Hibernation Burrows
"During the winter of 1968-69, the air temperature fell as low as -31° C at the hibernating site, but within the chambers the air temperature did not drop below 0° C. The presence of snow protected the hibernating toads from extremely low air temperatures, and prevented freezing of the brook.
The ground-water near the stream did not freeze and flowed throughout the winter. The high heat capacity of the slowly flowing ground-water beneath the porous floor of the chamber maintained the air temperature of the burrow at slightly above 0° C and undoubtedly kept the humidity within the chamber high, contributing to the survival of the hibernating toads."
-Excerpt from James B. Campbell's, Ph.D. dissertation, Life History of the Bufo boreas boreas in the Colorado Front Range, 1970.