Glacial Lake Mackenzie, located in the middle reach of the Mackenzie Valley, extended 800 km as a long narrow lake between the Rabbitskin River, 50 km east of Fort Simpson, and the Ramparts cliffs near Fort Good Hope; part of the lake extended an additional 75 km west from the Ramparts. The lake averaged 15 km in width, but broadened to 75 km in the northern sector and 50 km near Fort Norman, and narrowed to 5 km near Wrigley. The depth was at least 80 m in the Fort Norman region and 60 m near Wrigley. Radiocarbon ages suggest that the lake had formed in the northwest by 11 760 BP, and that the last phase of the lake occurred in the south near Camsell Bend no later than 10 290 BP. The lake formed as a result of a bedrock (limestone) barrier at the Ramparts near Fort Good Hope and glacial depression of the basin. The demise of the lake is ascribed to outlet incision into the limestone barrier, sediment filling, and isostatic rebound. Differential postglacial rebound raised the Fort Simpson region at least 97 m higher than the Ramparts over the past 11 500 years.