Geoscience Reference
In-Depth Information
water spills out onto the floodplain, the velocity of flow decreases and sediment begins to
settle, causing fresh deposits of alluvium on the floodplain.
Certain patterns of alluvial river channels have been seen on every continent and are di-
vided at the most basic level into straight, meandering, and braided. Straight channels are
rare in nature and, for the most part, are a function of the scale of assessment. They are
described as straight at the regional scale, but at more local scales they are winding or sinu-
ous to some degree. The most common river channel pattern is a series of bends known as
meanders, named after the River Menderes in southwestern Turkey, which is well known
for its sinuosity. Meanders develop because erosion becomes concentrated on the outside
of a bend and deposition on the inside. As these linked processes continue, the meander
bend can become more emphasized, and a particularly sinuous meander may eventually be
cut off at its narrow neck, leaving an oxbow lake as evidence of its former course. Alluvial
meanders migrate, both down and across their floodplain, a process that can be monitored
by comparing old maps and repeated photography. This lateral migration is an important
process in the formation of floodplains.
Braided rivers can be recognized by their numerous flows that split off and rejoin each oth-
er to give a braided appearance. These multiple intersecting flows are separated by small
and often temporary islands of alluvium. Braided rivers typically carry abundant sediment
and are found in areas with a fairly steep gradient, often near mountainous regions. The
reason why one channel meanders and another is braided has been the subject of consid-
erable research. Important factors that influence the channel pattern include the volume of
water and velocity of flow, which are related in turn to the gradient of the channel and the
nature of the channel, particularly the ease with which its bed and bank is eroded, which
affects the supply of sediment to the river. These factors can change, over time and through
space. For example, the Milk River in North America is a classic meandering river as it
flows across southern Alberta in Canada but changes abruptly to a braided pattern shortly
after entering Montana, USA. The change is probably due to differences in the material
that makes up the bed and banks and a widening of the channel in the braided reach which
reduces the power of the river.
