Great Slave

Excerpt from Mackenzie River Basin: State of the Aquatic Ecosystem Report 2003 by the Mackenzie River Basin Board (p. 143-168). Full report available at


The Great Slave sub-basin includes the Slave River drainage from the Peace-Athabasca Delta and all other tributary inflows into Great Slave Lake. It covers more than 379,000 square kilometers. Approximately 75% of the sub-basin is in the southwestern NWT and 20% is in Northern Alberta. The Great Slave sub-basin consists of fourteen major drainage systems. It straddles two distinct physiographic regions: the erosion-resistant Precambrian Shield to the east, and the Interior Plains to the west. As a result of geological and vegetative differences between these areas, annual runoff is greater in the Shield than in the Interior Plains.

The Dene have lived in the Great Slave sub-basin for thousands of years, and are the principal Aboriginal group. First Nations people comprise about 38% of the population of approximately 28,000 people. Eighty-seven percent of the population resides in the Northwest Territories, the remaining 13% live in Alberta. There are twenty-two communities; the largest is Yellowknife with a population of 18,500.

Discovery of gold during the 1930s on the northern shore of Great Slave Lake brought the first major industry to the sub-basin. Over the years, other mines (Lead, zinc) were developed within the region and brought the railroad north in 1964. There is high potential for mineral development in the Great Slave sub-basin, but by global standards, the area remains relatively undeveloped. However, there are nineteen old mines, sixteen of which are considered abandoned. Many were not properly cleaned up when they closed; consequently, some sites in the immediate vicinity of the mines became polluted. Clean up and reclamation of these sites is underway, with some indications of success. Governments have taken steps to ensure that mines do not leave a legacy of pollution to the future.

There are six hydroelectric generating facilities in the Great Slave sub-basin, located on the Snare, Yellowknife, and Taltson river systems. These facilities provide power to most of the communities and mines within the sub-basin.

The petroleum industry has expanded northeast from the oil and gas fields of Alberta.

Many people have observed the water quality has deteriorated in the upstream portion of the Slave River system and near the dam on the Taltson River. One of the most common observations has been that there is more algal growth than there used to be.

Although turbidity and concentrations of certain metals in rivers of the Interior Plains of the Great Slave sub-basin routinely exceed Canadian Environmental Quality Guidelines for protecting freshwater aquatic life, the causes are likely natural in origin. Aquatic plants and animals that are native to these rivers have likely adapted to these conditions.

The amount of arsenic in the local environment surrounding Yellowknife is the legacy of sixty years of gold mining and a product of the area’s geology. Since gold mining began, arsenic levels have increased in some of the region’s aquatic ecosystems. Except for a couple of small lakes and creeks on the mine properties, arsenic levels are within guidelines. Several government and industry-led initiatives are addressing the issue of arsenic management and containment in the Yellowknife area.

Some Aboriginal inhabitants of the Great Slave sub-basin have reported decreases in water levels and changes to water fluctuations and flooding of certain locations in the sub-basin. In some cases, these changes have interfered with people’s traditional lifestyles.

A number of factors, including operation of the Bennett Dam and climate variability have likely altered the Slave River’s flow regime and sediment load—factors that can influence aquatic habitat and communities. Recent changes in climate and flow regulation at the dam appear to have affected water levels in Great Slave Lake. While some recent changes have been observed in the ecology of the Salve River Delta, it is uncertain whether they are the result of flow regulation by the Bennett Dam, of climate change, or just natural variation that is common in the delta ecosystems.

Spring fishnet and annual peak flow on certain rivers in the sub-basin are occurring a few days earlier than in the past. Climate warming is a possible explanation for this trend. There is a need for more information on the impact of climate change, both on the aquatic ecosystem, and on human communities.

The fisheries of Great Slave Lake provide jobs, food, and recreation. Comprehensive, updated information on the Aboriginal food fishery and sport fishing in the East Arm is needed to assess the effects of these activities on the fisheries of Great Slave Lake. In light of incomplete information on fish stocks and harvest, management has adopted a precautionary approach that appears to be working thus far.

Hydro development has proceeded slowly. Moreover, the scale of development has been quite small compared to mega-projects that have occurred elsewhere in Canada. Nevertheless, changes in aquatic ecosystems associated with changes in flow patterns are an inevitable consequence of hydro development. There is currently insufficient information to assess such changes in the great Slave sub-basin. Through environmental assessments, licensing, and public involvement in future developments, the negative effects of hydro development can be minimized.

Insights into the health and population trends of wildlife in localized areas of the sub-basin have been gained through traditional knowledge. While beaver populations have increased, populations of muskrat and certain species of fish and waterfowl have decreased in some areas. Localized observations of deteriorating fish health and fewer muskrat, fish, and waterfowl are causes for concern. Much of the traditional knowledge of aquatic species and habitat is based on information prior to 1995. It would be useful to update this information.

Commercial gillnetting caused declines in the lake trout population in the West Basin thirty to fifty years ago. Intense fishing pressure reduced inconnu and walleye populations in some of the major tributaries to Great Slave Lake. On the other hand, conservation management of the trophy lake trout sport fishery in the East arm seems to have resulted in stable populations. Whitefish stocks appear to be stable, and the commercial harvest of the species seems to be sustainable. The stock status of most species is relatively unknown. This is unfortunate, given the magnitude and regional importance of the fisheries.

There are relatively few species at risk within the portion of the Mackenzie River Basin that lies within the Northwest Territories compared to other areas in Canada. The new federal Species at Risk Act and other recent initiatives will help to prevent the further endangerment of wildlife in this area.

Fish consumption advisories, although not widespread in the sub-basin, are indicative of a problem with environmental quality. Additional assessment of fish from more lakes is required to further examine the extent to which contaminants in the environment pose a risk to human health.


Sonia Wesche, PhD

University of Ottawa