Mackenzie River
At , the Mackenzie River drainage basin encompasses nearly 20 percent of Canada. About , or 54 percent of the basin, lies above Great Slave Lake. Permafrost underlies about three-quarters of the watershed, reaching up to deep in the Mackenzie Delta. As a whole, the Mackenzie basin receives only meager to moderate rainfall, averaging over the entire basin, though mountain areas experience much higher precipitation, and areas near and north of the Arctic Circle receive much lower precipitation.
Water supply and sanitation in Canada
Metro Vancouver's water system is one of the largest in North America. Almost 600 square kilometers of mountainous land is closed to public access to protect the large reservoirs which collect rain and snow melt. 
Three watersheds - the Capilano, Seymour and Coquitlam Watersheds are the water source for more than two million residents in the most populated urban region of British Columbia. Each watershed supplies about one-third of the region’s tap water. The regional district’s long-standing closed watershed policy is one component of a water supply system that provides multiple barriers to contamination, from source to tap.