River Processes

Rivers are one of the most powerful agents of landscape change. From tiny upland streams to mighty lowland rivers, they shape the land through erosion, transportation, and deposition. This lesson examines these processes in detail, including how they change as a river flows from its source to its mouth.

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The River as a System

A river can be understood as an open system — it has inputs, stores, transfers, and outputs.

Component	Examples
Inputs	Precipitation (rain, snow); groundwater seepage; tributary streams
Stores	Water held in the channel, floodplain, lakes
Transfers	Water flowing downstream; sediment transported by the river
Outputs	Water reaching the sea (or a lake); evaporation; water abstraction by humans

The river's ability to erode and transport material depends on its energy, which is determined by the volume of water (discharge) and the speed of flow (velocity).

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River Erosion

Rivers erode their channels through four main processes — the same four that operate at the coast, but adapted to the river environment.

Erosion Process	Description	Where Most Effective
Hydraulic action	The sheer force of flowing water forces its way into cracks in the river bed and banks. Air is compressed in these cracks, and when released, the pressure shatters the rock.	Most powerful during floods and in the upper course where flow is turbulent
Abrasion (corrasion)	Rocks, pebbles, and sand carried by the river scrape against the bed and banks, wearing them away like sandpaper. This is the most effective erosion process in most rivers.	Throughout the river, but especially in the upper course where the load is angular and heavy
Attrition	Rocks and pebbles carried by the river collide with each other, breaking into smaller, smoother, more rounded particles.	Throughout the river — particles get smaller and rounder downstream
Solution (corrosion)	Slightly acidic river water dissolves soluble minerals in the rock (especially calcium carbonate in chalk and limestone).	Where the river flows over or through soluble rock

Types of River Erosion

Rivers erode in two main directions:

Type	Description	Where Dominant
Vertical erosion	The river erodes downwards, deepening its channel	Upper course — where the gradient is steep and the river has high potential energy
Lateral erosion	The river erodes sideways, widening its channel	Middle and lower course — where the gradient is gentler and meanders develop

graph LR
    subgraph "Upper Course"
        A["Steep gradient"] --> B["Fast, turbulent flow"]
        B --> C["VERTICAL erosion<br/>dominates"]
        C --> D["Deep, narrow<br/>V-shaped valley"]
    end
    subgraph "Lower Course"
        E["Gentle gradient"] --> F["Slower, smoother flow"]
        F --> G["LATERAL erosion<br/>dominates"]
        G --> H["Wide, flat<br/>floodplain"]
    end

Exam Tip: Do not say a river flows "faster" in its upper course — this is a common misconception. The river actually flows faster in its lower course because there is less friction (the channel is wider and deeper, so a smaller proportion of water is in contact with the bed and banks). In the upper course, turbulent flow gives the impression of speed, but friction from the rough, rocky bed slows the water significantly.

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River Transportation

Rivers transport material in four ways, depending on the size and weight of the particles and the energy of the river.

Transport Method	Particle Size	Description
Traction	Very large (boulders, cobbles)	Heavy material is rolled along the river bed by the force of the current. Only occurs when the river has high energy (e.g., during floods).
Saltation	Medium (pebbles, coarse sand)	Particles are bounced along the river bed in a hopping motion. The current lifts them briefly, but they are too heavy to remain suspended.
Suspension	Small (fine sand, silt, clay)	Fine particles are carried within the water column. This gives rivers their muddy, brown appearance. This is the most common transport method by volume.
Solution	Dissolved minerals	Dissolved material (e.g., calcium bicarbonate from limestone) is carried invisibly in the water. Cannot be seen.

How Transport Changes Downstream

Feature	Upper Course	Middle Course	Lower Course
Load size	Large boulders and cobbles	Pebbles and coarse sand	Fine sand, silt, and clay
Load shape	Angular, rough	Sub-angular, smoother	Rounded, smooth
Dominant transport	Traction and saltation	Saltation and suspension	Suspension and solution
Volume of load	Small	Medium	Large

This change occurs because attrition progressively breaks particles into smaller, smoother fragments as they travel downstream.

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The Hjulström Curve

The Hjulström curve is a graph that shows the relationship between river velocity and the erosion, transport, and deposition of particles of different sizes.

Key Features of the Hjulström Curve