Small circular depressions in the
riverbed, carved out of solid rock.
Commonly found in upland areas, close to the source of
the river, where the channel flows directly over bedrock.
Formed when rock fragments are 'drilled' into holes and
cavities by turbulent whisking-like action of the river.
This type of erosion is called corrosion.
Rapids and Waterfalls
Tougher, more resistant rocks, are less easily eroded than weaker rocks,
and they will often form irregular steps in the long profile of a river. These
irregularities may create a very turbulent stretch of river, called rapids.
Form white-water, as the river plunges over jagged
rocks and mini-waterfalls to form dangerous whirlpools
and fast-flowing tubes of water.
A waterfall is a single, more
pronounced, 'step' in the long profile of
a river.
Most commonly formed when a
river flows over a relatively
tough band of rock.
Directly beneath the waterfall is a plunge pool, formed by
the erosion of hydraulic action to the less resistant rock
underneath.
Gradually the waterfall will undercut the rock
behind it. A narrow steep sided gorge will be
formed.
Meanders
Sweeping bends made by a river, and are
often found in lowland areas in the river's
middle course
Thalweg is the line of greatest velocity or fastest
flow within the river.
Erosion on the outside bend,
deposition on the inside bend.
Helicoidal flow is the corkscrew-like flow of water in a meander
Alternating sequence of shallow, fast-flowing sections called
riffles. And deeper, slower moving sections called pools.
Riffles and pools commonly form in channels during
low-flow conditions, and are closely linked to the
very complex pattern of Helicoidal flow,
The old meander loop is called an
oxbow lake. This is a lake that has
resulted from the deposition and
erosional processes working
antagonistically.
Braiding
A river described as being braided
when it becomes sub-divided into
many separate channels.
Channel becomes overloaded, results in the
channel depositing sediment in the channel
to form islands.
Common in rivers that experience large
variations in discharge, and where the load is
relatively large e.g. rivers that flow from the
snouts of glaciers.
Floodplains and Levees
A floodplain is an extensive flat area of
land on either side of the river which
periodically becomes flooded
Floodplains are most extensive in
lowland areas, where they can be
several km wide
Silt deposits that form the floodplain are
fertile, good for farming.
The river flows over sediment that have been deposited on
the valley floor by previous floods - every time the river
floods, it deposits a fresh layer of silt on top of the existing
floodplain.
When the river floods, the water overtops onto the floodplain and larger and
heavier sediment is deposited on top of the banks. This is because water velocity
at this point is much less than in the main channel.
The build up of sediment gradually raises the
bank and these are called levees.
Deltas
A river loses energy very rapidly when it enters the sea (or
lake) and as a result, vast amounts of sediment are
deposited in a fan shape.
The build up of deposited material breaks
through the water surface and this forms new
land called a delta.
A river in a delta is forced to split into different
rivers called distributaries. This is the rivers
response to the massive deposition o sediment
and reduction in the gradient.
An arcuate delta is a gently curved delta.
A birds foot delta is shaped like a 'birds foot' and
represents deposition along the edges of several
distributaries.
A cuspate delta is shaped like an arrow towards the sea.