Water Quality

Water Quality

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Water Quantity

Moderate changes in water quality have been observed in the Great Slave sub-basin. Some Indigenous communities have observed an increase in sediment in the water, warmer surface water temperatures, and more lesions and parasites in fish. Scientific studies indicate elevated levels of some heavy metals in the Slave River, including mercury and aluminum, but have not detected trends, and thus have attributed levels to naturally elevated sediment loads. High turbidity has been reported in water flowing into Great Slave Lake from the Marian and Slave Rivers, and significant increasing trends in ionic strength (salts) and spring temperatures were recorded in several rivers throughout the sub-basin suggesting a broader regional influence. The sub-basin is not densely populated with human settlements and effluent volumes remain minimal. However, human activity linked to mining, forestry and other industries has increased over time, in particular the Hay River watershed. Significant concerns among Indigenous communities remain for how contaminants from industrial development activities affect water quality and ecosystem health.

The following table summarizes the availability of information for each Water Quality indicator.

Signs and Signals

Indigenous Knowledge Information and Data1

Indigenous Knowledge Availability

Science Information and Data

Science Data Availability2

Water Quality

Local observations and oral histories of good water, poor water, seasonal differences, land-based consumption practices

Many observations from several locations.

Ambient surface and ground water concentrations

Many data and reports available and trend analysis completed

Benthic Invertebrates

Not assigned to a Sign or Signal

Not assessed.

Relative abundance of aquatic macroinvertebrates

CABIN* database contains benthic data and GNWT is collecting data, but was not assessed for this report version.

Land Use Changes

Stories and oral histories of land use cover and practices

Many observations from several locations.

Map and statistics of current vs. past land cover and land use

Limited maps of land cover available.

Effluent Discharge

Not assigned to a Sign or Signal

Not assessed.

Volume of effluent discharges

National reporting programs (NPRI, WSER) and Water Board Registries contain effluent data, not collected for this project.

1 Qualifiers for the availability of local and Indigenous Knowledge observations in publicly available sources: Limited = 1-2 observations; Some = 3-4 observations; Many = 5 or more observations
2 Qualifiers for the availability of science data in publicly available sources: Low = Individual studies or locations; Many = Network of monitoring stations across the basin
*CABIN = Canadian aquatic biomonitoring network[44].

Water Quality

An increase in contaminants and sediment alongside warmer water temperatures and changes in the appearance of water have been observed by many Indigenous communities. Scientific trend analyses indicate warmer spring temperatures and increases in ion concentrations in Slave River.

Increased contamination and sediment in waterbodies have been observed by some Indigenous communities in the Great Slave sub-basin. Some residents of Fort Resolution have noticed an increase in sediment in rivers, lakes, and side channels in the Slave River Delta, which may affect fish spawning habitat.[45],[46] Possible contamination of waterbodies in the Kakisa and Hay River watersheds linked to oil and gas development in the Cameron Hills area[47] led members of Ka’a’gee Tu First Nation and K’atl’odeeche First Nations to raise concerns for the safety of drinking water in the mid-2010s; however, oil and gas development in the Cameron Hills region has since ceased.[48],[49]

High turbidity (including suspended sediments) and heavy metal loads in the Slave River have similarly been recorded in scientific studies, but no trends over time have been detected. The Slave River is known to have high levels of suspended sediment with values ranging from < 3 to 5600 mg/L. This is expected as the two major tributaries, the Athabasca and Peace Rivers, flow through the Interior Plains, an area that is underlain with easily eroded sedimentary rocks and soils. Total cadmium, chromium, copper, iron, lead, and mercury exceeded the Canadian guidelines for protection of aquatic life more than 25% of the time, but were likely related to the elevated suspended sediment levels. A greater proportion of these metals was in the particulate form and therefore likely less biologically available. However, for cadmium and copper the dissolved fraction accounted for a large percentage of the total, suggesting that a greater proportion of cadmium and copper is potentially more biologically available. Total arsenic, barium, manganese, nickel, selenium, silver, vanadium, and zinc exceeded the Canadian guidelines for protection of aquatic life less than 25% of the time.[50]

The Hay River near its confluence with Great Slave Lake. Image source: Alan Sim via Flickr Creative Commons (copyright-free).

