River Physical Processes

How river processes work together

A river landscape includes both the river channel and the surrounding valley sides. Its shape changes through connected processes: rock breaks down, material moves downhill, and flowing water wears away, carries and deposits material.

Weathering breaks down rock where it is. Mass movement transfers material downhill under gravity. Within the channel, erosion, transportation and deposition remove, move and lay down material. For example, weathering can loosen rock on a valley side, a slide can deliver it to the channel, and the river can then transport it downstream.

Weathering weakens rock in place

Mechanical weathering breaks rock into smaller pieces without changing its chemical composition. Freeze–thaw weathering is an example. Water enters a crack, freezes and expands, putting pressure on the surrounding rock. When the ice thaws, water can enter further into the crack. Repeated freezing and thawing widen the crack until pieces of rock break off.

Chemical weathering changes or dissolves the minerals in rock. Rainwater is naturally slightly acidic and can react with susceptible rocks. Carbonation, for example, involves a reaction between slightly acidic water and rocks such as limestone, gradually dissolving them. Not all rock types respond equally: their mineral composition affects their resistance.

Biological weathering is rock breakdown caused by living organisms. Plant roots can grow into cracks. As the roots grow thicker, they exert pressure and widen the cracks, eventually splitting the rock.

These processes weaken valley sides and make material available for erosion or mass movement; weathering itself does not carry that material away.

Sliding and slumping move material downhill

Mass movement happens when gravity pulls rock or soil down a slope. Weathering can loosen the material beforehand. River erosion can also undercut the foot of a slope, removing support from the material above.

Water can make a slope less stable. Heavy or prolonged rainfall saturates the material, increasing its weight, while water along a slip surface can reduce friction and make movement easier.

In sliding, a mass of material moves downslope along a relatively straight, often flat slip surface. In slumping, material moves along a curved slip surface and rotates as it moves. Slumping commonly occurs in weak material such as clay and leaves a curved scar behind.

Both processes reshape valley sides and can supply sediment to the river. This links processes on the slopes to the load carried within the channel.

Four processes erode river material

The river’s bed is the bottom of its channel; its banks are the sides. Flowing water and the material it carries can wear these surfaces away.

• Hydraulic action: the force of moving water breaks material away from the bed and banks.
• Abrasion: rocks and other sediment carried by the river scrape or grind against the bed and banks, wearing them down.
• Attrition: transported particles collide with one another and chip or break apart. They become smaller, smoother and more rounded.
• Solution: soluble rock, such as limestone or chalk, dissolves in river water. This is also called corrosion.

Abrasion and attrition involve different collisions: abrasion wears away the channel, whereas attrition wears down the river’s load. Erosion directed downwards deepens a channel; erosion directed sideways widens it.

Four ways a river transports its load

A river’s load includes solid sediment and dissolved material. How a particle travels depends on its size and weight, as well as the strength of the flow.

Traction moves large particles, such as boulders, by rolling or pushing them along the bed. Saltation moves smaller particles, such as pebbles, in short hops or bounces along the bed.

Suspension carries fine particles, such as silt and clay, within the flowing water. Solution transports dissolved substances within the water itself. Unlike suspended sediment, dissolved material is not present as separate solid particles.

The transport method is not a fixed rule based on size alone: stronger flow can move larger particles than weaker flow. Material dissolved by solution erosion can subsequently be transported in solution.

Deposition happens when carrying ability falls

Deposition occurs when a river no longer has enough energy to carry some of its sediment. As flow slows, larger and heavier particles generally settle first, while finer particles can remain in transport for longer.

Flow may slow where the gradient becomes gentler, on the inside of a river bend, or where the river enters a lake or the sea. Reduced water flow can also reduce its carrying ability. Deposition may occur if the sediment supplied exceeds what the river can transport.

Deposited material can later be picked up again if the flow becomes stronger. River landscapes therefore develop through an ongoing interaction between the supply of material, its movement and its deposition.