Lowell Glacier, a large valley glacier in the St. Elias Mountains of southwestern Yukon Territory, advanced across Alsek Valley and blocked south-flowing Alsek River many times during the Neoglacial interval. The resulting lake, termed Neoglacial Lake Alsek, extended east of the front of the St. Elias Mountains and inundated parts of Dezadeash Valley that presently are populated. Lake Alsek, at its maximum, was about 200 m deep at the glacier dam and over 100 km long. Staircase flights of beaches, wave-cut benches, layers of driftwood, and thin lacustrine sediments provide evidence of the former lake. Giant dunes and flood terraces on the floor of Alsek Valley indicate that the lake emptied catastrophically when the Lowell Glacier dam failed.Several phases of Lake Alsek have been identified. Each was preceded and followed by intervals during which the lake was empty and the present southward drainage pattern in Alsek Valley prevailed. Historical records, radiocarbon and tree-ring dates on driftwood, and radiocarbon dates on buried soils separating lacustrine units indicate that Lake Alsek extended into Dezadeash Valley sometime between A.D. 1848 and 1891, between A.D. 1736 and 1832, twice between 250 and 500 years ago, and at least once between 800 and 2900 years ago. In addition, a small lake may have existed in Alsek Valley after 1891, but before 1917, although the evidence for this is equivocal. During each ponding phase, Lake Alsek may have emptied and filled repeatedly, perhaps on a regular cycle, as is common for some existing glacier-dammed lakes.Lowell Glacier, during a future surge, might again block Alsek River and form a new lake. However, populated areas in Dezadeash Valley probably would not be inundated because Lowell Glacier has thinned and receded somewhat since Lake Alsek last extended beyond the St. Elias Mountains in the nineteenth century.