Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 275

serve as good indicators of the amount of erosion and other geomorphic activities taking place in
mountains. (Photo by L. W. Price.)
Alluvial fans are cone- or fan-shaped deposits occurring at the mouths of mountain
valleys or canyons. Water running down the valley loses velocity, and therefore drops
some of its load, since the gradient at the apex of the fan is reduced. Moreover, the wa-
ter is no longer confined to a steep-walled valley, so it may spread laterally. Its energy
is diffused, and a braided pattern may develop in the middle of the deposited sediment.
The resulting debris accumulation builds up in the form of a concave-upward fan or an
outward slope to a semicircular perimeter (Fig. 5.20). The larger material is dropped
first because of the lower gradient and resulting loss of velocity; smaller-sized material
is transported farther (McPherson and Hirst 1972). Alluvial fans are essentially com-
posed of the portion of the bedload which mountain streams have been unable to trans-
port farther. In general, large upland watershed and source regions produce larger al-
luvial fans. In Death Valley, California, high-resolution airborne laser swath mapping re-
vealed that alluvial fan roughness becomes smoother until a threshold is reached, and
incision dominates on the older surfaces (Frankel and Dolan 2007).
Alluvial fans are key areas for agriculture and settlement in mountainous regions
because of their relatively smooth surfaces, productive soils, and availability of water.
Streams are sometimes dammed in the mountains and the water diverted for use. In
other areas, as alluvial fans provide excellent aquifers and are good sources of ground-
water, wells are drilled into the fan.
Drainage Patterns
The most common drainage pattern on Earth is the dendritic pattern, which resembles
the branches of a tree. Headwater tributaries or branches feed into the mainstream of
a trunk. The dendritic stream pattern develops under homogeneous surface conditions
and is generally accepted as evidence for lack of significant structural control. In moun-
tains, however, where stream flow is more strongly controlled by slope and structure,
other types of stream patterns develop. The presence of any given type of stream pat-
tern may aid in understanding the geomorphic history of the landscape. For example,
a dome or volcanic peak will frequently display a radial stream pattern, in which the
streams flow outward from a central peak. If the mountain mass is domed upward but
with a series of encircling ridges, as in the Black Hills of South Dakota, an annular
drainage pattern may prevail. Streams follow the circular outcrops of less resistant
rocks until they find a path through the ridges of resistant rock. In folded mountains
with parallel ridges and valleys, such as the Appalachians or Jura, the common pattern
is for a main stream to occupy the center of the valley with many short tributary streams
joining it at right angles, forming a trellis pattern. Faulting and jointing greatly affect
the direction of stream flow. Many parts of the Adirondack Mountains in New York dis-
play a rectangular pattern because the streams are forced to make rightangle bends
in response to the regional joint system. Other types of stream patterns could be men-
tioned, but these are the principal types for mountains.