The World's Foundations; or, Geology for Beginners · Ch. XXIII · Unit 4: Part III: Water at work today
26. Rivers
Goal: Your child can name the three jobs of running water, and show with a sand tray how water carves channels faster on steep slopes.
You'll need
- The book, open to this chapter
- A large baking tray, plastic storage box lid or a patch of bare garden
- Damp sand or sandy soil
- A watering can
- A brick or books to tilt the tray
- A notebook for a "rock journal" and a pencil
What's in the book
Chapter XXIII opens Part III. It describes how running water wears land away, carries it off and drops it, how streams cut deeper into their beds, how swirling stones drill pot-holes like the Earl of Mar's Punch-bowl, and examples of fast valley-cutting: a new stream in a Swiss vineyard, a gully in Georgia, the River Simeto through hard lava, and Niagara Falls moving backward.
The lesson, step by step
A question to start
SayWater is soft and rock is hard. Can water ever win?
Read to find out.
Read the chapter together
Read Chapter XXIII (pages 219 to 227). It begins with three jobs:
First, water wastes, breaks up, crumbles, or wears away land. Secondly, it carries the wasted material, whether rocks, stones, pebbles, sand, or soil, towards the ocean. Thirdly, it drops or deposits that material on the sea-bottom.
Erosion tray
Outdoors, pack damp sand into the tray in a smooth slope. Tilt it a little with a brick. Sprinkle water gently from the watering can at the top for a minute. Watch: little channels, sand carried down, a fan of sand dropped at the bottom.
Smooth it, tilt it more steeply, and pour the same amount. Which slope wore away faster? Then try a heavy pour versus a gentle sprinkle.
Steep water works fastest
Read the book's point that a stream on a nearly level plain does little damage, but rushing down a steep hillside it tears away earth fast. Your tray just showed it. Ask your child to point to the "carrying" and the "dropping" in the tray.
The punch-bowl
Explain the pot-hole: a boulder rocking on a riverbed makes a small dip; water swirls in the dip, spinning pebbles round and round, and over a long time they drill a deep round hole, even through the bottom of the "bowl". Ask your child to draw it in three stages.
Fast and slow
Talk about the book's examples: a stream that cut a vineyard in two in about eleven years, and the River Simeto that cut forty to fifty feet into hard lava in about two hundred years. Then read:
A river may therefore be said to have two beds — its narrow constant bed, and its wide occasional bed.
Tell it back
Ask your child to explain, using the tray, the three jobs of running water. Then: Why does the book say we cannot be sure how fast Niagara Falls moved back in the past?
What we know now
Niagara Falls has worn its gorge back about 7 miles since the end of the last Ice Age. Rivers really can cut through rock given enough time: the Grand Canyon was carved mainly by the Colorado River over the last several million years as the land around it slowly rose.
Afterwards
Notice it this week: after rain, look for little channels and fans of sand on paths, in flower beds or along curbs. These are tiny valleys and deltas.
Tip
Do the tray outside, or over a towel in the bathtub. Damp sand holds a shape better than dry.
Answers to the book's problems
Water wears land away, carries the material off, and drops it. We cannot be sure about Niagara because careful measurements are recent, and the rocks it has cut through in the past may have been harder or softer than today's.
Worked out for this site from the scan. If a number in your copy differs, trust the book.