is very young in geologic terms – its channel formed over a period of no more than several thousand years as the ice sheet retreated. Prior to the ice ages, only the Peel River tributary flowed through what is now the Mackenzie Delta into the Arctic Ocean. The other tributaries of the Mackenzie combined into the "Bell River" which flowed east into Hudson Bay. During glaciation the weight of the ice sheet depressed northern Canada's terrain to such an extent that when the ice retreated, the Mackenzie system was captured to lower elevations in 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 sediment load, transporting about 128 million tonnes each year to its delta. The Liard River alone accounts for 32 percent of the total, and the Peel River about 20 percent. Essentially all of the sediment is contributed by areas downstream of Fort Providence, since upstream sediment is trapped in Great Slave Lake. Ecology The Mackenzie River's watershed is considered one of the largest and most intact ecosystems in North America, especially the northern half. Approximately 63% of the drainage basin, or , is forested (mostly boreal forest). Wetlands comprise about 18%, or about , of the basin. More than 93% of forested areas are virgin old-growth forest. However, human activities such as oil extraction have threatened water quality in the headwaters of the Mackenzie River. In addition, a warming climate in northern parts of the watershed is melting permafrost and destabilizing soil through erosion. Most of the taiga consists of black spruce, aspen and poplar forest. In the north, the river's shores are lined with sparse vegetation like dwarf birch and willows, as well as extensive areas of muskeg and peat bogs. South of Great Slave Lake, there are much larger reaches of temperate and alpine forest, prairie, and fertile floodplain and riparian habitat. There are