Page 573 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 17 glacial and Periglacial Landscapes 537
a typical avalanche trigger and victim
is a backcountry skier, snowboarder, snowmobiler, or climber. The Canadian avalanche Centre (www.avalanche.ca) reports that an average of 14 people were killed in avalanches annually in Canada from 1998 to 2007, with snowmobiling being the backcountry activity that resulted in the most avalanche-related fatalities.
Snowpack conditions are highly variable across a mountain range and even along a single slope. Unwary people recreating in the winter backcountry tend to trigger avalanches from weaker areas of the slope, where the snow is often thinner. a skier or snowboarder approach- ing the middle of a slope may cause a cascading fracture that triggers a slab avalanche that is difficult if not impossible to escape. Since snow scientists have
not yet developed tools to definitively identify these areas of weaker snowpack, assessing avalanche conditions remains challenging.
Avalanche Control, Forecasting, and Safety
avalanche control by ski patrollers at ski areas and by highway workers on moun- tain passes consists of using hand-thrown explosives and in some cases military artillery to safely trigger avalanches that remove unstable snow. after avalanche control work is done, these areas can
be safely opened to the public. This work makes it possible for ski resorts to
▲Figure 17.1.2 Avalanche paths in the Rocky Mountains, Kananaskis Country, Alberta. [Marlene Ford/alamy.]
operate with less risk in avalanche terrain and for mountain passes to remain open in winter.
in Canada, the Canadian avalanche Centre issues updates and bulletins
on avalanche conditions. in the United States, there is a network of avalanche centres that provide forecasts; for infor- mation see www.avalanche.org. To remain safe in the winter backcountry, people need to be familiar with the current
general conditions, pick terrain appropri- ate for those conditions, evaluate the specific avalanche danger on the slopes where they are recreating, and expose only one person to the danger at a time so that companions are available for res- cue in the event of an avalanche.
A portion of the transported debris may also be carried on its icy surface, visible as dark streaks and bands. This surface material is known as supraglacial debris, which originates either from rockfalls and other gravity-driven processes that carry material downward from above or from processes that float material upwards from the glacier’s bed.
A tidewater glacier, or tidal glacier, ends in the sea. Such glaciers are characterised by calving, a process in which pieces of ice break free to form floating ice masses known as icebergs that are usually found wherever gla- ciers meet an ocean, bay, or fjord (Figure 17.4). Icebergs are inherently unstable, as their centre of gravity shifts
Georeport 17.1 Global Glacial Ice Losses
Worldwide, glacial ice is in retreat, melting at rates exceeding anything previously recorded by scientists. The european alps have lost more than 50% of their ice mass over the past century, with an acceleration in melt rates since 1980 and nearly
20% of their ice lost during the past 20 years. in alaska, 98% of surveyed glaciers are in what scientists describe as a “swift retreat.” Similar ice losses and reduced snowpacks are reported for the rockies, Sierra nevada, Himalayas, and andes. High latitude glaciers are also affected, as Canadian scientists have documented a trend of ice losses in nunavut over the last several decades. The USgS is capturing dramatic changes in glacial mass in a repeat Photography Project (see nrmsc.usgs.gov/repeatphoto/).