Subnivean animals such as voles are vulnerable to rain or meltwater that percolates through the snowpack because of loss of insulation to both winter nests and animal fur. Percolation of rain and spring meltwater through the snowpack was measured by means of a specially designed funnel apparatus that allowed monitoring of percolated water without disturbing the snowpack.

Great spatial variability was found in terms of where and how water percolated through the snowpack during a January thaw with 15.8 mm of rain. Lateral movement occurred at the surface or in icy layers where downward movement was obstructed. In the spring, the data collected indicated that about 80% of the meltwater moved vertically, and was strongly linked to the development of a firnified pack through freeze-thaw metamorphism. Flooding of the ground and the formation of shallow ponds in even the slightest of surface depressions was observed during both thaw episodes.

Our results suggest that the high incidence of mortality of subnivean small mammals that is often observed in the spring results from the saturated conditions at the surface of the ground. In areas with little relief, flooding can be extensive if the thaw is rapid and there is little opportunity for infiltration.