GCSE Geography | Landforms in the Upper Course (River Landscapes 4)

Erosional processes in the upper course of the river lead to the formation of v-shaped valleys, interlocking spurs, waterfalls, rapids and gorges.

How do interlocking spurs form?

Interlocking spurs poke out into the valley from alternate sides - they are found in narrow steep v-shaped valleys with convex slopes, where the river channel takes up most of the valley floor. They are formed by vertical erosion (hydraulic action) deepening the channel.

V-shaped valleys are made of hard rock, such as slate or granite. This rock is resistant to erosion but is subject to freeze-thaw weathering (water gets into rocks, which freezes and expands and then thaws again) which weakens the rock over time causing fragments to break loose and fall down the slopes, creating scree slopes. Once this weathered material reaches the valley floor it will be carried away by the river.

The river has to take a winding path through the valley because of hard rock obstacles. The river will go around these obstacles which causes slopes to project into the valley from alternating sides, forming zip-like interlocking spurs.

How do rapids form?

Rapids are known as 'white water' and are fast-flowing turbulent parts of the river, where the river bed is very uneven and has a steep gradient. The reason the river bed is uneven is due to alternating bands of rock - soft rock is less resistant so erodes easily, lowering the river bed, whereas the hard rock is much more resistant so is left protruding up from the river bed. As the water flows over these uneven sections the river becomes turbulent, and is great for white water rafting!

How do waterfalls and gorges form?

Waterfalls form where water falls down a vertical drop in the river channel - rocks are laid horizontally, but a drop will form where there is a gap between hard rock capping upstream and softer rock downstream. Differential resistance means that the soft rock will erode much quicker and the water will fall down to the soft rock from the hard rock above.

Once the waterfall has become established the sheer volume of the water tumbling down will erode the rocks behind the fall through hydraulic action, this will break of pieces of soft rock and will form a notch or indentation at the base of the waterfall. Over time this soft rock will erode more leaving the hard rock capping overhanging the drop with no support below, which will eventually break off and fall to the base of the waterfall.

The collapsed rock gets picked up by the turbulent flow of the water here and will help erode the base further, creating a plunge pool. The erosion here means that undercutting continues, creating a new overhang, that again will collapse due to gravity, repeating the process and causing the waterfall to retreat upstream, leaving a steep-sided gorge downstream.

A gorge is a narrow, steep and high-sided valley, with bare, rocky walls which will become longer each time the waterfall retreats. The river channel takes up most of the valley floor and is full of large boulders and is fast-flowing turbulent white water.