Mackenzie River from the Peace-Athabasca system are significantly slowed by the delta area at the western end of Lake Athabasca causing the lake to rise, and the excess water can only flow out after the rivers have receded. The same phenomenon occurs at Great Slave Lake, which naturally regulates the flow from the Slave River into the Mackenzie. There are river gages at several upstream points along the Mackenzie River. The average flow rate at the outlet of Great Slave Lake is . At Fort Simpson, below the Liard River, it is . At Norman Wells it is , and at the
Mackenzie River the northwest, establishing the present flow direction to the Arctic. Fluvial deposits and other erosional evidence indicate that around the end of the Pleistocene, about 13,000 years ago, the Mackenzie channel was scoured by one or more massive glacial lake outburst floods unleashed from Lake Agassiz, formed by melting ice west of the present-day Great Lakes. At its peak, Agassiz had a greater volume than all present-day freshwater lakes combined. This is believed to have disrupted currents in the Arctic Ocean and led to an abrupt 1,300-year-long cold temperature shift called the Younger Dryas. The Mackenzie carries a very large