A watershed on the map
A watershed is all the land whose water drains to the same place, such as a river or a lake. Rain falls on the hills and runs into small streams. These small streams are tributaries, and they flow into a bigger river, which flows on to a lake or the sea. Because all the water bodies are linked, what happens upstream affects the places downstream. A map with arrows helps us to follow the flow and to predict. If dirty water enters a tributary, it will reach the river and the sea, but it will not reach another tributary that joins the river further downstream. The Nile has a huge watershed that begins far to the south of Egypt. A map cannot tell us everything: it does not show how much pollution there is, how fast it moves, or what happens in groundwater.
| Event | Affected | Not affected |
|---|---|---|
| a factory pollutes stream C | C, E, F | D |
| litter is blown into stream D | D, E, F | C |
| litter is dropped into river E | E, F | C, D (they are upstream) |
| A map can show | A map cannot show |
|---|---|
| which way water flows | how much pollution arrives |
| which places are downstream | how fast it travels |
| where tributaries join | what happens underground |
Colour the flow (a grown-up helps). Draw your own watershed on paper with two hills, two streams, a river and a sea. Choose a colour for each of three events, such as litter in one stream. Follow the water with the colour and mark every body of water affected. Swap maps with a friend, and predict their results before you check.