With reference to an example you have studied, explain the formation of a land
form formed by fluvial processes.
I’m going to discuss the formation of a waterfall. Waterfalls are vertical falls on the river
found usually in the upper course where the river has most energy. The main processes
at work are erosion, in particular differential erosion, hydraulic action, abrasion and
solution. An Irish example is the Torc waterfall in Killarney.
Diagram of waterfall formation
Waterfalls are formed when two bands of different rock are present in the river bed.
The softer rock (such as shale) gets eroded faster than the harder rock (granite) - this
is known as differential erosion. Hydraulic action, which is the force of the flowing
water down hill, erodes the softer rock and a vertical fall begins to develop. The
eroded rock adds to the rivers load and this is turn caused more erosion, which is
known as abrasion. The load contains stones and silt which wear away the soft rock on
the river bed making the fall even deeper.
Eventually, as the vertical fall develops, a plunge pool will form at the bottom of the
waterfall. This is the result of hydraulic action (the force of the water) and abrasion (the
river’s load). The plunge pool will get deeper and the splash back from the plunge
pool will erode the wall of the waterfall causing an overhang to develop. The
erosion of the wall is known as cavitation and is caused by solution. Solution is
chemical erosion caused by the chemicals in the water reacting with the rock. The
overhang will eventually crack off and fall into the plunge pool, causing further erosion.
These processes continue and the waterfall will retreat upstream - this is known
as head-ward erosion. In some cases, head-ward erosion will cause the formation of
a steep sided river valley known as a river gorge. Niagara fall on the US / Canada
border is an example of a gorge.
Niagara falls