Dammed Lakes and outburst Floods in Alaska
By Austin Post and Lawrence R. Mayo
U.S. GEOLOGICAL SURVEY
Hydrologic Atlas 455
Floods resulting from Alaska glacier dammed lakes present a serious and
increasing hazard to the growing population of the State. The number of
lakes varies widely during the seasons and from year to year; the 750 lakes
identified in this report indicate their abundance. Large lakes are most
frequently found where valley glaciers block off ice-free side valleys; most
commonly the lakes are small and located along the glacier margins below the
Although most smaller lakes fill in early summer and have drained by late
August, the time of dumping of glacier dammed lakes occurs when the basins
fill to critical levels and has little relationship to weather patterns.
Thus flooding may occur at any time of year. Floods superimposed on high
discharge due to meltwater runoff or storms create unusual hazards in
summer; in winter even small floods can raise the ice on rivers causing ice
jams and serious flooding.
Usual methods of storm flood frequency analysis should not be applied to
floods from glacier dammed lakes. Due to changes in the glaciers, new lakes
may form, old ones may be destroyed, or the frequency and/or volume of
discharge may be altered. Each lake has a unique past and future, and
up-to-date data on the lakes and glaciers are required for even short-term
flood predictions. Flood histories of Salmon, Knik, Kenai, Tazlina, and
Bering Rivers illustrate how variable the lake discharge regimes can be.
New lakes may form where glaciers advance; Hubbard Glacier may block
Russell Fiord around 1990 creating a lake 45 kilometers (28 miles) in
length. Lakes may also form subglacially or in depressions on or adjacent to
receding glaciers. In the past 200 years several very large former lakes
have disappeared due to melting or retreat of glaciers; a large former lake
dammed by Taku Glacier probably will not reform in 50 years even though the
glacier has been advancing strongly since 1890. Recent Lake Alsek, largest
glacier dammed lake in Western North America in recent centuries which rose
to high levels about 120 and 200 years ago, could reform if Lowell Glacier
were to surge strongly. Many other surging glaciers form lakes when
advancing; a strong surge of Black Rapids Glacier could block the Delta
River creating major floods.
Large floods can also be released from glaciers with no visible lakes.
Lakes dammed by river icings in interior Alaska can create outburst floods.
Glacier sheathed volcanoes present serious outburst flood hazards
particularly when eruptions occur; recent eruptions of Redoubt Volcano and
Mount Spurr produced floods resulting from glacier melt.
Post, Austin, and Mayo, L.R., 1971, Glacier-dammed lakes and outburst
floods in Alaska: U.S. Geological Survey Atlas HA-455, 10 p., 3 pl.
[Report, text only, Acrobat PDF (202 KB)]
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Maintainer: Rod March
Monday, December 27, 2010 05:05 PM