Great Slave – Water Quantity

Water Quantity

Water quantity in the Great Slave sub-basin has undergone moderate change. Observations by Indigenous communities and scientists suggest that ice is less thick and unstable in some areas, with earlier break-ups and later freeze-up dates in many waterbodies, including the Slave River and Slave River Delta. Scientific assessments indicate snow mass has decreased since 1980 over much of the northern and southern portions of the sub-basin, while observations by Indigenous communities suggest changes in snow texture and quality. Water levels in lakes, rivers, and creeks are more variable than in the past, notably in the Horn River and the Slave River Delta. Flows are generally lower than in the past in the Slave, Taltson and Mackenzie Rivers. These changes are largely the result of a combination of flow regulation on the Peace River by the W.A.C. Bennett Dam and the effects of climate change. In contrast, flows in rivers west and northwest of Great Slave Lake have increased, along with significant expansion of lake surface areas, related to climate change. These changes in water quantity threaten to further disrupt the aquatic ecosystem health and the ability for Indigenous communities to practice a traditional way of life in the sub-basin.

Information Availability

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

Signs and Signals	Indigenous Knowledge Information and Data	Indigenous Knowledge Availability¹	Science Information and Data	Science Data Availability²
Snow and Ice	Local observations and oral histories of changes in snow quality, ice thickness, presence and break up of ice jams	Many observations from several locations.	Available modeling information / stats on snow quality, ice thickness, presence and break up of ice jams	Snow data and analyses available from many weather stations. Ice thickness data available but not analyzed. Some research on ice breakup
Water Flows and Levels	Local observations and oral histories of changing flow / water levels in rivers / lakes and aquifers over time	Many observations from several locations	Seasonal statistics including changes in flow / water levels in rivers / lakes and aquifers over time	Data available from many Water Survey of Canada stations; analyzed six representative sites across the basin
Climate	Local observations and oral histories of temperature, precipitation normals and extremes over time	Some observations from few locations	Temperature, precipitation normals and extremes over time	Data available from many weather stations in the basin, sub-basin analyses completed
Water Use	Not assigned to a Sign or Signal	Not assessed	Number of water licenses, purpose, volume allocated, and volume actually used vs. water flow / level; Water demand from various sectors, including dams, agriculture, oil and gas, etc., trends in water use over time	Information available. Water Licences issued by Water Boards, not collected for this report

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; Few = 1-3 locations; Several = 4 or more locations
²Qualifiers for the availability of science data in publicly available sources: Low = Individual studies or locations; Many = Network of monitoring stations across the basin

Snow & Ice

Changes in ice and snow quality and more variability in the timing and intensity of ice break-up, freeze-up and ice jams have been observed in many parts of the Great Slave sub-basin. Ice break up-patterns and related ice jam floods appear to be related to Slave River flows and snow patterns in the upper watersheds.

Changes in ice quality and thickness have been observed in the Slave River Delta and on Great Slave Lake

Ice quality and thickness is more variable than in the past and has declined in many places. In the Slave River Delta, research scientists and Indigenous harvesters from Fort Resolution and Fort Smith have observed how ice in the Delta does not freeze as thick or smooth as in the past, attributed to climate change and altered water flow patterns from upstream hydroelectric dams. Indigenous harvesters from Fort Resolution and Fort Providence have similarly observed thinner and more unstable ice conditions,while harvesters from Fort Simpson have observed changes in the consistency of ice when it freezes that make it difficult or impossible to travel along the shore in the winter. Members of the Lutsel K’e Dene First Nation have observed that the ice on Great Slave Lake does not exceed a thickness of more than 3 to 4 feet (1 to 1.2 m) in most places today, although it used to reach 6 to 8 feet thick (1.8 to 2 m).Ice thickness data collected at Yellowknife show the same trend, with an average decline of over 15 cm thickness from 1958 to 2016 (this study).

