Mountain Landforms and Geomorphic Processes - Mountain Geography: Physical and Human Dimensions - page 273

FIGURE 5.19 A braided river channel (foreground) extending from Exit Glaciers (background) in
Kenai Fjords National Park, Alaska. Most of the bed material is too coarse for the stream to carry,
so it simply works its way through the material, dividing and reuniting. (Photo by J. R. Janke.)
Braided channels have a low sinuosity and are wide, shallow, and straight, with nu-
merous strands of water that are divided by coarse-grained bars. The water strands
frequently change their location and numbers because of pulsating flow. Total channel
width is large compared to channel depth, and the gradient is generally steeper than
that of meandering rivers (Fig. 5.19). A typical misconception found in introductory
physical geography or geology textbooks is that braided streams have too much sedi-
ment and that, therefore, aggradation occurs. In reality, this is not the case, as there are
several factors associated with braiding. The most important variable is the erodibility
of the banks. If banks are easily eroded and have low cohesion, widespread bank erosion
occurs and a braided pattern emerges. Abundant bed material, especially coarsesized
bedload, is also required, as well as rapid, frequent variations in discharge that make
it difficult for vegetation to establish and stabilize banks. If there is too much coarse
bedload material for the stream to transport in a single deep channel, bars or mini-
ature islands begin to develop. This raises the stream bed, increasing the slope and
downstream velocity. At the same time, the channel is made shallower, which increases
the flow velocity along the streambed and allows a larger amount of bedload to be
carried by the same discharge. Braided sections of streams display gradients several
times steeper than sections with single channels. Consequently, the shallow but relat-
ively steep channels of braided streams facilitate the transport of coarse particles along
the stream bottom. Braiding, therefore, can be viewed as a natural response by streams