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examBoard: Cambridge
examType: IGCSE
lessonTitle: River Transportation - Saltation and Solution
Rivers are nature's conveyor belts, constantly moving materials from one place to another. After erosion breaks down rocks and soil, rivers transport these materials downstream. How they do this depends on the size of the material, the speed of the river and several other factors. Today, we'll focus on two important transportation methods: saltation and solution.
Key Definitions:
Rivers transport materials in four main ways:
Today we'll focus on saltation and solution, which represent different ends of the transportation spectrum.
The material transported by rivers is categorised into three types:
Each type of load affects the river differently and creates distinct landforms.
Saltation comes from the Latin word 'saltare' meaning 'to dance' – and that's exactly what these particles do! Sand and small pebbles are picked up by the current, carried a short distance and then dropped back to the riverbed, creating a bouncing or jumping motion.
Imagine a game of hopscotch played by sand grains. Here's what happens:
The flowing water exerts force on sand-sized particles. When the force is strong enough, the particle is lifted slightly off the riverbed.
The particle travels a short distance through the water, moved by the current. Gravity is constantly pulling it back down.
The particle lands back on the riverbed, often knocking other particles into motion and continuing the process.
If you've ever stood in a fast-flowing stream and felt tiny particles hitting your legs, you've experienced saltation first-hand! These particles can move surprisingly quickly during floods, contributing to erosion and the reshaping of river channels.
While saltation is visible and dynamic, solution is the quiet, invisible transportation method. It involves the chemical dissolution of rocks and minerals into the water itself.
Solution occurs when water acts as a solvent, breaking down certain materials at a molecular level:
When rainwater falls, it absorbs carbon dioxide from the air, forming a weak carbonic acid. This slightly acidic water is excellent at dissolving certain rocks, especially limestone and chalk. As the river flows over these rocks, it gradually dissolves them, carrying the minerals away in solution.
Common dissolved materials include:
You can't see the dissolved load with your naked eye, but there are clues:
Several factors determine whether material will be transported by saltation, solution, or other methods:
Faster-flowing rivers can transport larger particles. As velocity increases, more particles move by saltation rather than just rolling along the bed. Solution occurs regardless of velocity, though faster flows can dissolve material more quickly by bringing fresh water into contact with rocks.
The size, shape and density of particles determine how they're transported. Sand-sized particles (0.06-2mm) are perfect for saltation. Solubility determines what can be transported in solution - limestone dissolves easily while granite barely dissolves at all.
Warmer water can dissolve more minerals, increasing the dissolved load. Rainfall patterns affect river discharge and therefore transportation capacity. The pH of water affects what dissolves - acidic water dissolves limestone more readily.
The River Thames provides an excellent example of both saltation and solution in action. In its upper course, where it flows over limestone in the Cotswolds, the river carries a significant dissolved load of calcium carbonate. This makes the water 'hard' and contributes to the famous white cliffs at its estuary.
In its middle course, the river's moderate velocity allows for significant saltation of sand and small pebbles, particularly after heavy rainfall. Researchers have found that during flood events, the amount of material transported by saltation can increase by up to 500%.
The Thames Water Authority regularly monitors the dissolved load of the river, finding seasonal variations that correspond to rainfall patterns and agricultural activities in the catchment area.
These transportation processes are crucial for understanding river systems and landscapes:
When studying rivers in fieldwork, you can investigate transportation processes through several methods:
You can observe saltation by:
To investigate dissolved load:
Let's compare these two transportation methods:
| Feature | Saltation | Solution |
|---|---|---|
| Particle size | Sand to small pebbles | Molecular (dissolved) |
| Visibility | Visible to naked eye | Invisible (water appears clear) |
| Movement pattern | Bouncing/jumping along bed | Moves with water flow |
| Dependency on velocity | Highly dependent | Less dependent |
| Material types | Quartz sand, small stones | Limestone, chalk, salt |
| Resulting landforms | Contributes to meanders, bars | Caves, sinkholes, tufa deposits |
Understanding these transportation processes helps us appreciate how rivers shape our landscape and how human activities might affect these natural systems. In your next river study, try to identify evidence of both saltation and solution!
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