What this lesson teaches
I can identify meanders, braided channels and deltas in the humid tropics.
Syllabus 9173, Topic 2.2, 3. Fluvial Landforms in the Humid Tropics. What fluvial landforms can be found in the humid tropics?
I can explain the processes and factors that form tropical river landforms.
Syllabus 9173, Topic 2.2, 3. Fluvial Landforms in the Humid Tropics. What factors influence the formation of fluvial landforms in the humid tropics?
Make a guess
A mountain river receives far more gravel from landslides than it can move. What is likely to happen?
- The gravel is carried straight to the sea.
- The river stops flowing.
- Gravel bars build up and split the flow.
- The river becomes one deep, straight channel.
Show the answer
Gravel bars build up and split the flow.
When bedload exceeds what the river can carry, sediment piles up as bars. Water flows around them in many shifting channels.
Rivers become braided when they carry lots of sediment, have changing flows and easily eroded banks, creating shifting bars.
A braided river has many linked channels separated by bars of sediment in the middle of the river. The bars are usually bare, because floods often rework them. Channels split and rejoin around them, and keep migrating.
When a river receives more bedload than it can move, sediment piles up as bars. Water flows around them and cuts new routes. Floods move and reshape the bars. When flow drops, the bars are exposed, and water gathers into fewer channels.
Braiding is favoured by lots of coarse sediment, changing flow, a wide shallow channel and weak banks. A drop in gradient also helps sediment settle. Plants can stabilise some bars.
Sediment can come naturally from landslides, eroding mountains and tributaries. Deforestation, mining and building can add more. Dams can reduce sediment downstream. What matters is the balance between supply, transport and bank strength.
- Braided river
- Many linked channels around shifting sediment bars.
- Mid-channel bars
- Usually bare and often reworked.
- Main cause
- More bedload than the river can carry.
Step by step
Supply beats transport
Sediment builds up on the bed.
Bars split the flow
Water finds routes around them.
Flow reshapes
Floods move bars and channels.
Worked example: The Brahmaputra in Assam
NASA images show the Brahmaputra widening and braiding as it leaves the Himalayan foothills for flatter land. Huge sediment loads and falling transport power explain the bars.
Watch out for this
Every river with several channels is braided.
Some rivers have stable, vegetated islands. Braiding means active, shifting bars.
Check your understanding
Which combination most strongly favours braiding?
- Very little sediment and strong, confined banks.
- Lots of sediment, changing flow and weak banks.
- Steady flow and little movable sediment on the bed.
Show the answer
Lots of sediment, changing flow and weak banks.
Yes. These let bars build and channels shift.