Stable isotopes in river ice: identifying primary over-winter streamflow signals and their hydrological significance

The process of isotopic fractionation during freezing in the riverine environment is discussed with reference to a multi-year isotope sampling survey conducted in the Liard-Mackenzie River Basins, northwestern Canada. Systematic isotopic patterns are evident in cores of congelation ice (black ice) obtained from rivers and from numerous tributaries that are recognized as primary streamflow signals but with isotope offsets close to the equilibrium ice-water fractionation. The results, including comparisons with the isotopic composition of fall and spring streamflow measured directly in water samples, suggest that isotopic shifts during ice-on occur due to gradual changes in the fraction of flow derived from groundwater, surface water and precipitation sources during the fall to winter recession. Low flow isotopic signatures during ice-on suggest a predominantly groundwater-fed regime during late winter, whereas low flow isotopic signatures during ice-off reflect a mixed groundwater-, surface water- and precipitation-fed regime during late fall. Copyright (C) 2002 John Wiley Sons, Ltd.