ice can persist as late as mid-June. The river typically freezes by late October or November, starting in the north. Year round, the Mackenzie's outflow has a major stabilizing effect in the local climate above the Arctic Ocean with large amounts of warmer fresh water mixing with the cold seawater. The average flow rate at the mouth is , the second largest in Canada after the St Lawrence, and the fourteenth largest in the world. About 60 percent of the water comes from the western half of the basin, which includes the Rocky, Selwyn, and Mackenzie mountain ranges out of which spring major tributaries such as the Peace and Liard Rivers, which contribute 23 and 27 percent of the total flow, respectively. In contrast the eastern half, despite being dominated by marshland and large lakes, provides only about 25 percent of the Mackenzie's discharge. During peak flow in the spring, the difference in discharge between the two halves of the watershed becomes even more marked. While large amounts of snow and glacial melt dramatically drive up water levels in the Mackenzie's western tributaries, large lakes to the east retard springtime discharges. Spring floods 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 gauges 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 Arctic Red confluence it is . Mackenzie monthly mean discharge at Arctic Red River (m3/s) Geology As recently as the last glacial maximum about 30,000 years ago, the majority of northern Canada was buried under the enormous continental Laurentide Ice Sheet. The tremendous erosive powers of the Laurentide and its predecessors, at maximum extent, completely buried what is now the Mackenzie watershed under thousands of metres of ice and flattened the eastern portions of the watershed. When the ice sheet receded for the last time, it left a long postglacial lake, Lake McConnell, of which Great Bear, Great Slave and Athabasca Lakes are remnants. Today's Mackenzie River