Trend analysis conducted on Environment and Climate Change Canada (ECCC) data from the Slave River at Fitzgerald from 2000 to 2018 showed that ion concentrations increased in spring and fall, as indicated by significant increasing trends in alkalinity, hardness, dissolved magnesium, calcium, and conductivity. A similar trend in conductivity was previously reported in data from 2012 to 2016 collected by the Government of Northwest Territories Community Based Monitoring Program in Slave River at Fort Smith and Slave River at Fort Resolution.[51] There are no guidelines for the protection of aquatic life for these parameters, thus these changes are not expected to impact aquatic biota.

Increasing trends in dissolved uranium have been observed for several individual months in the Athabasca River (March, May, July, November), Peace River (April, May, June, September) and Slave River (April, October). Other trends of potential concern is in Slave River at Fitzgerald are increasing dissolved selenium in April and October, increasing total selenium in January, February, and July and increasing water temperatures in March and April.  Total selenium and uranium have remained well below the Alberta guideline for protection of aquatic life, indicating no impact on aquatic communities.

Analysis of ECCC data from 1974 to 2011 indicated an increasing trend in flow-adjusted total phosphorus data. For dissolved phosphorus, a significant increasing trend was found in the spring and winter in both the non-flow-adjusted and flow-adjusted data.[52] These trends were not found in the analysis of 2000 to 2018 data. Water quality data in several watercourses in the Great Slave Lake watershed showed low dissolved to total phosphorous ratio, indicating that a low proportion of phosphorus is readily available for uptake by plants and thus would not be of consequence for aquatic productivity in Slave River or the downstream Great Slave Lake[53].

Trend in October specific conductance in Slave River at Fitzgerald (2000-20018). Data from Environment and Climate Change Canada. Significance and slope of the trend were estimated using the Mann Kendall Trend Test. The Sen Slope (change in µS/cm per year) and % change per year are displayed on the chart.

Mercury levels and other toxins in the Mackenzie River are concerning for members of Dehcho First Nations. In one study, most Dehcho respondents indicated “they could not drink the water” from the region due to the amount of contamination.[54] Scientific analyses indicated increasing trends in major ions in the Mackenzie River in certain months of the year, particularly in June, similar to trends found in the Slave, Athabasca, and Peace Rivers. On the other hand, there is no scientific evidence of water quality impairment in the Upper Mackenzie River. Environment Canada data showed no trends in mercury in these rivers. Decreasing trends were evident for several total metals, turbidity and total suspended solids in Mackenzie River at Strong Point in several monthly datasets.[55] Community-based monitoring data collected from 2012 to 2016 showed no indication of water quality impairment in the Mackenzie River headwaters, and total mercury was below the detection limit in 94% of the samples.[56]

Harvesters from Fort Resolution have observed that water temperatures are warmer in Great Slave Lake, causing fish to congregate in different areas.[57] Warmer water temperatures have also been observed by members of Deh Gah Got’ie First Nation, who report catching ”soggy” fish in recent years linked to warmer water.[58] Consistent observations of warmer water temperatures leading to softer fish are also reported in the Yellowknife area, including by members of the Akaitcho First Nations.[59] While no long-term record of lake water temperature was identified as part of this report, scientific data have indicated warmer air temperatures and shorter ice cover duration (see climate section) that would promote warmer lake temperatures.

