4.3 Changes in River and Lake Ice
Because of its importance to many human activities, freeze-up and breakup dates of river and lake ice have been recorded for a long time at many locations. These records provide useful climate information, although they must be interpreted with care. In the case of rivers, both freeze-up and breakup at a given location can be strongly affected by conditions far upstream (for example, heavy rains or snowmelt in a distant portion of the watershed). In the case of lakes, the historical observations have typically been made at coastal locations (often protected bays and harbours) and so may not be representative of the lake as a whole, or comparable to more recent satellite-based observations. Nevertheless, these observations represent some of the longest records of cryospheric change available.
Observations of ice thickness are considerably sparser and are generally made using direct drilling methods. Long-term records are available at a few locations; however it should be noted that, just as for sea ice, changes in lake and river ice thickness are a consequence not just of temperature and radiative forcing, but also of changes in snowfall (via the insulating effect of snow).