River Landforms » Upper Course River Landforms

The upper course: a young river with a steep slope

Near its source, a river is small, with a rough, rocky bed, and sits on a steep slope. It has little water but a lot of energy to cut downwards. Its bed is rough and full of large, angular stones. This is the part of the river where vertical erosion is strongest, so the main landforms here are made by erosion. The processes of hydraulic action and abrasion do the work, as you saw in the lesson on river erosion, transportation and deposition.

A landform is a natural feature of the land. For each one below, learn what it looks like first, then the steps that make it.

Key terms:

• Landform: a natural feature of the Earth's surface, such as a valley or a waterfall.
• V-shaped valley: a narrow valley with steep sides, shaped like the letter V in cross-section.
• Interlocking spurs: ridges of land that stick out from alternate sides of a valley and fit together like the teeth of a zip.
• Waterfall: a place where a river drops vertically over a step in the river bed.
• Plunge pool: a deep hollow at the bottom of a waterfall.
• Gorge: a deep, narrow valley with very steep, rocky sides.
• Rapids: a stretch of river where the water is shallow, fast and turbulent over a rocky bed.
• Pothole: a round hole in a rocky river bed.

V-shaped valleys

How it forms:

1. In the upper course the river has a steep gradient and lots of energy, but not much water. It cuts downwards by vertical erosion.
2. Rocks carried along the bed grind against it (abrasion), and the force of the water breaks rock away (hydraulic action). This deepens the channel.
3. The valley sides are not cut by the river. They are broken up by weathering (rock broken down where it is) and moved downhill by mass movement, such as soil creep and slumping.
4. The loose material falls into the river and is carried away.
5. Over thousands of years the river cuts deeper and the sides stay steep, leaving a narrow V-shaped valley.

Interlocking spurs

How they form:

1. The upper course river has little energy to erode sideways, because most of its energy is used for vertical erosion.
2. The river cannot cut through hard, high ridges of land in its way.
3. So it twists and bends around them, flowing from side to side.
4. The ridges that remain stick out from alternate sides of the valley. These are the interlocking spurs.

Waterfalls and gorges

How a waterfall forms:

1. The river flows over a band of hard rock that lies on top of softer rock.
2. The soft rock erodes faster than the hard rock, because it is weaker.
3. Over time the soft rock is worn away and the river bed develops a step.
4. Water now falls over the step. Its force and the swirling stones dig out a deep hollow at the bottom, the plunge pool.
5. The soft rock below the hard rock is undercut, which leaves a hard-rock overhang with nothing under it.
6. The overhang becomes too heavy and collapses into the plunge pool.

How a gorge forms:

1. The overhang collapses and the process repeats again and again.
2. Each time, the waterfall moves back (retreats) upstream.
3. Over thousands of years the waterfall moves a long way upstream.
4. It leaves behind a deep, steep-sided, narrow valley: the gorge.

Rapids

Rapids look like white, rough, fast-moving water, where the river is shallow and flows over many rocks. You can often see rocks poking above the surface.

What to label: the bands of resistant rock, rocks poking above the surface, the shallow turbulent water and an arrow for the flow direction.

How they form:

1. A river flows over alternating bands of rock that have different resistance.
2. The softer bands erode faster, so the bed becomes uneven.
3. Bands of resistant (hard) rock stick up from the river bed.
4. Water flows quickly over and around these bands, so it becomes shallow and turbulent. The result is rapids.

Think of rapids as a very small, gentle version of a waterfall. There is a drop, but the water flows down a slope rather than over a vertical step.

Potholes

A pothole is a rounded hole, often deep, in a hard rock river bed. You can sometimes see a few smooth pebbles still sitting inside it.

How they form:

1. A small crack or dip in the river bed traps a few pebbles.
2. Fast-flowing water makes eddies (swirling currents) that spin the pebbles round and round.
3. The spinning pebbles wear away the rock by abrasion.
4. The hollow gets deeper and wider. Over time it becomes a smooth, rounded pothole.

What to label: the rounded hollow, the trapped pebbles, arrows showing the swirling eddy and the rock river bed around it.

Exam-style question

Fig. 1.1 is a photograph of a waterfall in a rocky valley. A band of dark, hard rock lies across the top. Softer, paler rock is visible below, with a deep pool at the bottom.

(a) Identify the name of the deep pool at the bottom of the waterfall. [1 mark]

(b) Describe two characteristics of a V-shaped valley. [2 marks]

(c) Explain how a waterfall forms and how it leaves a gorge behind it. [6 marks]