Akaitcho members all around the lake and on the river have observed warmer water and thinner ice. [Lutsel K’e Dene Dene First Nation] members remember the ice being 6-8 feet thick in places; currently it doesn’t get much more than 3-4 feet in most places,
Lutsel K’e Dene First Nation in Parlee and Maloney, 2017

Ice thickness for Great Slave Lake at Yellowknife 1958-2016, based on ECCC data. Black dotted line indicates linear trend.

Communities have observed more variable ice break-up, freeze-up and ice jams, and scientific studies link shorter ice cover to climate change

Although people of Deh Gah Got’ie First Nation have not had any trouble traveling on the ice so far, they have noticed changes in the ice. Spring break-up is occurring at different times, sometimes earlier than usual and sometimes later,
Guyot et al., 2006

Many Indigenous communities report changes in the timing and nature of ice break-up, freeze-up and ice jams. Indigenous harvesters from Fort Resolution and Fort Smith report that ice-jam floods in the springtime are smaller, and springtime ice break-up in the Slave River Delta is “less dramatic now than in the past”. Residents of the Delta also report how the pattern and timing of spring break-up is shifting. Break-up used to take longer and occur in mid-June; however, in recent decades break-up occurs more quickly and as early as mid-May. Deh Gah Got’ie harvesters from Fort Providence have similarly noticed that the timing of spring ice break-up is more variable, reporting in one study that break-up occurs either earlier or later than in the past. A scientific assessment of break-up and freeze-up dates in the Slave River did not identify a statistically significant trend in river ice freeze-up or break-up over the years 1950-1999.

Shifts in the timing of freeze-up and break-up of lakes are reported in Indigenous Knowledge and scientific studies. Members of Yellowknives Dene First Nation have observed changes in the freeze-up of lakes, noting that in recent years freeze-up has shifted from mid-October to early November as lakes are experiencing numerous freeze-thaw cycles linked to an increase in precipitation events in the autumn that delay ice build-up.Scientific assessments indicate lake break-up and freeze-up dates are influenced by large-scale climate patterns related to Pacific Ocean temperatures (termed climate teleconnections), with certain phases of these teleconnections shifting break-up earlier (and quicker), and to a lesser extent, shifting freeze-up dates later. A mid 1970s shift in Pacific-related climate teleconnections (e.g., El Nino Southern Oscillation, Pacific Decadal Oscillation) resulted in a regional shift in break-up and freeze-up dates with lakes much more strongly impacted than rivers. While the effects of global warming on the teleconnection patterns examined in that study remain uncertain, climate models suggest that as climate warms, El Nino-type patterns may become more frequent and would likely result in shorter ice durations, particularly in western Canada.

Ice receding from the shores of Great Slave Lake. Image source: Gary Danvers Collection via Flickr Creative Commons (copyright-free).

Reduced snow mass was estimated for the southern and northern areas of the sub-basin, possibly related to warmer winters

Snow mass has decreased over much of the northern and southern portions of the Great Slave sub-basin from 1980 to 2015, with some slight increasing trends in the western portion of the sub-basin. This could be viewed contradictory to the observed long-term precipitation increases across the sub-basin since 1950. The snow mass decreases, however, were observed over a shorter time period, with possibly stronger influence of climate oscillations. In addition, the trend may be influenced by other factors, such as reduced proportion of precipitation falling as snow and increasing winter air temperatures leading to winter snow melt events.

Change in Average March Snow Mass (as Snow Water Equivalent, in mm/decade) in the Great Slave sub-basin 1980 to 2015. Positive (yellow) values indicate increases in snow mass, negative (dark green) values indicate declines in snow mass.

Snow mass trends were derived using a multi-dataset approach described in Mudryk et al. 2020, and provided by Environment and Climate Change Canada.

Some Indigenous communities have observed changes in snow quality in some parts of the Great Slave sub-basin

Members of the Akaitcho First Nations report that the snow “doesn’t pack the same, and there are no hard snowdrifts anymore,” and it is more difficult to travel across the snow by foot or dog team.

