60 



M. C. Miller et al. 



Freeze-thaw Events 



The onset of the pond thaw is determined by air temperatures and 

 also by the snow cover that effectively reflects most of the solar radiation. 

 For example, in 1971 a laboratory hut was placed about 10 m upwind from 

 Pond C. As a result, there were snow drifts on Pond C the following spring 

 and thaw was delayed 1 to 2 weeks. When there is little snow, as in 1970, 

 the ponds may become windswept and snow-free in places. Thus, in 1970 

 Ponds B and C had snow-free patches in May while Pond E was snow- 

 covered. There was up to 5 cm of melt at the bottom of the ice in B and C 

 as early as 20 May, giving these ponds a 15% longer thaw period (mid- 

 May to mid-September) than Pond E. However, this early start did not 

 appreciably affect the time of disappearance of the ice as Ponds B and C 

 became ice-free only two days before Pond E. While it is difficult to 

 determine a mean date for ice melt, it is obvious that the winters of 1970, 

 1971, and 1972 had much lower than normal snowfall (Table 2-2). 

 Therefore, windswept ponds are not a usual occurrence and the first melt- 

 water should occur in the first week in June (Table 3-4). On the average, 

 the ponds should melt completely between 10 and 17 June. 



"T 1 1 1 r 



10 



E 

 u 



S- 20 





30 



40 



10 20 



Jun 



10 20 



Jul 



10 



Aug 



20 



FIGURE 3-6. Sediment thaw depth, 1970 and 1973. 



