File:Mackenzie River Enters Beaufort Sea.jpg
The Mackenzie is the largest and longest northward-flowing river in North America, and the second largest on the continent (after the Mississippi). The watershed drains a huge, but mostly unsettled portion of Canada; population density along the 4,200 kilometer (2,600 mile) river is just 1 person per square kilometer.
About 7 percent of the fresh water that flows into the Arctic Ocean each year comes out the Mackenzie and its delta, and much of that comes in large pulses in June and July after the freshet—when inland ice and snow melts and floods the river. The spring flood carries with it tremendous amounts of dissolved organic material and sediment.
More info & refs at source.