What it shows
When a river reaches the sea it slows down suddenly and drops the sediment it was carrying. The sediment builds a fan of land called a delta that splits the river into many channels, which is how the Ganga-Brahmaputra and Godavari deltas formed.
The science
A river's power to carry grains depends on its speed. In the still sea the speed falls almost to zero and the coarse sand drops first, then finer silt further out. The dropped load piles up at the mouth, blocks the channel, and the river splits round it into distributaries. Each new channel builds its own lobe, and the triangular fan grows seawards year by year.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A long tray, raised at one end
- Sand and a little soil to make it muddy
- A clear plastic box at the low end filled with water as the sea
- A jug of water
- A straw to make the channel
- A ruler
Safety
This project uses safe, everyday materials. Still, read the rules below with a parent before you start.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Pack damp sand and soil in the raised part of the tray and make a straight channel with the straw to the edge of the water box.
Pour water slowly and steadily into the top of the channel from the jug.
Watch the water turn muddy as it picks up grains and then clear as it enters the still box.
Mark on the box side where the first pile of sand builds up.
Keep pouring and watch the channel split into two or three around the growing pile.
Measure how far the delta has grown into the box after each jug.
Stop, let the water settle and look at where coarse and fine grains ended up.
Draw the delta from above and label the distributaries and the sea.
What you should see
A fan-shaped deposit that grows a few centimetres into the box with every jug, with the channel splitting into distributaries and finer mud settling furthest out.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why does the river drop its load at the sea?
It slows down and can no longer carry the grains.
2What is a distributary?
One of the channels a river splits into on its delta.
3Name the largest delta in the world.
The Ganga-Brahmaputra delta in Bangladesh and West Bengal.
4Why is delta soil good for farming?
It is fine, fresh and rich in minerals brought from upstream.
Take it further
Add a little salt to the "sea" and compare how quickly the mud settles; clay clumps faster in salt water.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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