11.3 Trends in Great Slave Lake levels

Last Updated: 
May 28, 2015

AThis indicator tracks the trends in water level of Great Slave Lake in the NWT.

The data were downloaded from the Water Survey of Canada Hydat website1. Text and analysis was done by ENR. Water Division.

Great Slave Lake, Yellowknife Bay.
Great Slave Lake, Yellowknife Bay.

Great Slave Lake, Yellowknife Bay. © B. Fournier.

NWT Focus

Monitoring water levels on Great Slave Lake is important to inform the public and boaters of changing water levels, to provide information on water levels downstream along the Mackenzie River for ferry and barge operations and for assessing overall ecosystem changes2.

Current view - status and trend

Water levels in Great Slave lake are influenced by the Slave River as it contributes about 77% of the inflows to the lake2. With regulation of the Peace River flow (Bennett Dame and Williston Reservoir operations) since 1968 and its effects on the Slave River, (see water: Indicator 11.2), the resultant changes on Great Slave Lake are investigated. Droughts or wet conditions upstream of rivers flowing into Great Slave Lake also affect its water level.

Although the total volume of the Slave River has not been changed by the operations of the Williston Reservoir, the flow regime change has reduced the average high level of Great Slave Lake by 9 cm. The average annual water level and the average annual low water level have both increased by 1 cm since flow regulations began on the Peace/Slave Rivers.

Annual maximum, minimum and mean water levels for Great Slave Lake were plotted for 1941 to 2013.
The 2010 water levels were the lowest on record.

Looking around

Although Great Slave Lake water levels were affected slightly by operations of the Bennett Dam and Williston Reservoir, the annual variations in levels are due to variations in winter snowpack/spring melt and summer rainfall throughout the watershed. More than 65% of the Great Slave Lake watershed is outside the NWT, in the northern areas of British Columbia, Alberta and Saskatchewan. Precipitation amounts in these areas greatly affect the levels of Great Slave Lake.

Find More


Ref. 1. Water Survey of Canada, Hydat. 

Ref. 2. Renewable Resources & Environment. 2010. Water Today: Water Quality and Quantity in the NWT. Indian and Northern Affairs Canada.

Updated: May 28, 2015