What this lesson teaches
I can evaluate the ecosystem services of deltas and human impacts on them.
Syllabus 9173, Topic 2.2, 3. Fluvial Landforms in the Humid Tropics. What is the importance of deltas in the humid tropics?
Make a guess
People on a delta pump lots of groundwater for farms and homes. How can that make the delta more vulnerable to the sea?
- It has no effect on the land itself.
- Pumping raises sea level directly.
- The soft sediment compacts and sinks.
- It makes the delta grow faster seawards.
Show the answer
The soft sediment compacts and sinks.
Pumping lets soft sediment compact, so the ground sinks. Relative sea level then rises from both rising seas and sinking land.
Deltas support millions of livelihoods through sediment, fresh water and wetlands, but activities upstream and on the delta can weaken these services.
Deltas provide fertile soil, water, crops and fish. These are provisioning services. Wetlands and mangroves shelter young fish, cycle nutrients and store carbon: supporting services. They also slow water, trap sediment and calm waves: regulating services. Landscapes and heritage provide cultural services.
Sediment keeps deltas renewed. Upstream dams trap it, and sand mining removes it. Embankments protect fields from floods, but stop fresh sediment reaching them. With less new sediment, the land falls further behind rising water.
Pumping groundwater can compact soft sediment, so the land sinks. So relative sea level rises from both rising seas and sinking land. Lower dry-season river flow also lets salt water push further inland, harming drinking water and crops.
Clearing mangroves for fish farms removes habitat and wave protection. Fertiliser, sewage and industrial waste pollute water. Ports and dredging change currents. These activities bring jobs and food as well as costs, and gains and losses fall unevenly on different groups.
Protecting deltas needs action across the whole river basin. A sea wall alone cannot replace sediment or stop land sinking. Managing sediment, limiting sand mining, changing water use, restoring wetlands and supporting other livelihoods can work together.
- Delta services
- Food, water, habitats, flood and wave control.
- Sediment loss
- Dams, sand mining and embankments starve deltas.
- Subsidence
- Pumping groundwater makes land sink.
Step by step
Name the service
Link water, sediment or habitat to a livelihood.
Trace the disruption
What upstream or local activity changes it?
Weigh across scales
Short-term income versus long-term land and water.
Real examples to use
Mekong Delta, Vietnam
Upstream dams and sand mining reduce sediment, while groundwater pumping makes land sink. Salt water pushed far inland in the droughts of 2016 and 2020, damaging rice crops.
Worked example: Vietnam's Mekong Delta
The World Bank describes combined pressures on the Mekong Delta from groundwater pumping, sand mining and less sediment. Flood-based farming in the upper delta shows that responses must fit local water patterns. Keeping out all floodwater would lose useful seasonal floods.
Watch out for this
Rising seas are the only reason a delta sits lower relative to the sea.
The land itself can sink, and less sediment limits renewal.
Check your understanding
Why might an upstream dam affect fishers far downstream?
- It weakens the ocean tides at the river mouth.
- It changes the floods and sediment delta habitats need.
- It warms the river water that reaches the delta.
Show the answer
It changes the floods and sediment delta habitats need.
Yes. The whole basin is connected.