The hamlet of Fort Providence, overlooking the partially frozen Mackenzie River. Image source: Leslie Philipp via Flickr Creative Commons (copyright-free).

Flood frequency in the Slave River Delta is influenced by snow melt patterns in the headwaters of the Mackenzie Basin

Trends in past flood frequency reconstructed from Slave River Delta lake sediment records showed that there were oscillating periods of high flood frequency followed by low flood frequency over the past 80 years. The W.A.C. Bennett Dam regulation is unlikely to the be primary factor in changing spring break-up flood frequency in the Slave River Delta, because flood frequency increased during the 1960s to 1980s. In addition to river flows, reduced snowpack and modified snowmelt timing in tributaries and headwater regions are considered important factors in reducing the frequency of floods caused by spring ice break-up. Snow accumulation and melt several hundred kilometres upstream of the Slave River Delta are understood to strongly impact the flood frequency and magnitude of the Slave River Delta. Additionally, they have significant impact on the occurrence of ice-jam flooding in the Delta.

Water Flows & Levels

Lower water levels and an increase in navigable hazards has been observed in some rivers in the southern portion of the Great Slave sub-basin, along with decreasing trends in river flows, especially in the summer months. River flows in the northwest area of the sub-basin have increased, due in part to a significant expansion of lakes located upstream from permafrost thaw.

Observations by Indigenous communities and scientific data analyses indicate that water levels and flows in the Slave and Taltson Rivers have declined

A general decline in water levels in the Slave River and its tributaries has been reported by Indigenous communities. Elders and harvesters from Fort Resolution and Fort Smith have observed lower water levels in the Slave River, making travel and navigation more difficult. Residents of Fort Resolution have observed lower water levels in the mainstem of the Slave River and report that side channels and tributaries of the Slave River and Taltson River are drying out or becoming impassable by boat. Members of the Akaitcho First Nations have noticed water levels on the Slave River between four and ten feet (1.2 to 3 m) lower in different locations, according to markings on rocks indicating former water levels. Elders from Lutsel K’e First Nation similarly report a decline in water levels since the early 2000s, with some suggesting that levels have dropped by five feet (1.5 m).

Indigenous communities report that flow rate has slowed in the Slave River and Taltson River, related to changes in flow patterns in the Delta. As a result, harvesters say it is now possible to paddle up the rivers, whereas before this was nearly impossible. Similar patterns are recorded in Water Survey of Canada flow data for Slave River at Fitzgerald, which indicate a decline in flows from 1921 to 2017 with significant decreases recorded in months from June to October. On average, flows decreased by 20 – 30 m³/s per year during summer months and by 11.2 m³/s per year, or by 0.33% annually. The Taltson River flows also have an apparent decreasing trend, particularly in the fall and winter months; these trends are not currently statistically significant, however, so more data are required to confirm this trend. These trends were in contrast with those measured in streams west of Great Slave Lake, as discussed below.

Residents [of the Slave River Delta] have noted increasing challenges and concerns with travelling on the Slave River in both winter and summer times, associated with lower water levels and changes in ice conditions,
Dagg, 2016

Trends in July Flows at Selected Water Survey of Canada Stations in the Great Slave sub-basin. Data labels are the Sen’s slope (annual change in mean monthly level in m) and annual percent change relative to long term mean monthly level. Data from the Water Survey of Canada

Communities have observed lower water levels in the Mackenzie River and more variable flows in the Horn River

Members of the Dehcho First Nations and Deh Gah Got‘ie First Nation of Fort Providence have observed lower water levels in the Mackenzie River in recent years which disrupt their ability to travel the river by boat. Water Survey of Canada flow data analysis for Mackenzie River at Fort Simpson (this study, data not shown) indicate declining trends in water levels for the months of June to September from 1972 to 2017; however, these trends are not statistically significant.

