown silt plumes can be seen across the centre of the lake.

A large silt plume spreading through the NWT’s Great Slave Lake is being attributed to high water levels in the Slave River, turning large areas of lakewater brown.

This summer’s heavy rainfall has created surface runoff from the surrounding land and increased the sediment in the river system, said a Department of Environment and Natural Resources (ENR) spokesperson by email.

The plume was first reported by NNSL. While a release of water took place at the WAC Bennett Dam on the Peace River in northern BC, the NWT government has not drawn any correlation between the two events.

Satellite imagery from Nasa shows the plume stretching across the body of Great Slave Lake. While silt from the Slave River and Hay River is a common occurrence, wind has helped to spread the plume throughout the lake, ENR scientists believe.



The department is in the process of taking water samples from the East Arm, Slave River, and Slave River delta to learn more about the contents of the silt and assess water quality within the plume.

Fishers and boaters on Great Slave Lake have taken notice of murkier-than-usual waters.

Troy Linington, a member of the lake’s Tu’cho’ Fishers’ Cooperative, said high water levels have made fishing more challenging this summer. Fish are moving in unusual patterns, he added, and more driftwood in the water makes it difficult to navigate.

“We hit [a piece of driftwood] and it caused serious damage … our boat started taking on water,” Linington said.

“Not to mention this Covid thing is making repairs take longer because parts are taking longer to get up here. It’s been a really tough year for fishers all around.”

Increased sediment in the water has various impacts on freshwater ecosystems like Great Slave Lake.



Dr Muhammad Yamin Janjua, a scientist specializing in arctic aquatic research at Fisheries and Oceans Canada, said the sediment could change the water quality, reduce light penetration, increase temperature, and potentially increase toxicity.

Though tolerance to silt varies considerably between species, Yamin said those changes can affect the wellbeing of fish in the water, making it harder to feed and move.

“It may clog the opercular cavities, may irritate gills, and it may result in respiratory difficulties and poor health,” he said.

There could be consequences for fish spawning, too. Sediment can reduce the oxygen available to eggs or prevent incubation and hatching.

“Fortunately, for species such as lake trout and whitefish, they spawn in the fall,” Yamin said. “Personally, I think that it may be a short-term increase and it will settle down in coming weeks.”