Discovery Education Science P4: Term 2, Unit 4, pp. 135–148
GoalExplain how rain, streams and rivers carve valleys and canyons, and what shapes them.
New words
stream · a small flow of water over the landriver · a big stream that joins many streamscarve · to cut a shape out of a material, little by little
Explanation
When it rains on high ground, gravity pulls the water downhill. The water gathers into small streams. Streams join to make bigger streams, and these make a river. A bigger, faster river erodes more than a small, slow one. The river picks up pieces of rock and uses them to scrape the bed and the sides, so it cuts deeper and wider. Over a long time it carves a valley. If the river keeps cutting down through hard rock for millions of years, the walls stay steep and tall, and the valley becomes a canyon. In its walls we can see layers of rock like pages of a book. The shape of a valley depends on the kind of rock and on how fast, how big and how old the river is. The land beside the canyon is not cut as deeply, because only the river’s path is eroded.
rain falls on high ground→small streams→a river→a valley or a canyon
Worked examples
Statement
True or false?
bigger streams cause more erosion
true
a canyon has low, gentle walls
false
rivers can change land very slowly
true
Agree or disagree? Farida checks four statements. “The bigger the stream, the more erosion” is true. “A canyon is a kind of valley with steep sides” is true. “Canyon walls are low with gentle slopes” is false. “Rivers can change land very slowly” is true.
Years
Depth cut
1 000
3 m
2 000
6 m
3 000
9 m
Maths of a river. Suppose a river deepens its bed by 3 mm every year. In 1 000 years it cuts 3 × 1 000 = 3 000 mm, which is 3 m, because 1 000 mm make 1 m. In a million years it could cut a very deep canyon.
Stream tray. Omar pours water slowly down a sand-filled tray and gets a thin channel. Pouring twice as fast gives a wider, deeper channel and a bigger sandy pile at the bottom. Faster water erodes more.
Class activity
Stream tray (a grown-up is near). Tilt a tray of damp sand on a book. Pour a cup of water slowly from the top and draw the channel. Smooth the sand and pour a cup twice as fast. Keep the same tray, sand, slope and amount of water. What changed? Draw both channels side by side and label the sand pile at the bottom.
Practice
1
What pulls rain downhill into streams?
Gravity.
2
How is a canyon different from an ordinary valley?
A canyon has tall, steep walls and is narrower and deeper.
3
A river cuts 2 mm a year. How deep in 1 000 years?
2 × 1 000 = 2 000 mm = 2 m.
4
Why are the layers of rock easy to see in a canyon wall?
