River Processes: Erosion, Transport & Deposition

Rivers are powerful agents of landscape change. They shape valleys, create landforms, and transport vast quantities of sediment from the mountains to the sea. This lesson covers the key processes that operate in river systems and how they change from the source to the mouth.

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The Drainage Basin System

A drainage basin is the area of land drained by a river and its tributaries. It is a type of open system with inputs, stores, transfers, and outputs.

Component	Examples
Inputs	Precipitation (rain, snow)
Stores	Interception (by vegetation), surface storage (puddles, lakes), soil moisture, groundwater
Transfers (flows)	Infiltration, percolation, throughflow, groundwater flow, surface runoff (overland flow), channel flow
Outputs	Evaporation, transpiration, river discharge (water flowing out to sea)

Key terms

Term	Definition
Source	Where the river begins (usually in upland areas)
Mouth	Where the river meets the sea
Tributary	A smaller river or stream that joins the main river
Confluence	The point where two rivers meet
Watershed	The boundary of the drainage basin (usually following a ridge line)
Discharge	The volume of water flowing in a river at a given point, measured in cumecs (m³/s)

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River Erosion

Rivers erode their channels through four processes — the same four that operate at the coast:

Process	Description
Hydraulic action	The force of the water itself dislodges particles from the bed and banks. Particularly effective during floods when the river has more energy.
Abrasion (corrasion)	The river's load (stones, sand, gravel) scrapes against the bed and banks, wearing them away. This is the most effective erosion process.
Attrition	Rocks and stones carried by the river collide with each other, becoming smaller, smoother, and rounder over time.
Solution (corrosion)	Slightly acidic river water dissolves soluble rocks such as limestone and chalk.

Vertical and lateral erosion

Type	Direction	Effect	Where it dominates
Vertical erosion	Downward into the bed	Deepens the channel; creates steep V-shaped valleys	Upper course (steep gradient, high energy)
Lateral erosion	Sideways into the banks	Widens the channel and valley; creates meanders and floodplains	Middle and lower course (gentle gradient)

Exam Tip: In the upper course, the river has a steep gradient and most of its energy is used to overcome friction with the rough, rocky bed. Vertical erosion dominates, cutting downward. In the lower course, the gradient is gentle, the river is deeper, and there is less friction — more energy is available for lateral erosion.

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River Transport

Once material has been eroded, the river transports it downstream. There are four transport processes:

Process	Description	Material size
Traction	Large boulders and rocks are rolled along the river bed	Very large
Saltation	Smaller stones and pebbles are bounced along the bed	Medium
Suspension	Fine particles of sand, silt, and clay are carried within the water	Small
Solution	Dissolved minerals are carried invisibly in the water	Dissolved

The Hjulstrom Curve

The Hjulstrom curve shows the relationship between river velocity and sediment size. Key points:

• Larger particles require more velocity to be eroded (picked up).
• Smaller particles (sand) require the least velocity to be eroded.
• Very fine particles (clay, silt) require surprisingly high velocity to be eroded because they are cohesive (they stick together).
• Once eroded, particles can be transported at lower velocities than were needed to erode them.
• When velocity drops below the deposition threshold, particles are deposited.

Exam Tip: You do not need to draw the Hjulstrom curve from memory, but you must understand the key principle: it takes more energy to pick up (erode) a particle than it does to keep it moving (transport it). This explains why rivers deposit sediment when they slow down.

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River Deposition

Deposition occurs when a river loses energy and can no longer carry its load. The largest, heaviest particles are deposited first, and the finest particles are deposited last.

Causes of deposition

Cause	Explanation
Reduced gradient	The river flows onto flatter ground and slows down
Reduced discharge	After a flood, water levels drop and the river has less energy
Increased load	A tributary adds more sediment than the river can carry
Entering a lake or sea	The river slows dramatically as it meets a standing body of water
Inside of a meander	The water is shallower and slower on the inside bend
Behind an obstacle	Vegetation, boulders, or man-made structures can cause localised deposition

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How the River Changes Downstream

As a river flows from its source to its mouth, its characteristics change systematically:

Feature	Upper Course	Middle Course	Lower Course
Gradient	Steep	Moderate	Gentle
Valley shape	Narrow, steep-sided, V-shaped	Wider, with a small floodplain	Wide, flat floodplain
Channel shape	Shallow, narrow, rocky bed	Deeper, wider	Deep, wide, smooth bed
Velocity	Can be fast but much energy lost to friction	Increasing	Fastest (least friction)
Discharge	Low	Increasing (tributaries join)	Highest
Load	Large, angular boulders and rocks	Smaller, more rounded	Fine sand, silt, clay
Dominant erosion	Vertical	Both vertical and lateral	Lateral
Dominant process	Erosion	Erosion and transport	Deposition

Why does discharge increase downstream?

As the river flows downstream, tributaries join the main channel, adding more water. Even though there may be less rainfall in the lower course, the cumulative effect of all the tributaries means that the total volume of water (discharge) increases.

Why does velocity often increase downstream?

This is counterintuitive — many students assume the upper course is fastest because it is steepest. In fact:

• In the upper course, the channel is rough and shallow, creating high friction. Much of the river's energy is used to overcome this friction, reducing velocity.
• In the lower course, the channel is smooth and deep, creating less friction. More energy is available for flow, so velocity is often higher despite the gentler gradient.

Exam Tip: This is a common exam question: "Explain why velocity often increases downstream despite the gradient becoming gentler." Always mention reduced friction due to a smoother, deeper channel in the lower course.

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The Long Profile and Cross Profile

Long profile

The long profile of a river shows the gradient of the river from source to mouth. It is typically a smooth, concave curve:

• Steep near the source (upper course)
• Gradually flattening in the middle course
• Very gentle near the mouth (lower course)