Freezing of surface water begins a unique hydrologic period in cold-region climates, one where the resultant ice covers create lotic and lentic environments that are distinctly different physically, chemically and biologically from their open-water counterparts. Many such differences result directly from the altered hydrology associated with the formation, growth and decay/ablation of surface ice. An effect of ice on rivers can readily be seen in dramatic short-term changes in flow and water levels. Examples are the dynamic flow and backwater associated with freeze-up and break-up on rivers. Break-up events frequently establish the annual maximum water levels, even though the maximum flow is more likely to result from spring snowmelt or summer rainfall. Although less obvious, but perhaps more important, ice causes seasonal disruptions to the hydrographs of entire basins. Pronounced seasonal hydrograph changes result from the storage and later release of significant quantities of water within river channels during both the freeze-up and break-up. Storage of water as ice during freezing, abstraction of flow into channel storage caused by increased hydraulic resistance by growing ice and storage of frozen winter precipitation all lead to reduced fall-to-winter yield from a basin. So dramatic are some channel storage events that the early winter freeze-up can produce the low-flow event of the year; a hydrologic feature that usually occurs in late winter when actual runoff production from the delivering landscape is at a minimum. Similar hydrologic conditions can occur on lakes where high water levels are produced by the ice-obstruction of outlet flow. Also, the release of water from storage through snow-load pumping from lakes during winter precipitation events can produce significant and rapid additions to flow. Rarely are the seasonal shifts produced by lake and river ice considered when hydrologists, or even cryohydrologists, analyse the hydrographs of cold-region basins. However, not considering such ice storage and release effects is likely to result in serious water source quantification errors, particularly of groundwater and snowmelt.
The above are just a few of the numerous hydrologic features produced by freshwater ice. These, as well as many others, are reviewed within the selected papers contained in this volume. Notably, however, as our understanding of the hydrologic significance of freshwater ice has expanded, an interest in the broader environmental relationships of ice hydrology has rapidly developed. For example, while interest in ecohydrology or fluvial geomorphology of open water has developed gradually, expansion of research into similar realms associated with ice hydrology has occurred within a few years. Much to the credit of our scientific societies, two separate international groups recognized this growing interest, the International Commission on