Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 254

The essential ingredient for solifluction is water originating from melting snow or
ground ice. The affected area is often saturated for several weeks in the spring and
early summer. Other important factors governing solifluction include soil texture, slope
gradient, surrounding geology, and vegetation. Slopes of only 2°-5° are sufficient to in-
duce solifluction in the Arctic, where there is ample permafrost and saturated soil, but
in most mountains it occurs on slope gradients of 5°-20° (though some have reported
low on slopes as low as 1°; Ritter et al. 2002). On steeper slopes, water will be lost
through overland flow. In general, the finer the soil, the more likely solifluction is to oc-
cur because of the greater water-holding ability, frost susceptibility, and potential for
flowage. At a site in Norway, Harris et al. (2008) found that gelifluction is an import-
ant component of small, near-surface mass movements. Summer rainfall events had the
potential to increase pore pressure, but did not initiate soil movement. The spatial and
temporal variation in movement was influenced by snow distribution.
Vegetation can play a role in solifluction by increasing the moisture content through
reducing runoff and decreasing evaporation, although solifluction lobes are best deve-
loped in areas with sparse vegetation or tundra grasses. Vegetation also acts as a bind-
ing agent, giving the downslope movement definition and form. Solifluction lobes and
terraces often resemble huge soil tongues moving downslope. They often coalesce and
form crenulated, lobate banks along the slope. Such features form a striking micro re-
lief and have important ecological implications (Price 1971a, 1971b).
In many midlatitude mountains, solifluction features may be inactive, with well-ve-
getated, stabilized lobes. Little sign of movement exists and erosion may be occurring.
Elsewhere, they may display evidence of reactivation. In any case, an understanding
of their disposition can provide insight into past and present environmental conditions
(Benedict 1966, 1976). Real-time monitoring techniques to detect the thermal status,
hydraulic condition, phase changes, soil volume strain, and soil shear strain of solifluc-
tion can provide high temporal resolution measurements to improve our understanding
of solifluction dynamics (Harris et al. 2007).
Mudflows
Mudflows consist of water-saturated heterogeneous material confined to a definite
channel that flows quickly downslope; they are a major geomorphic process in moun-
tains. They should not be confused with mudslides, such as those that occur in Cali-
fornia, which involve the massive failure of large sections of slopes. Mudflows have a
much greater speed of movement (up to several meters per second) than solifluction.
The name mudflow is actually a misnomer, because the material is composed largely of
rocks, resembling an aggregate of fresh concrete. However, mud provides the flow mat-
rix and transporting medium. In mountainous environments, there are many other types
of flows, all characterized by their high moisture content and sudden movement. Flows
of volcanic origin are called lahars and have been observed after several volcanic erup-
tions, including those that occurred at Mount Saint Helens. Earthflows are typically less
fluid than mudflows, contain more earth material, and exhibit slumping that produces a
step-like terrain. A special term, debris flow, has been used to describe a watery type of
flow that commonly occurs after forest fires in the western United States. Without hav-