If a stream is flowing along straight, the strongest, fastest
flow will be in the center of the stream well above the bottom of
the bed or channel but below the surface. Friction with the
bottom and sides of the channel acts to slow the water against
it, and that in turn slows the adjacent water, but not as much.
The surface water experiences friction with the air, but it is
not as great as that with the ground. See the illustrations below
of the map view and the cross section of the stream.
A bend in a stream is called a meander. On a meander, the main
flow of water is forced to the outside of the bend, just like
your body is pulled toward the outside of a curve as you go
around it in a car. This means the main force of the water is
going to be directed against one side of the bank. Here, the
water will hit against the bank and create turbulence. Faster,
more turbulent water has a greater erosional capacity than
slower, less turbulent water. On a meander, then, the stream will
actively cut away or erode the outside bank making it steeper.
This steepened, eroded slope is called an UNDERCUT SLOPE. On the
opposite side of the stream, the water will be less turbulent.
This slower, gentler water is less capable of carrying the load
or material being transported by the water, so it is a location
of deposition. This deposition will fill in part of the bend
making a gentle slope called a SLIPOFF SLOPE. Material deposited
by a stream is called ALLUVIUM.
This relationship between velocity and carrying capacity is an
extremely important aspect of the fluvial process. A stream
carries or transports a load. Some of this load is just being
rolled or pushed along the bottom of the stream (traction), some
is being bounced along the bottom (saltation), some is being
picked up and carried (suspension), and some of the load has been
dissolved (solution). If the velocity increases, the size
(defined by weight) of the particles moved by traction,
saltation, and suspension will increase. This means that more
material will be picked up and moved, if it is available. The
load will increase. If, however, the velocity drops, the amount
of material being transported by each of these will be reduced.
As the velocity rises or falls, so does the amount of load.
Consider the situation of where a stream flows into a body of
water. As the stream flows downslope, it is carrying a load based
on the gradient and velocity of the stream. When the stream
empties into the water body, that gradient is lost. The water
slows down, and it can no longer carry all of that load. The
result is that it drops or deposits this material. The landform
that develops is called a DELTA. The stream drops the material
which eventually can clog the channel. Part of the water
overflows and follows another path. The stream ends up branching
apart and extending the deposits further into the body of water.
These stream branches are DISTRIBUTARIES.
Another similar situation with relationship to gradient reduction
results in the formation of an ALLUVIAL FAN. Where a stream goes
from a steep slope to a lesser slope, such as when flowing from
mountains onto a plain, the sudden drop in gradient promotes the
deposition of material. A delta shaped deposit of material is
added onto the plain as the water slows down, drops part of its
load, and then branches around the blockage through new
distributaries. Note that the first major difference between an
alluvial fan and a delta is that the alluvial fan forms on land
and the delta forms in water.
Braided streams form in a similar manner. When a stream has a
sudden decrease in volume or a sudden increase in load, it may be
unable to handle all of this material. Like in the above two
situations, it drops the material and then some of the water must
seek an alternate route. The stream develops multiple, smaller
channels through the area. The channels intersect resembling a
with the lesson