Thermokarst Topography

Permafrost terrain consists of a seasonally thawed active layer underlain by perennially frozen ground. Permafrost immediately below the base of the active layer is commonly ice rich. Thermokarst terrain is the pitted relief formed by the melting of ground ice, and develops throughout the permafrost regions after surface disturbance or climate warming. In the boreal forest, the most widespread surface disturbances are caused by forest fires, and commonly landslides occur in areas that have been burned. Thermokarst depressions may contain ponds that grow into lakes as the surrounding permafrost thaws and the depression enlarges. Where ice-rich ground is exposed by river, lake, or coastal erosion, rapid ablation of the permafrost occurs. Retrogressive thaw slumps, consisting of a steep headwall of melting ice-rich ground and a foot slope of lower gradient, develop following such erosion. Where near-surface ground ice thaws on hillslopes, excess pore-water pressures may lead to a reduction in effective stress and failure of the slope. The sedimentary records of most thermokarst features are disorganized beds of organic debris mixed with mineral deposits. Thermokarst lake development in northwest North America has been associated with periods of relatively warm climate, but the initiation of individual lakes is the result of site-specific disturbances