Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 274

to an increased bedload consisting of material too large for transport by a single chan-
nel.
Braided channels reach their highest development in mountain regions where runoff
is rapid and periodic. Many glacial streams display extensive braiding because of the
abundant debris contributed to the stream. Braiding is also found in intermediate-sized
streams that are fed by smaller streams originating on steeper slopes. The small but
turbulent streams often transport coarser material to the valley bottom than the inter-
mediate stream can carry in a single normal channel. Nonbraided channels may also
become braided because of human disturbance such as logging, agriculture, or mining
in areas with steep slopes.
Braided channels typically have wide valleys without confining walls. In many cases,
the braided stream has inherited a valley greatly widened and deepened by glaciers.
Braided channels are composed of poorly sorted material, varying from fine silt, sand,
and gravel to cobbles and boulders. Mid-channel bars composed of this loosely consol-
idated material are subject to continuous erosion as the current impinges upon their
flanks. When discharge is high, erosion takes place on the upstream ends of the bars,
with deposition occurring at their downstream ends. Thus, the bars continually change
shape and migrate downstream. The channels themselves display much greater vari-
ation, since they are subject to rapid change during periods of high runoff. The White
River draining the Emmons Glacier on Mount Rainier, Washington, has moved laterally
more than 100 m in eight days (Fahnestock 1963). The Kosi River, which rises near
Mount Everest in Nepal, has migrated westward a distance of 112 km over the last 200
years. The river has also shifted 19 km in a single year (Leopold et al. 1964). In 2008,
the Kosi broke through its embankments and reverted to its old course, causing serious
flooding and many deaths in Bihar State, India.
FIGURE 5.20 An alluvial fan in the St. Elias Mountains, Yukon Territory. The fan has been deposited
onto the floodplain of the Slim's River, indicating a relatively young age. Note that the left side of
the feature is forested, while the right side is largely bare, owing to recent activity in the area.
The stream now appears to be moving to the left and encroaching on the forest. Alluvial fans