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