Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 210

Such areas frequently become locally important agricultural regions. The annual con-
tribution of glacial meltwater streams to the runoff of many watersheds amounts to mil-
lions of liters. On the negative side, glacial streams are commonly so choked with sed-
iment that the water is not immediately usable by people. The sediments can be trans-
ported long distances, leading to increased deposition and infilling of the stream or lake
into which they empty. The quality of the lake water is affected in several ways, the most
obvious of which is that the normal crystal blue of the lake is transformed to a murky
gray around the mouth of the stream (Bryan 1974a, 1974b).
Rock Glaciers
In periglacial environments, water can remain perennially frozen inside a matrix of
loose talus and scree to form rock glaciers. Rock glaciers can flow and deform in much
the same manner as ice glaciers, and are they often found as residual forms in the later
stages of glacial retreat. The San Juan Mountains of southwestern Colorado serve as
a classic example where many valleys formerly filled with glaciers now contain active
rock glaciers. Rock glaciers are addressed more completely in Chapter 5.
Glacial Lake Outburst Floods
Glaciers can produce lakes on several scales, from small meltwater ponds sitting direc-
tly on the ice surface to very large lakes where major rivers are dammed by adjacent
ice-sheet glaciers or by valley glaciers issuing from lateral tributaries. As an example of
the latter, the Lowell Glacier descends from the St. Elias Mountains as a tributary of the
Alsek River, a major drainage in the southwestern Yukon Territory, Canada. The Lowell
has surged at least five times in the past 3,000 years, extending its length beyond its
valley mouth and thus blocking the flow of the Alsek. The resulting glacier-dammed lake
(called Neoglacial Lake Alsek) has backed up as far as present-day Haines Junction, 60
km to the northeast. This lake formed as recently as 1850 (Clague and Rampton 1982).
A consequence of lakes forming behind ice dams is the likelihood of failure and sub-
sequent flooding if the dam is overtopped, melts, or otherwise changes configuration, or
if the terminal moraine itself fails. Such failures are termed glacial lake outburst floods
(or GLOFs ) (Post and Mayo 1971). Closely related failures are jökulhlaups (Björnsson
2002; Clarke 1982) from water bodies located under the ice and marginal lake drainage
from water bodies along the edge of the ice. Regardless of the specific form, glacier out-
burst floods can be extremely hazardous for people living in the valleys below (Hewitt
1982). While GLOFs can occur in any state of climate change, the risk of such outbursts
is on the rise along with global temperature (Ives et al. 2010).
One notable GLOF in recent history occurred in the spring and summer of 2002 when
the Hubbard Glacier surged, damming the adjacent Russell Fjord where the two valleys
converge on Yakutat Bay along the coast of the Alaskan panhandle. The Russell is nor-
mally a coastal estuary with ecosystems adapted to a mixture of salt and fresh waters.
As the advancing Hubbard Glacier blocked the Russell's outlet, the proportion of fresh
to sea water increased and the water level rose within the fjord, impacting numerous
habitats. At one point, water from the Russell almost overtopped a mountain gap lead-