Sign up to access the complete lesson and track your progress!
Unlock This Course
Rivers aren't just flowing water - they're busy highways that move tons of material from mountains to seas! After erosion breaks down rocks and soil, rivers transport these materials downstream. You met the four erosion processes in River Erosion: Hydraulic Action, Abrasion, Attrition and Solution. This lesson follows that eroded material as it is carried along and, finally, dropped. How a river moves its load depends on the size of the material, the speed of the river and several other factors.
Key Definitions:
Rivers are nature's conveyor belts! They pick up material through erosion, carry it downstream and eventually deposit it. This process shapes our landscapes, creates habitats and even provides resources like sand and gravel that we use for building. Without transportation, rivers couldn't carve valleys or build deltas!
River transportation is part of the water cycle and rock cycle. Material eroded from mountains might end up in river deltas, beaches, or the sea floor. Over millions of years, some of this material might become sedimentary rock, only to be uplifted and eroded again. Rivers are key players in this never-ending cycle of Earth's materials.
Erosion, transportation and deposition work together along the whole length of a river. Rivers transport material in four main ways, from the heaviest cargo to the lightest:
Each type of load affects the river differently, and all four methods often happen at the same time in the same stretch of river.
Traction happens when particles are too heavy to be lifted into the water flow but can still be pushed along the riverbed. Think of it as the river's way of moving its heaviest cargo!
During traction, large particles like pebbles, cobbles and boulders roll, slide, or are pushed along the riverbed. This usually happens when:
The particles moving by traction make up part of the river's bed load. These materials often show signs of their journey - they become rounded and smoother as they bump and scrape against other rocks and the riverbed. Much of this movement happens during floods, when the river has the most energy.
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. Like traction, saltation moves part of the bed load.
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.
Suspension is when smaller, lighter particles like silt and clay are carried within the flowing water. These tiny particles give rivers their muddy or cloudy appearance, especially after rainfall.
For particles to be carried in suspension:
The amount of material a river can carry in suspension (its suspended load) depends on the river's velocity and turbulence. Faster, more turbulent rivers can carry more material and larger particles in suspension.
While saltation is visible and dynamic, solution is the quiet, invisible transportation method. Minerals that have been dissolved from rocks such as limestone and chalk (by the erosion process of solution) are carried along in the water itself as the dissolved load.
Common dissolved materials include:
Because these materials are dissolved, they do not settle out when the river slows down. They stay in the water until it reaches the sea or the minerals precipitate out.
You can't see the dissolved load with your naked eye, but there are clues:
Several factors determine how much material a river can transport and by what method:
The speed of the water is crucial for transportation. As velocity increases:
Solution happens 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, while silt and clay are carried in suspension. Solubility determines what can be transported in solution - limestone dissolves easily while granite barely dissolves at all.
Turbulence helps keep particles suspended. It creates upward currents that counteract gravity. You can see this in action at rapids or waterfalls where the water appears frothy and muddy - that's suspended sediment being churned up!
River discharge (the volume of water flowing past a point in a given time) affects transportation. Higher discharge during floods means more energy and more material transported - both in suspension and as bed load.
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 Yellow River gets its name from the huge amount of yellow silt it carries in suspension - over 1.6 billion tons annually! This material comes from the easily eroded loess soil of northern China. The river is so full of suspended sediment that it's considered the world's muddiest major river. This suspended load eventually builds up, raising the riverbed and causing the river to flow above the surrounding land in its lower course. This has led to devastating floods throughout Chinese history, earning it the nickname "China's Sorrow."
The four transportation methods are related and often happen at the same time. Let's compare them:
| Feature | Traction | Saltation | Suspension | Solution |
|---|---|---|---|---|
| Particle size | Pebbles, cobbles, boulders | Sand to small pebbles | Clay, silt, fine sand | Molecular (dissolved) |
| Movement pattern | Rolls or slides along the bed | Bounces/jumps along the bed | Carried within the water flow | Moves with the water flow |
| Type of load | Bed load | Bed load | Suspended load | Dissolved load |
| Contact with the riverbed | Constant | On and off | Little or none | None |
| Visibility | Visible stones on the bed | Visible to naked eye | Water looks cloudy or muddy | Invisible (water appears clear) |
| Dependency on velocity | Needs high energy, often only in floods | Highly dependent | Needs turbulence to stop particles settling | Less dependent |
| Material types | Large rounded stones | Quartz sand, small stones | Silt and clay | Limestone, chalk, salt |
When you're looking at a river, or studying one in fieldwork, you can often find evidence of each transportation method:
Next time you're near a river, observe its water. Is it clear or cloudy? Can you see the riverbed? Look for signs of transportation - are there rounded stones? Is there evidence of material being moved? After rainfall, notice if the river's appearance changes - this can indicate increased suspended load. These observations help you understand river transportation in action!
Rivers don't just erode and transport material - they also drop or deposit it. Deposition happens when a river loses energy and can no longer carry its load. Understanding deposition helps us make sense of many river features and landscapes we see today.
Key Definitions:
Rivers deposit material when their energy decreases. This happens when:
As a river slows down, its competence falls, so it deposits material in order of size - largest particles first, then progressively smaller ones. Boulders and pebbles are dropped first, then sand, then silt. The finest clay can stay in suspension until the water is almost still, and the dissolved load is not deposited at all. This sorts the load, so deposits are often coarse near the channel and finer further away.
The Hjulström Curve is a graph that shows the relationship between river velocity and the erosion, transportation and deposition of different sized particles. Particle size runs along the bottom and velocity up the side. The graph is split into three zones: above the top line particles are eroded (picked up), between the lines they are transported, and below the bottom line they are deposited.
Key points:
Deposition happens wherever a river loses energy. Common places include:
Deposition builds some of the most important river landforms. Each one has its own lesson: Meander Formation and Features, Oxbow Lake Development, Levee and Floodplain Formation and Delta Formation and Types.
Understanding how rivers move and drop their load matters for the people who live and work along them.
People manage depositional processes in various ways:
Rivers transport their load in four ways. Traction rolls the largest stones along the bed and saltation bounces sand and small pebbles, together moving the bed load. Suspension carries fine silt and clay as the suspended load, and solution carries dissolved minerals as the dissolved load. Velocity, particle size, turbulence, discharge and water chemistry control which methods happen and how much is carried. When a river loses energy it deposits its load, heaviest material first, as the Hjulström Curve shows. Remember that transportation and deposition work alongside erosion to create the dynamic river environments we see today.