The Mackenzie River displays a sharp change in geomorphic character at The Ramparts, a postglacially-developed rock-walled canyon near Fort Good Hope, N.W.T. Upstream for at least 70 km the river is cut into late-glacial and early postglacial lacustrine, deltaic, and floodplain sediments with radiocarbon dates of 11 000 to 11 500 years. These sediments are overlain by peat dating back 6120 radiocarbon years. Downstream the river reoccupies, for more than 350 km, a Pleistocene channel containing glacial and fluvial sediments yielding only dates beyond the range of carbon dating. Abandoned channels west of The Ramparts record the overflow, in early postglacial time, from a rock-rimmed lake athwart the newly developed Mackenzie drainage. The deepest of the channels, believed to have been abandoned in mid-postglacial time, contained a major waterfall with its plunge-pool floor close to present sea level. Free drainage from the foot of the falls to the ocean at the time it operated is indicated. Erosion continues along the Mackenzie River above The Ramparts but some aggradation occurs below it. Permafrost thaw and heavy rainfall contribute to landsliding along the river banks in both segments.