Members of Deh Gah Got’ie First Nation also report that flows are more variable in the Horn River than in the past, with high flows leading to more springtime floods, quickly followed by low flows and abnormally dry conditions. Members attribute these changes in part to an increase in beaver dams in the watershed that alter natural flow patterns.

The Mackenzie River, near the hamlet of Fort Providence. Image source: Alan Sim via Flickr Creative Commons (copyright-free)

Scientific monitoring data indicates that flows have increased across most of the Great Slave sub-basin in the winter months

Most of the basin has experienced consistent increases in winter flows in response to warmer winter temperatures and an increase in winter precipitation as rainfall. Exceptions to these trends are Lockhart River at the outlet of Artillery Lake and Slave River at Fitzgerald. The increases in winter flows were mainly in January, February, and March, with increases of 2 – 2.5% annually for Hay River and Trout River since the 1960s, and an increase of approximately 0.6 – 0.7% annually for the Yellowknife River at the inlet to Prosperous Lake from 1939 to 2017.

Trends in February Flows at Selected Water Survey of Canada Stations in the Great Slave sub-basin. Data labels are the Sen’s slope (annual change in mean monthly level in m) and annual percent change relative to long term mean monthly level. Data from the Water Survey of Canada

Indigenous communities have encountered more sandbars, gravel bars, and other navigable hazards in the Slave River

An increase in the number of sand bars and shallower water conditions makes it challenging or impossible for people to access some areas by boat. Residents of Fort Resolution have stated it is increasingly difficult to travel to the community of Fort Smith by boat due to the emergence of sand bars in the mainstem of the Slave River. Members of the Akaitcho First Nations have also reported more encounters with navigable hazards on the Slave River, particularly reefs, which make it more dangerous to travel on the river. As observed by members of Dehcho First Nations, the erosion of river banks, gravel bars in rivers, and generally lower water levels are negatively affecting river navigability and make it more difficult and dangerous to travel to some areas.

Great Slave Lake near Yellowknife, NT. Image source: Alan Sim via Flickr Creative Commons (copyright-free).

A significant expansion has occurred in the size of some lakes located west of Great Slave Lake

Some lakes, such as Falaise Lake, Trio Lakes, Jackie Lake and Chan Lake west of Great Slave Lake have increased in area by up to 800% since 1986. The area covered by water in this region has approximately doubled in size. This recent lake expansion represents a fundamental alteration of the structure and function of this ecosystem and its use by Mackenzie wood bison, in response to climate change. The increases in lake area were related to climate teleconnections, regional river discharge, and summer sea surface temperature trends, however additional research is needed to determine the exact processes at play. These patterns are supported by the analysis of Water Survey of Canada flow data from La Martre River at the Outlet of Lac La Martre. This site showed increasing monthly average and maximum flows for its period of record (1975 to 2017). La Martre River annual flows at the outlet of Lac La Martre increased by 1.7% (0.6 m³/s) per year and increased up to 2.29% in the spring months. Trout River, a southern tributary to the upper Mackenzie River that originates in Trout Lake, also showed increased annual flows by 0.8% (0.33 m³/s) per year from 1969 to 2017 (this study).

Climate

An increase in air temperatures and more variability in precipitation patterns have been observed by Indigenous communities and scientists in the Great Slave sub-basin.

