Geography Reference
In-Depth Information
a further threshold above which they will braid. These thresholds
decrease with increasing discharge. Thus, braided sections are
usually found on large rivers or on small rivers with steep slopes.
Wandering and braided rivers will occur where there is coarse sedi-
ment, erodible banks and where the main sediment transport
mechanism is bedload transport rather than suspended transport.
The characteristics of river beds tend to change downstream.
Often there are bedrock channels in the upper section of a river
network or large boulders and cobbles. There is a sharp decline of
bed material size downstream. This pattern is because smaller parti-
cles are transported downstream more easily and because of abra-
sion of larger sediments within the river by colliding and grinding
causes material to get smaller and more rounded downstream.
However, these patterns are not seen everywhere and may be dis-
turbed by local sediment inputs to the river network.
Within rivers there are several erosional and depositional land-
forms. In bedrock channels potholes can be found formed by:
mechanical wearing and grinding of small particles enlarging an
existing small depression or weakness in the rock; pressure changes
due to bubble collapse in turbulent flow; and chemical weathering.
Within rivers that have gravels on their bed the most common
landforms on the bed are sequences of pools and
riffles
. Pools are
deep sections with relatively slow flowing water with fine bed
material. Riffles
are formed by accumulation of coarse sediment
with shallow, fast flowing water. The spacing of pools and riffles is
often five to seven times the channel width but these do vary. The
bed of sandy sections of rivers can have small ripples in the sand
which are less than 4 centimetres in height and then also larger
dune features. The size and shape of these dune features change
with discharge during rainfall events. The dunes and ripples tend to
migrate downstream as sand is carried up the upstream facing side
of the ridge of the feature and then falls down the downstream
facing side. At very high flow velocities, a flat river bed can be
formed or dunes can even migrate upstream since
erosion from the
downstream side of the dune allows suspension of material in the
water which occurs faster than it can be replenished from
upstream.
River channels change their slope, cross-section, planform, and
bed forms in response to environmental change. Humans have
