🚩 Why Study Sediment Changes?
Understanding how sediment changes helps engineers design better bridges, predict where rivers might flood and manage water quality. It's also crucial for understanding how landscapes form over thousands of years.
River Processes and Landforms » Sediment Size and Shape Changes
What you'll learn this session
Study time: 30 minutes
- How sediment size changes as it travels downstream in rivers
- Why sediment becomes more rounded during transport
- The processes that cause these changes in rivers
- How to measure and describe sediment characteristics
- Real examples from UK rivers showing these changes
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Unlock This CourseIntroduction to Sediment Size and Shape Changes
When rivers flow from their source to the sea, they carry millions of pieces of rock, sand and mud called sediment. This sediment doesn't stay the same - it changes dramatically as it travels downstream. Understanding these changes helps us predict river behaviour and manage flood risks.
Key Definitions:
- Sediment: Particles of rock, sand and mud carried by rivers.
- Load: All the material a river carries, including sediment.
- Attrition: The process where rocks knock against each other, wearing them down.
- Abrasion: When sediment scrapes against the river bed and banks, wearing them away.
- Downstream: The direction towards the river's mouth.
- Calibre: The size of sediment particles.
How Sediment Size Changes Downstream
One of the most important patterns in rivers is that sediment gets smaller as you move downstream. This happens for several reasons and it's so reliable that geographers can often tell where they are along a river just by looking at the sediment size.
The General Pattern
In the upper course of a river (near the source), you'll find large boulders and rocks. In the middle course, the sediment is typically pebble-sized. By the time you reach the lower course (near the mouth), most sediment is fine sand, silt, or clay.
⛰ Upper Course
Large boulders and rocks (over 256mm). Angular shapes with sharp edges. Limited transport due to high energy needed.
🛀 Middle Course
Pebbles and cobbles (64-256mm). More rounded shapes. Easier to transport as river energy increases.
🌊 Lower Course
Sand, silt and clay (under 2mm). Very rounded and smooth. Easily transported in large quantities.
Processes Causing Size Changes
Several processes work together to reduce sediment size as it moves downstream. These processes are constantly active whenever the river is flowing.
Attrition - The Collision Process
Attrition is like a giant natural tumble dryer. As sediment bounces along the river bed, pieces knock into each other. Each collision chips off small fragments, gradually reducing the size of the original rock. The more collisions, the smaller the sediment becomes.
Abrasion - The Grinding Process
Abrasion occurs when sediment scrapes against the river bed and banks. Think of it like sandpaper - the constant grinding wears down both the sediment and the river channel. This process is particularly effective during floods when the river has more energy.
Case Study Focus: River Tees, North Yorkshire
The River Tees demonstrates classic downstream changes. At High Force waterfall in the upper course, boulders over 1 metre across are common. By Middlesbrough in the lower course, the average sediment size is less than 2mm. Studies show the sediment reduces in size by approximately 50% every 10 kilometres downstream.
How Sediment Shape Changes
Shape changes are just as important as size changes. Fresh rock fragments start angular and sharp, but gradually become rounded and smooth through the same processes that reduce their size.
The Roundness Scale
Geographers use a standard scale to measure how rounded sediment becomes:
- Angular: Sharp corners and edges (typical of upper course)
- Sub-angular: Slightly worn corners (upper to middle course)
- Sub-rounded: Noticeably rounded but some edges remain (middle course)
- Rounded: Smooth curves, few sharp edges (middle to lower course)
- Well-rounded: Very smooth, almost spherical (lower course)
⏱ Time Factor
The longer sediment spends in the river, the more rounded it becomes. A pebble might take thousands of years to change from angular to well-rounded, depending on the river's energy and the rock type.
Factors Affecting the Rate of Change
Not all sediment changes at the same rate. Several factors influence how quickly size and shape changes occur.
Rock Type Matters
Soft rocks like limestone change much faster than hard rocks like granite. Sedimentary rocks often break along natural weak points, whilst igneous rocks are more resistant to wear.
River Energy and Velocity
Faster-flowing rivers cause more collisions and abrasion. During floods, when velocity increases dramatically, a river can accomplish months of normal wear in just a few hours.
Distance Travelled
The further sediment travels, the more opportunities for attrition and abrasion. However, most size reduction happens in the first few kilometres of transport.
Case Study Focus: River Severn Sediment Study
Research on the River Severn showed that limestone pebbles lost 40% of their mass in the first 5km of transport, but only another 20% in the next 50km. This demonstrates that most size reduction happens early in the journey, with the rate slowing as sediment becomes smaller and more rounded.
Measuring Sediment Characteristics
Geographers use several methods to study sediment changes, from simple field techniques to sophisticated laboratory analysis.
Field Measurement Techniques
The most common method is the pebble count, where researchers measure the longest axis of randomly selected stones. They also assess roundness using visual comparison charts and record the rock type of each sample.
Laboratory Analysis
Detailed studies involve sieving sediment through different mesh sizes to create precise size distribution graphs. Scanning electron microscopes can reveal microscopic surface features that show wear patterns.
📈 Data Presentation
Results are often shown as scatter graphs plotting distance downstream against average sediment size, or histograms showing the distribution of different size classes at specific locations.
Exceptions to the General Pattern
Whilst the downstream decrease in sediment size is very common, there are important exceptions that prove the rule.
Tributary Inputs
When a tributary joins the main river, it can introduce new, larger sediment. This temporarily increases the average sediment size before the normal downstream pattern resumes.
Selective Transport
Rivers can only move sediment when they have enough energy. During low flow periods, only the smallest particles move, leaving larger ones behind. This can create temporary accumulations of coarse sediment.
Human Interference
Dams trap sediment, creating artificially clear water downstream that can erode the river bed. Quarrying can introduce new angular material, disrupting the natural pattern.
Case Study Focus: River Thames Barrier Impact
The Thames Barrier has altered sediment transport patterns in the lower Thames. Upstream of the barrier, fine sediment accumulates during closures. Downstream, the reduced sediment supply has led to increased erosion of the river bed, demonstrating how human interventions can disrupt natural sediment processes.
Implications for River Management
Understanding sediment size and shape changes is crucial for managing rivers effectively and predicting their behaviour.
Flood Risk Assessment
Areas where sediment suddenly becomes finer often experience deposition, which can raise the river bed and increase flood risk. Monitoring sediment changes helps predict where flood defences might be needed.
Habitat Management
Different fish species prefer different sediment sizes for spawning. Salmon need clean gravel, whilst some invertebrates prefer fine sand. Understanding natural sediment patterns helps create appropriate habitats.
Engineering Considerations
Bridge foundations must account for local sediment characteristics. Areas with active sediment transport may experience scour around bridge piers, requiring special design considerations.