Introduction - Freezing of Lakes and the Evolution of their Ice Cover - page 2

Fig. 1.1 Ice break-up takes place in spring in boreal lakes. In a few weeks before, the ice cover is
weak and patchy and impossible to cross by foot or boat

A lake is a liquid water body in a depression on the surface of the Earth. It is associated
with a water balance, which largely determines the lake volume and the quality of the
water. Most lakes contain fresh water, 1 and their size ranges from small ponds to large
intra-continental basins with area more than 10 4 km 2 . The global water storage of lakes is
125,000 km 3 , and about 80 % of that is fresh water, accounting for the fraction of 0.3 % of
the Earth s fresh water resources (Henderson and Henderson 2009). In the boreal zone,

tundra, and high mountain regions in lower latitudes, lakes freeze over in winter. The ice
season may last more than half a year, and the thickness of the ice can reach 1 m. In high
polar zones and at very high altitudes, there are perennially ice-covered lakes, with ice
cover several meters thick. Sub-glacial lakes are located at the base of the 3

'

4 km thick

-

Antarctic ice sheet and represent an extreme ice coverage condition.
Frozen lakes have belonged to people

' s normal life in the cold regions. Solid ice cover

has been an excellent base for traf c across lakes and to travel and to transport cargo,

provided the ice has been thick enough. Indeed, ice cover isolates island inhabitants from
the mainland in periods, when the ice is too weak to walk but still limits boating (Fig. 1.1 ).
These periods bear in Finnish language an old word kelirikko (from

' keli ' = traf c

1 Fresh water refers to natural water with low concentration of dissolved substances. There is no
strict upper limit for the concentration but 0.5 ‰ (or about 0.5 g L - 1 ) serves as a convenient
reference. This limit is also often used for the upper limit of the salinity of drinking water.