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
On a river bend, which bank usually erodes more?
- Neither, because bends keep their shape.
- The inner bank, because it is the shortest route.
- Both banks erode equally.
- The outer bank.
Show the answer
The outer bank.
The fastest, deepest flow is usually near the outer bank, so it erodes into a steep river cliff. Slower water on the inside deposits a point bar.
A meander moves sideways as its outer bank erodes and its inner bank builds up, driven by how water swirls around the bend.
A meander is a winding bend in a river. As water goes round the bend, the fastest, deepest flow is usually near the outer bank. This erodes the outer bank, forming a steep river cliff.
Water also spirals around the bend, called helical flow. Surface water moves towards the outer bank, and water near the bed moves towards the inner bank, carrying sediment. Slower flow on the inside lets sediment settle, building a gentle point bar.
Outer-bank erosion and inner-bank deposition slowly shift the bend sideways and downstream. In floods, the neck of a meander can narrow until the river cuts through. The old loop may become an oxbow lake, but not every bend does.
Put these in order
Put the steps in the life of a meander in order.
- The abandoned loop may become an oxbow lake.
- Slow flow on the inner bank deposits a point bar.
- The bend migrates and the neck of the meander narrows.
- The river cuts through the neck during a flood.
- The fastest, deepest flow swings towards the outer bank of a bend.
- The outer bank erodes into a steep river cliff.
Show the answer
- The fastest, deepest flow swings towards the outer bank of a bend. Fast water has the energy to erode, so it wears the outer bank away.
- The outer bank erodes into a steep river cliff. Helical flow carries sediment near the bed towards the slower inner bank.
- Slow flow on the inner bank deposits a point bar. Erosion on one side and deposition on the other shift the whole bend sideways.
- The bend migrates and the neck of the meander narrows. In a flood, high water can break through the narrow neck along a shorter route.
- The river cuts through the neck during a flood. The river now takes the shorter path, leaving the old loop cut off from the main flow.
- The abandoned loop may become an oxbow lake.
Discharge, sediment supply, bank strength, plants, slope and floodplain space all shape meanders. Firm or root-bound banks slow migration. Very resistant banks or narrow valleys limit sideways movement.
Bank protection and straightening can stop local erosion, but may move it downstream and destroy floodplain habitat. So a river plan should consider the whole reach and its floodplain, not just one threatened bank.
- River cliff
- Steep outer bank, eroded by fast flow.
- Point bar
- Gentle inner bank, built by deposition.
- Helical flow
- Water spirals round the bend, moving sediment inward.
Step by step
Outer bank erodes
Faster, deeper flow attacks the bank.
Inner bank builds
Slower flow deposits a point bar.
The bend migrates
Repeated erosion and deposition move the channel.
Worked example: Reading an aerial photo
Bare sediment on the inside of a bend shows a point bar. A steep bank on the outside shows a river cliff. One photo shows the features. Photos from different years are needed to measure movement.
Watch out for this
The inner bend erodes most because it is the shortest route.
The fastest flow usually swings to the outer bank. Sediment builds up on the inside.
Check your understanding
Where is a point bar normally found?
- On the outer bank where flow is fastest.
- At a river's source, where flow is slow.
- On the inside of a meander bend.
Show the answer
On the inside of a meander bend.
Yes. Slower flow there deposits sediment.