Elders from Fort Resolution and Fort Smith have stated that the appearance and vitality of water has degraded in recent years.[60] Dehcho First Nations members have observed a change in the colour of water in the Mackenzie River.[61]

Several respondents had concerns about the quality of the water in the Mackenzie system and noted a change in the colour of the Mackenzie River,

Water quality in the Hay River differs from the other rivers in the sub-basin due to unique watershed characteristics. For example, there are higher levels of nutrients and conductivity in the Hay River due to rich soils and wetlands in its watershed. Among samples taken from 2012 to 2016, hydrocarbon levels were highest in 2014 and then decreased, likely due to extensive forest fires in this region. Still, all levels remained well below values that would harm fish.[62]

Hay River water quality showed few consistent trends over time. There were some total metals (iron, nickel, zinc) that showed an increasing trend in April paired with a decreasing trend in these parameters and total suspended solids in May and June, possibly due to a slight shift to earlier freshet. This is supported by increasing water temperature trends in Hay River in April and June[63], but not by monthly flow datasets collected by Water Survey of Canada data for Hay River at Hay River, which didn’t show any trends over time (see flows and levels section). These trends therefore require verification.

Benthic Invertebrates

Limited analyses on benthic communities were identified and therefore the status of benthic invertebrates in the Great Slave sub-basin was not assessed.

Few studies of benthic communities in the Great Slave sub-basin were found. A study in the late 1990s found that benthic invertebrate communities in the Slave River were dominated by chironomids and oligochaetes, indicating fine sediments. Environmental factors, such as water depth and organic carbon, determined the distribution of the organisms.[64]


Land Use

Declines in the quality of resources are leading to changes in Indigenous land use practices in the Great Slave sub-basin. Land cover in the sub-basin is dominated by lakes and forests; however, there is some human footprint from historical mining practices, oil and gas development, active exploration and forestry.


The downstream effects of industrial development are concerning for Indigenous communities in the Great Slave sub-basin. Flooding around Nonacho Lake as a result of the Talston River Hydroelectric Project led to elevated mercury levels and changes in fish quality such that the Lutsel K’e Dene no longer consider them safe to eat.[65] Increased mining activity has also led some Lutsel K’e Dene members to stop fishing out of concerns for contamination in some areas, such as Stark Lake.[58] Similarly, members of Akaitcho First Nations have expressed concern for high arsenic levels and other toxins from gold mines in Great Slave Lake and the surrounding region, leading some Akaitcho communities to limit the amount of fish they consume.[66]  Active oil and gas activities in the Cameron Hills area of the Northwest Territories has also raised concern for members of Ka’a’gee Tu First Nation and K’atl’odeeche First Nation, who have hunted, trapped, and fished in the region for generations.[67]

Concerns about contamination of the fish with arsenic and related toxins from gold mining activity as well as other kinds of contaminants are a source of anxiety in that region and have limited the amount of fish consumed in local [Akaitcho] communities,

Parlee and Maloney, 2017

Abandoned gold mine near Yellowknife, NT. Image source: Marke Clinger via Flickr Creative Commons (copyright-free)

Land cover in the Great Slave Lake sub-basin is primarily forest (42%) comprised mainly of temperate or sub-polar needleleaf forest. Additional cover is water (20%) and grassland (16%). The Hay River basin is sparsely populated, but supports oil and gas development as well as forestry.[68]


% Land Cover

















Land Cover in the Great Slave Sub-basin (2015)


Oil and gas is the main development pressure in the sub-basin and is most active in the portions of the sub-basin within British Columbia and Alberta. Forestry is the second most active sector and primarily occurs in the upper Hay watershed. There is little activity from the transportation, agriculture, municipal, and mining sectors in southern sub-basin. Some active and abandoned mines are located in the eastern and northern areas of the sub-basin. Local development pressure predominantly occurs in the portions of the sub-basin within British Columbia and Alberta.[69]

Effluent Discharges

The volume of effluent discharges by municipalities into lakes and rivers in the Great Slave sub-basin remains minor.

The largest communities discharging treated effluent into the Slave River sub-basin are the City of Yellowknife (2019 population of 21,200), and the Towns of Fort Simpson (1,250), Fort Smith (2,640), and Hay River (3,750)[70]. The Town of Fort Smith currently discharges treated sewage into the Slave River, and the Town is in the process of upgrading its sewage treatment plant.[71] The City of Yellowknife discharges lagoon effluent into a system of connected wetlands and lakes and ultimately to Great Slave Lake. There are no major effluent discharges in the Hay River basin.[72]

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