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Rivers rarely maintain a straight route as they travel to
larger rivers, lakes or oceans -- most make a series of bends
and smooth loops that snake across the landscape. The bends,
known as meanders, reflect the way in which a river minimizes
resistance to flow and spreads energy as evenly as possible along
its course.
Here's how it works: If you've ever tried to swim across
a meandering stream you probably quickly noticed that the velocity
of the moving water was not the same everywhere you swam. Velocity
is lowest along the bed
and walls of streams
or rivers because it is there that water encounters the most
friction, and therefore the flow is reduced. Along a straight
channel segment, water moves the fastest in mid-channel, near
the surface. But as water moves around a bend, the zone of high
velocity swings to the outside of the channel. As water rushes
past the outer part of the bend, sediment is continuously eroded
from the riverbank and is swept downstream. With the slower flow
concentrated around the inner side of each bend, coarse sediment
accumulates and forms distinctive point
bars. Thus, a meandering pattern is created along the course
of the river, with shallow water and point bars on the inside
bends and steep banks on the outside.
Since the material lining the banks does not remain uniform
the entire length of a river system, another landform -- an oxbow
lake -- can develop. If river water runs into resistant sediments,
the movement of the meander can slow downstream. As other meanders
continue to migrate through softer sediments upstream, they eventually
intersect the slower-moving meander and cut off the channel between
the two, forming an independent loop that will become a lake
(see below).
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