11.2 Trends in Slave River flows

Last Updated: 
May 28, 2015


This indicator tracks the trends in river flows of the Slave River.

Slave River with white pelican
Slave River with white pelican

River flow trends are examined using a) total annual flow calculated from daily mean data in cubic metres per second, b) daily mean flows averaged over the years of record and, c) extreme and mean flows (highest and lowest daily means and annual mean) in cubic metres per second for each year. The Slave River flow is somewhat controlled by a dam in its upstream portion. The naturalized total annual flow is also examined. Naturalization of flow measures adjusts for flow regulation by the upstream dam, but not for other human activities such as diversions, consumption, and reservoir evaporative losses.

Data are from the Water Survey of Canada NWT/NU Hydrometric Network1 gauging station at Fort Fitzgerald and Hudson Hope, and from BC hydro reservoir inflows2. Text and charts prepared by ENR Water Division.

NWT Focus

Monitoring the Slave River flow is important to inform on potential transboundary issues and to provide information for river traffic, flood monitoring and aquatic ecosystem change. The Slave River is the largest tributary of Great Slave Lake and contributes more than 75% of the flow into Great Slave Lake.

Current view - status and trend

Total annual flows measured for 1960-2013 period, and naturalized for 1960-2012, show a large inter-annual variability and a slight decreasing trend. Total annual flow was significantly reduced in 1968-1971 while Williston Reservoir on the Peace River in British Columbia (BC) was being filled. Other than during the filling period, total annual volume of flow in the Slave River has not been changed significantly by the operations of Williston Reservoir. Since hydro operations began in 1972, the lowest total annual flow was experienced in 2010 after several years of extremely dry conditions in the northern regions of BC, Alberta, and Saskatchewan. Comparable lows since 1972 have occurred previously in 1981, 1995, and 2006.

However, the flow regime (timing of flow and extremes) of the Slave river has changed since the Bennett Dam was constructed on the Peace River and the Williston Reservoir began operations began operations for hydro-electricity generation. Runoff from the Rocky Mountains in BC is held in the Williston Reservoir during the spring freshnet and summer for release back into the system during the autumn and winter when hydro-electricity demand is the greatest.  By the end of winter, the reservoir is generally drawn down to its lowest level and the refilling cycle continues wtih each spring melt and summer precipitation. Natural flows entering the Peace and Slave River system downstream of the Bennett Dam will dampen the effects of the Williston Reservoir operations. The pre-dam and post-dam hydrographs show the effect on the Slave River from the Williston Reservoir operations on the Peace River.

The annual and extreme flow graph shows a large reduction in maximum flows (usually a spring event) and an increase in minimum flows (usually a winter event) since the Bennett Dam was constructed and the Williston Reservoir has been operated for generating hydro-electricity.

The flow regime of the Peace River was changed with filling and operation of the Williston Reservoir for hydro-electricity generation. Downstream of the reservoir controls, natural flows start to mask the effects of regulation but the effects are seen all along the main stem of the Peace River and on the Slave River at Fitzgerald, where the Slave River crossed the boundary into the Northwest Territories.

Looking around

"Other major rivers flowing across the NWT bordered are also measured to determine the effects of upstream developments.  Transboundary rivers with gauging stations include the Peel, Liard, Hay, Coppermine, Thelon and Back rivers." - Quote from Water Today:  Water Quality and Quantity in the NWT3.

Looking forward

The flow regime changed with the filling and operation of the Williston Reservoir.  Another hydro-electric dam and generating station are being proposed for "Site C" on the Peace River.  The river flow regulated by the Williston Reservoir at the Bennett Dam will be re-used for operation of the proposed "Site C" thus, there will be very little additional flow regime change.

Find more


Found an error or have a question? Contact the team at NWTSOER@gov.nt.ca.


Ref. 1. Water Survey of Canada, Hydat.

Ref. 2. Scott Weston. March 11, 2011. Technical Report - A review of inflow quality control procedures at BC hydro.

Ref. 3. Renewable Resources & Environment. 2010. Water Today: Water Quality and Quantity in the NWT. Indian and Northern Affairs Canada.

Updated: May 28, 2015