River and Lake Ice
Summary
Floating freshwater ice is a key component of cold-regions
river and lake systems. Ice creates and controls unique
aquatic habitats and related biological productivity and
diversity. It also poses major challenges (for example,
flood threats) and opportunities (for example, transpor-
tation) for communities. Changes in freshwater-ice cover
have largely mirrored trends in air temperature, with
River and lake ice: floating ice formed in rivers and lakes.
large regions of the Northern Hemisphere experiencing
Distribution: high latitude and mountain regions mainly in
the Northern Hemisphere.
reductions in ice-cover duration characterized by earlier
spring break ups and, to a lesser degree, later autumn
freeze ups, particularly over the last 50 years. Although key sources of winter transportation and, in the case of
more dramatic changes in the timing and duration of the rivers, capable of causing extensive and costly damage
ice season are projected for the future, our understand- to human infrastructure
4
. Because the various forms
ing of how climate has affected or will alter the more and processes of freshwater ice are directly controlled
important freshwater-ice processes (such as ice-cover by atmospheric conditions (temperature and precipita-
composition, thickness and break-up dynamic,) remains tion), their spatial and temporal trends can be used as
poor. Improving our knowledge of these climate-ice rela- indicators of climate variability and change. Given the
tionships is the key to being able to properly adapt to, or broad ecological and economic significance of river and
even mitigate, future environmental change. lake ice, scientific concern has been expressed regarding
how future changes in climate might affect ice-covered
hydrologic and aquatic systems
5–7
.
Introduction to river and lake ice
Freshwater ice is a major component of the terrestrial Trends and outlook
cryosphere. It affects an extensive portion of the global
hydrologic system, including the rivers and lakes found Limited by the availability of detailed observations, most
throughout high-latitude and alpine areas, mainly in the historical evaluations of changes in freshwater ice have
Northern Hemisphere. Seasonal ice cover can develop as focused on relatively simple characteristics, such as the
far south as 33°N in North America and 26°N in Eurasia timing of autumn freeze up and spring break up, and
producing effects on 7 of the world’s 15 largest rivers
1
, maximum ice-cover thickness. Based on 27 long-term
and 11 of the 15 largest lakes. (about 150-year) records from around the Northern
Hemisphere, Magnuson and others
8
(Figure 8.1) discov-
River and lake ice are important modifiers of numer- ered that freeze up has been delayed by approximately
ous biological, chemical and hydrologic processes
1–3
, six days per hundred years and break up advanced by a
202 GLOBAL OUTLOOK FOR ICE AND SNOW