Fluvial Processes – How does a river erode, transport and
why does it deposit sediment?
Fluvial erosion –
Fluvial erosion is the removal of rock and other mineral particles from the
channel bad and banks by stream flow.
There are various types of erosion;
Abrasion Abrasion is the grinding effect of a
channel caused by sediments in the
process of being transported. The
material being transported
undermines river banks and valley
slopes. However, the sediment itself
is eroded by colliding with other
pieces of material, this is known as
Hydraulic action Hydraulic action is the erosive effect
of flowing water without the
assistance of rock particles. The sheer
force of the flow causes erosion to
occur. This type of erosion is most
effective in areas of the channel
which consist of incoherent materials,
such as; sand and gravel.
Capitation Cavitations occur when tiny bubbles
of air implode in fissures and cracks
in the channel banks. The tiny shock
waves that result in the weakening of
river banks and result in the collapse
of the landforms.
Corrosion Corrosion is the chemical action of
stream water, which dissolves
carbonate rocks such as chalk and
A river has three main stages, sediment supply zone, sediment transport zone
and the sediment storage zone. Within these three stages various erosion
occurs within each stage.
Within the sediment supply zone there are four types of erosion occurring;
hydraulic action, corrosion, corrosion and attrition. Within this stage the
erosion is mainly directed vertically and head ward, this is due to the river not
having a lot of spare energy as it is using 90% of its energy to overcome
obstacles such as; large rocks and boulders. This erosion leads to landforms
such as; rapids, small water falls and Steep River bed to form.
Within the sediment transport zone, it is mostly corrosion and attrition which
occurs due to the sediment being transported and colliding with both the
channel bed and with each other. However, there is little hydraulic power and
corrosion still taking place within this stage. Due to the river having fewer
obstacles to over come it has more spare energy to use to erode allowing it to
erode more laterally rather than vertically, widening the river channel. This
erosion leads to the development of landforms such as; rapids, small
meanders, small floodplains, pools and riffles.
In the sediment storage zone less to no erosion occurs, besides some lateral
erosion on the outside bends of meanders. This leads to the development of
larger meanders and floodplains.
Fluvial Transport – Solid and soluble particles eroded from the channel,
together with materials input by mass movements and weathering from valley
slopes, are transported down the stream (from the upper stage to the lower
stage). The material transported by a river is known as the rivers LOAD. The
load is divided into three fractions;
Type of load: Sediment Transport process:
Bedload Coarse caliber particles Intermitting sliding,
such as; boulders, cobbles rolling and hopping
and gravel. along the stream bad at
high velocity – for large
particles, lift and
eddying are very
Suspended or wash load. Fine-caliber particles of silt Silts and clays are
and clay, and medium size entrained at high flow
sand particles. velocities and
distances in suspension
– sand sized particles
move at lower velocities,
bouncing along the
stream bed and lifted
into the current and are
Solution load. Dissolved minerals from Minerals in solution –
the weathering and transport occurs
erosion of carbonate rocks continually and is
that crop out in the independent of velocity
channel and in the and discharge.
Material may be transported by a river in four main ways: solution,
suspension, saltation and traction (see diagram).
The type of transport taking place depends on 1. The size of the sediment
and 2. The amount of energy that is available to undertake the transport.
In the upper course of the river there is more traction and saltation going
on due to the large size of the bedload, as a river enters its middle and
lower course there is a lot of finer material eroded from further upstream
which will be carried in suspension.
River Deposition –
Deposition is where material carried by the river is dropped. This will occur
when there is no longer sufficient energy to transport material. Deposition
of material may result in the formation of distinctive features such as slip
off slopes (on the inner bends of meanders); levees (raised banks) and of
course the floodplain itself. Remember - it is the largest material that will
be dropped first as it requires the most energy to be transported. Eroded
from further upstream which will be carried in suspension.
Here is a website which has an animation which shows the sediment
deposition as a river enters a lake -