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.
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
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
Not assigned to a Sign or Signal
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
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
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
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.
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.
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.
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.
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.
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.
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.
Temperature trends in the Great Slave Sub-basin (1948 – 2016). From: Bonsal et al. 2020
Temperature Change (°C)
Great Slave Lake
Note: Spring: Mar-Apr-May, Summer: Jun-Jul-Aug, Fall: Sep-Oct-Nov, Winter: Dec-Jan-Feb
Precipitation trends in the Great Slave Sub-basin (1948 – 2012). From: Bonsal et al. 2020
Precipitation Change (%)
Great Slave Lake
Water use by local communities and other users in the Great Slave sub-basin is minimal compared to available water resources.
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 m3 threshold, 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.