ABSTRACT: Late Quaternary drainage diversions in the southern Mackenzie region, Canada.

CGRG Bibliography of Canadian Geomorphology
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Abstract

The Late Pleistocene glaciation of the southern Mackenzie Mountains and Mackenzie River region caused a number of major changes to the landscape. At some point, glaciers blocked and diverted major rivers such as: Redstone, Root, North and South Nahanni rivers. In pre-glacial times, the paleo-Redstone River drained eastward across the Franklin Mountains, of which there is a discernible geomorphic signature superimposed upon the Paleocene uplift of the region in the form of relict tributaries within intermontane valleys. Root River previously drained across the present-day Mackenzie valley, and eastward up Willowlake River (which now drains west into the north-flowing Mackenzie River). The paleo-Root River watershed drained the southern reaches of the Franklin Mountains and the northern limits of the Cretaceous/Paleocene Horn Plateau. The North Nahanni River drained eastward obliquely across Camsell Bend of the Mackenzie River. This river likely continued in an easterly direction, where it would have drained the southern reaches of the Horn Plateau. Geomorphic and stratigraphic evidence of these two paleo-river systems, as well as others that dissected the plains and foothills regions of the southern Mackenzie River region, have been interpreted from drift isopach maps presently being constructed from a regional seismic shothole database. The South Nahanni River was also changed by glaciers. The Laurentide Ice Sheet filled the valley up to the locality of Virginia Falls and lower Flat River, blocking eastward drainage of South Nahanni River. This led to the formation of a glacial lake inundating an approximate maximum area of 900 km2. For much of the last glaciation, this lake drained southwest into the Pacific Ocean through an outlet into Yukon Territory. Recorded thickness of glaciolacustrine sediments at various sites along South Nahanni River reveal between 110 and 120 metres east of Virginia Falls and about 90 m west of the falls. The falls have a height of 90 m, suggesting that this knick-point likely formed, or was tectonically reactivated in postglacial times. During retreat of the continental ice sheet and advance of montane and Cordilleran ice, glacial Lake Nahanni cut an outlet to the east in the vicinity of First Canyon and through the Labyrinth Scablands. This outlet drained into Glacial Lake Mackenzie, joining the north-flowing drainage that eventually reached the Arctic Ocean at about 22 ka BP.

Bibliography of Canadian Geomorphology