1Freeze-up and break-up of rivers and lakes

The plate contains four maps showing the mean river freeze-over date, the mean lake freeze-over date, the mean river ice-free date and the mean lake ice free date. The four maps depict, in a general way, the average dates on which freshwater bodies in Canada become completely ice-covered in the fall, and become completely ice-free in the spring. The formation of an ice cover on a water body is called freeze-up; and the melting and dissipation of this ice cover is called break-up. Freeze-up begins when surface water is cooled to 0 degrees Celsius and ice crystals begin to form; it ends when the water body has attained its maximum ice coverage. Most lakes freeze over completely; rivers may or may not, depending on their location, size, and flow characteristics. The final stage of the freeze-up process may be termed "freeze-over". Break-up normally begins when air temperatures rise above 0 degrees Celsius, and when surface and internal melting of the ice sheet begins. The process is aided by the action of winds and currents, which results in mechanical breaking of the ice. Break-up ends when the water body becomes completely clear of all ice. Many rivers and lakes in the Arctic region, however, may never become completely ice free because of the shortness of the melting season. In general, rivers freeze over later and clear earlier than lakes in the same area. This is due to the effect of river currents, which retard freezing in the fall and aid the breaking up of the ice in spring.