GNWT releases water quality results for Great Slave Lake, Hay and Slave Rivers

News Releases

Yellowknife — January 15, 2021

Results of water quality monitoring completed by the Government of the Northwest Territories (GNWT) indicate that concentrations of some metals and hydrocarbons in the Slave and Hay Rivers were elevated in July, due to unprecedented high flows, but the majority returned to their normal range in August and are not expected to pose a health risk to aquatic life.

Snowmelt and heavy rain in upstream watersheds of the Mackenzie River Basin in 2020 resulted in more suspended sediment (dirt, silt and clay) in surface waters this summer than normal. As a result, concentrations of some metals and hydrocarbons were higher than normal in July for the Slave and Hay Rivers and for a few substances in August in Great Slave Lake.  Substances that are attached to dirt generally do not affect the health of bugs and fish.

Water sampling and analysis in the Slave and Hay Rivers in August and September found that concentrations had returned to their normal range.

It is normal to see increased suspended sediment, metal and hydrocarbon concentrations when water levels are high as particles within and along the river are carried downstream. These increases are observed every spring with snowmelt, and have lasted longer this year due to prolonged high water levels across most of the Mackenzie River Basin, including the Athabasca, Peace, Hay, Slave and Liard Rivers. 


“Healthy water is of the utmost importance to the NWT and its residents. The high water levels we’ve seen this summer and fall in the Northwest Territories were unprecedented and temporarily increased levels of suspended sediments, and some metals and hydrocarbons in the Slave and Hay Rivers.  The data and information we have gathered suggests that impacts to fish are not expected.  We continue to gather information on our water, through the GNWT’s regular water quality and quantity monitoring program, and will continue data sharing efforts as part of our transboundary water agreements to help ensure the waters of the NWT remain clean, abundant and productive for all time.”

- Shane Thompson, Minister of Environment and Natural Resources

Quick facts

  • Each year the GNWT monitors water quality in the NWT, including at transboundary sites on the Slave, Hay, Liard and Peel Rivers. The GNWT also monitors Great Slave Lake at Resolution Bay, near the mouth of the Hay River, and Yellowknife Bay in partnership with monitors from the NWT-wide Community-based Water Quality Monitoring program each year. Long-term monitoring provides a record of changes over time, and will help detect changes going forward.
  • The Slave and Hay Rivers were sampled in July, August and September 2020. Great Slave Lake was sampled in August 2020.
  • Heavy rains and run off during the spring and summer of 2020 led to increased suspended sediment in upstream watersheds such as the Peace and Athabasca Rivers as well as our transboundary watersheds such at the Slave and Hay Rivers.  The Slave and Hay Rivers caused the large, persistent milky brown plume observed in Great Slave Lake.
  • Typically, the Slave River has an average water flow of 4,730 cubic metres per second. In July 2020, the average water flow was 7,449 cubic metres per second. That’s an average of almost 7.5 million litres of water passing by the Town of Fort Smith each second.

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