Why is permafrost widespread north of the Lower Tunguska River?

Permafrost is widespread north of the Lower Tunguska River primarily because of the region's cold subarctic climate and the lack of sufficient snow cover to insulate the ground.1

In the Yenisey River basin, the light snow cover provides little protection against the extreme cold, causing the soil and subsoil to remain frozen to a considerable depth for long periods. Generally, permafrost forms and persists in areas where the mean annual air temperature is below 0 °C (32 °F), preventing the ground from completely thawing during the summer. Over time, this thermal imbalance—where geothermal heat is insufficient to offset the cold temperatures at the surface—allows perennially frozen ground to develop and thicken.12