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 large river carries lots of sediment into a sea with very strong waves and tides. Will it build a big delta?
- Yes, in a neat triangle.
- No, not necessarily. Waves may carry it away.
- Yes. A sediment-rich river builds a delta.
- No, because deltas form in lakes.
Show the answer
No, not necessarily. Waves may carry it away.
A delta grows only where deposition outpaces removal. Strong waves and currents can spread sediment along the coast or out to sea.
A delta grows where a river deposits sediment at its mouth faster than waves, tides and currents remove it.
When a river enters a sea or lake, it spreads out and slows, so it drops sediment near its mouth. A delta forms where enough sediment stays to build land, partly above and partly below water. Strong waves, tides and currents can spread sediment out or carry it away.
A delta has three parts. The delta plain is the low, flat land above water, crossed by channels and wetlands. The delta front is the sloping zone offshore. The prodelta is the deeper area farther out, where fine sediment settles.
Sand bars at the mouth split the river into distributaries. As sediment builds up, the delta grows seaward. Channels can switch course. Coarse sediment settles near the mouth, and fine clay travels farther. Salt water helps clay particles clump and sink. This seaward growth is called progradation.
Where the river dominates, the delta shows long channels. Waves smooth and spread sediment along the coast. Strong tides carve tidal channels. Most deltas are a mix. So a delta's shape is evidence of the balance between river, wave and tide processes, not a fixed triangle.
Rising sea level and sinking land create more space to fill. Deposition must keep up. Dams and sand mining reduce sediment, so parts of a delta can shrink. This extra room for sediment is called accommodation space. Uplift, basin geology, vegetation and changes in river flow also affect how a delta develops.
- Delta plain
- Low, flat land above water.
- Delta front
- Sloping zone offshore.
- Prodelta
- Deeper area where fine sediment settles.
Step by step
Deliver sediment
The river brings load to its mouth.
Deposit and reshape
Sediment drops; waves and tides move it.
Check the balance
Does deposition keep up with erosion and sinking?
Worked example: Two river mouths
Two made-up rivers bring the same sediment. At one, strong waves carry it away, so no lasting delta forms. At the other, it builds up. If the land then sinks faster, even the growing delta needs more sediment to keep its height.
Watch out for this
A delta is always a triangle entirely above sea level.
Its shape depends on rivers, waves and tides, and much of it lies underwater.
Check your understanding
Which change is most likely to slow delta growth?
- A point bar forms far upstream, with no change in sediment delivery.
- A dam traps sediment while waves keep removing coastal deposits.
- More sediment reaching the river mouth.
Show the answer
A dam traps sediment while waves keep removing coastal deposits.
Yes. Less sediment arrives while removal continues.
Check your understanding
The Mississippi delta has a bird's-foot shape. What does this suggest?
- Strong tides dominate at its mouth.
- River processes dominate over waves and tides.
- Strong waves dominate along its coast.
Show the answer
River processes dominate over waves and tides.
Sediment builds out along each distributary.