River channel processes and landforms

Abrasion (corrasion) is when the river's load is dragged and hurled against the bed and banks, scraping and wearing away the channel.

Attrition is when the load particles collide with one another and break down, becoming smaller and rounder over time.

The key difference is what is being worn away: abrasion erodes the CHANNEL, whereas attrition wears down the LOAD itself.

Laminar flow is the movement of water in smooth, parallel layers with little or no mixing between them.

Turbulent flow is chaotic, with eddies and swirls that mix the water and lift sediment into suspension.

Turbulent flow is far more common in natural rivers; laminar flow occurs only in very shallow, slow, smooth channels.

An oxbow lake forms from a pronounced meander. Helicoidal flow directs the fastest current against the outer banks of the bends, where hydraulic action and abrasion cause erosion, while deposition occurs on the inner banks.

As erosion continues on the outer banks of two adjacent bends, the neck of land between them becomes progressively narrower.

During a flood, when the river has high energy and discharge, the river erodes through the narrow neck and takes the shorter, steeper, straighter route across it. This is the cut-off.

With the faster water now flowing along the straight new channel, the river deposits sediment across the entrances to the old bend, sealing it off from the main channel.

The abandoned, curved loop of water left behind is the oxbow lake. Over time it receives no flow, becomes stagnant and gradually dries up, leaving a meander scar.

When a river floods, water overflows the channel banks and spreads out across the floodplain. As soon as the water leaves the confined channel it loses velocity and therefore energy.

Because its competence falls suddenly, the river deposits its coarsest, heaviest sediment first — right at the edge of the channel.

The finer sediment is carried further out before it is deposited across the wider floodplain.

Over many flood events, the repeated deposition of coarse material at the channel margins builds up raised banks called levées, which stand above the level of the floodplain.

These natural levées can later raise the river above its floodplain, which is significant for flood risk if they are overtopped or breached.

Rivers create their landforms through both erosion — the wearing away of the channel — and deposition — the laying down of sediment when energy falls. This essay assesses their relative importance and argues that, although individual landforms may be dominated by one process, the most distinctive river landforms are products of erosion and deposition operating together, with the balance shifting along the river's course.

Some landforms are clearly erosion-dominated. Waterfalls and gorges form by the differential erosion of hard rock over soft rock, with hydraulic action and abrasion cutting plunge pools, undercutting the cap rock and driving upstream retreat. River cliffs and potholes are likewise products of vigorous erosion. These landforms are concentrated in the high-energy upper course, where steep gradients and turbulent flow maximise the river's erosive power, suggesting erosion is the more important process there.

Other landforms are deposition-dominated. Levées form as floodwater leaves the channel, loses velocity and drops its coarsest load at the bank; floodplains are built from layers of alluvium deposited during repeated floods; and point bars accumulate on the slow inner bank of meanders. These features dominate the low-energy lower course, where gentle gradients and reduced velocity favour deposition.

However, the most characteristic river landforms require BOTH processes working together. A meander is simultaneously a product of erosion (the outer-bank river cliff) and deposition (the inner-bank point bar); without both, the landform would not exist. Floodplains, too, are widened by lateral erosion as meanders migrate AND built up by deposition during floods. This interdependence shows that treating erosion and deposition as competing processes is misleading.

The relative importance therefore depends on context — on the river's energy and stage. Erosion dominates in the steep, high-energy upper course; deposition dominates in the gentle, low-energy lower course; and the two operate together in the middle and lower courses to create meanders and floodplains.

Overall, neither process is universally more important. Erosion is the leading process in the upper course and deposition in the lower course, but the river's most distinctive landforms — meanders, oxbow lakes and floodplains — arise from the two acting in combination. The fairest judgement is that their relative importance varies systematically with the river's energy along its course, rather than one process being inherently more important than the other.

River landforms are shaped by the interaction of the river's energy — its velocity and discharge — and the rock over which it flows. This essay assesses whether energy matters more than rock type, arguing that energy controls the type and rate of process while rock type controls where and how distinctly certain landforms develop, so the two are deeply interlinked rather than one simply outweighing the other.

There is strong support for the importance of river energy. Energy, derived from velocity and discharge, determines whether erosion, transport or deposition dominates. High-energy upper courses, with steep gradients and turbulent flow, generate the powerful hydraulic action and abrasion that carve gorges and river cliffs. As energy falls downstream, the river loses competence and capacity and switches to deposition, building floodplains and levées. The Hjulström relationship confirms that velocity governs whether a given particle is eroded, transported or deposited. In this sense, energy sets the 'process regime' that creates landforms.

However, rock type is also fundamental. Waterfalls and gorges exist precisely BECAUSE of differential resistance — a band of hard rock over softer rock — so without the geological contrast the landform would not form, however energetic the river. Permeable, soluble rock such as limestone allows solution and distinctive features, while resistant rock slows erosion and preserves steep valley sides. The pattern and resistance of rock also guide the river's course and the location of rapids and gorges.

The two controls clearly interact. Energy provides the force, but rock type determines how that force is expressed: the same high-energy river will cut a deep gorge in jointed hard rock but a broad valley in weak clay. Equally, a low-energy river cannot exploit even the most contrasting geology. Landform formation is therefore the product of energy acting upon a particular rock structure.

Weighing the two, river energy is the more universal control because it governs the fundamental process at every point along the river, whereas rock type's influence is decisive only where geological contrasts exist. Yet for specific landforms such as waterfalls and gorges, rock type is the necessary condition without which they cannot form.

Overall, I partly agree. River energy is the more pervasive control on landform formation because it dictates the dominant process throughout the river's course. But rock type is essential for explaining particular erosional landforms, and the most accurate judgement is that landforms result from river energy acting upon rock structure — neither factor operates alone.