ABSTRACT
In winter, rivers located in northern and mountain environments are severely affected in cold weather and subsequent accumulations of river ice. This chapter aims to show how the concept of a geomorphic threshold can be applied to river ice processes and to a better understanding of the ensuing geomorphologic effects. Several distinct geomorphic thresholds involving different river ice processes commonly occur in northern and mountain rivers. The first geomorphic threshold applies to river ice drives, with the threshold of channel width as the dominant physical parameter. The second threshold involves river ice jams during a breakup of an ice drive. The third threshold involves anchor ice which forms on river beds. Ice jams which commonly occur during ice drives in large northern rivers are influenced by a complex of channel morphologic factors, such as sharp meander bends, channel constrictions, and islands. Channel icings are influenced chiefly by flow depth in winter and occur commonly in mountain braided rivers.