Indigenous communities and scientific studies have reported a rise in air temperatures in the Great Slave sub-basin

Air temperatures are generally on the rise in the Great Slave sub-basin, particularly in the winter. Indigenous communities have reported a general increase in air temperatures in the past fifty years, particularly that winters are often warmer and shorter than in the past. Members of the Akaitcho First Nations have reported more frequent extreme weather events, including unseasonably warm air temperatures. Unusual weather events are also reported by members of Deh Gah Got’ie First Nation in the region surrounding Fort Providence, such as more frequent, stronger storms in the summer months and less wind in the month of February. Members have noticed a trend for warmer winters in recent decades and some suggest the winter season is delayed by approximately two months. This is supported by observations and scientific data of delayed freeze-ups (see section on ice and snow). Climate data indicates similar patterns, with increased air temperatures recorded in all seasons in the Great Slave sub-basin from 1948 to 2016. The greatest temperature increase was observed in the winter (4.9°C). The fact that cold regions and the cold season warm faster than warm seasons and warm regions is explained by the “Arctic amplification”. It is driven, amongst others, by the retreat of seasonal snow and ice that exposes darker surfaces and land cover underneath, introducing an additional warming effect across the region.

Many local residents have reported changes in temperature, in particular that winters are warmer and shorter now than in the 1950s,
Dagg, 2016

Temperature trends in the Great Slave Sub-basin (1948 – 2016). From:

Sub-basin	Season	Temperature Change (°C)
Great Slave Lake	Spring	2.6
	Summer	1.9
	Fall	1.8
	Winter	4.9
	Annual	2.9

Note: Spring: Mar-Apr-May, Summer: Jun-Jul-Aug, Fall: Sep-Oct-Nov, Winter: Dec-Jan-Feb

Scientific studies suggest total precipitation has increased across the sub-basin

Climate data indicates total annual precipitation has increased in the Great Slave sub-basin, predominantly in the spring (20.5%) and summer (26.2%) months. This represents the largest increase in total annual precipitation at the sub-basin scale across the Mackenzie River Basin.

Precipitation trends in the Great Slave Sub-basin (1948 – 2012). From:

Sub-basin	Season	Precipitation Change (%)
Great Slave Lake	Spring	20.5
	Summer	26.2
	Fall	17.1
	Winter	17.7
	Annual	24

Communities have observed more variable precipitation patterns

Local residents [of Fort Resolution] have observed significant changes in climatic phenomena over the past half-century relating to temperature, precipitation, seasonality, and wind,
Wesche et al., 2016
Changes in precipitation patterns are observed by some Indigenous communities in the Great Slave sub-basin. In the Slave River Delta, some communities have reported more frequent hailstorms, high rainfall events and freezing rain. Some communities also report that summers typically have more rainfall and winters are drier than in the past in the Slave River Delta region. Lutsel K’e Dene First Nation elders have noticed there is less overall rainfall in recent years, which is thought to be causing or contributing to lower water levels in the region surrounding Great Slave Lake. Deh Gah Got’ie First Nation members have observed that although precipitation patterns are more variable, in recent years they have noticed more rainfall than normal.

Local residents [of Fort Resolution] have observed significant changes in climatic phenomena over the past half-century relating to temperature, precipitation, seasonality, and wind,
Wesche et al., 2016

Water Use

Water use by local communities and other users in the Great Slave sub-basin is minimal compared to available water resources.

Water use by local communities and industry in the Slave River is minimal relative to streamflow

Due to minimal water use on the Slave River, water quantity objectives have not been set by the Governments of Alberta or the Northwest Territories. The two governments have an agreement to re-evaluate water usage in the Slave River if one of the following occurs: annual consumptive use in Alberta exceeds a 2 billion m³threshold, the 2 billion m³ threshold no longer represents 1.9 % of the long-term mean annual discharge, or 50% of the consumptive use in Alberta is for use outside of the Mackenzie River Basin.

Surface and groundwater allocations in the Slave River watershed, a portion of the Great Slave sub-basin, decreased between 2015 and 2017, from a total annual allocation of 1.08 billion m³ to 1.03 billion m³. Approximately 16% of water allocations in the sub-basin are from groundwater and 84% from surface water.

The oil and gas sector is the main water user in the Hay River watershed

About 71% of both surface and groundwater allocations in the Hay River watershed are attributed to the oil and gas sector, with the remaining allocations split between agriculture, commercial, forestry, and municipal sectors.

References

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