Aspects of river ice hydrology in Japan

Rivers in northern Japan are subjected to ice formation each winter. They are typically very steep with rapid changes in channel slope. As a result, ice covers are usually discontinuous with open water sections. Winter discharges in Japanese rivers are usually very small. Water temperature and ice production in these streams are very sensitive to the change in air temperature. The open water sections enable the formation of frazil and anchor ice during the winter. Owing to the relatively stable winter weather and heavy snow cover, premature break‐up and ice jams rarely occur, even though the channel geometry of these rivers is favourable for their occurrence. In this paper, hydrometeorological factors related to ice‐cover formation, frazil and anchor‐ice development, and ice‐jam formation, as well as measurements of the undercover discharge in rivers in northern Japan are discussed.