---
title: "Water Shapes the Land"
book: "Primary & Middle School Earth and Space Sciences"
subject: earth
language: en
chapter: 10
exercises: 11
source: https://one-course.com/books/earth/1/en/chapter/10-water-shapes-the-land
license: CC-BY-NC-SA-4.0
credit: "One Earth and Space Sciences Book, One Course (one-course.com)"
---

# Chapter 10 — Water Shapes the Land

All night long, the storm drummed on the roof of the school. In the morning the sky is clearing, and Lena and Tom run to the little stream behind the playground. Yesterday it was clear and quiet: they could count the [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) on its bed. Today it is wide, fast and brown, the colour of milky chocolate. It carries twigs, leaves, and something they cannot see one by one: [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), torn from its banks and from the fields upstream.

“Where is all that [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) going?” asks Tom. Downstream, a little further every minute. The stream is carrying the land away, grain by grain. In this chapter we follow those grains, from the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) they come from to the place where they settle.

![A stream the morning after a storm: the water is brown with the mud and sand it carries.](https://one-course.com/images/onecourse/chapters/earth-1/g5-water-shapes-the-land/img-78e73fd7dee5.jpg)

*A stream the morning after a storm: the water is brown with the [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) it carries.*

**You already know.**

[Rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) is the hard material of the Earth; broken into pieces it gives stones, [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) rounded by water, [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) whose grains we can see one by one, and [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) whose grains are too small to see ([Chapter 5](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#ch-g3-stones-pebbles-sand)). A [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) flows all the time in a long channel, from higher land down to a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) or to the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) ([Chapter 2](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#ch-g1-landscapes-around-us)). [Rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) that falls on the land runs off into streams and [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), which bring the water back to the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) ([Chapter 3](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#ch-g2-water-on-earth)).

## 10.1 Rock breaks where it stands

Even the hardest [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) does not last for ever. Where it lies in the open, under the sun, the [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) and the frost, it slowly cracks and crumbles.

**Definition 10.1 (Weathering).**

*Weathering* is the slow breaking-up of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) where it stands, by frost, by water, by the roots of plants and by heat and cold. The [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) is broken into pieces, but the pieces stay in place.

Frost is one of the strongest tools of [weathering](#def-g5-water-shapes-the-land-weathering). [Rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) water seeps into a thin crack in the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). On a cold night it freezes, and when water freezes it takes up a little more room: the ice pushes on the sides of the crack and opens it a tiny bit wider. The next day the ice melts, more water seeps in, and the next night it freezes again. After many winters, a piece of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) breaks off.

![Frost breaks rock. Each night of frost, the ice in a crack pushes a little; after many winters, the crack is wide enough for a piece to break away.](https://one-course.com/images/onecourse/chapters/earth-1/g5-water-shapes-the-land/fig-33071164c8d2.svg)

*Frost breaks [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). Each night of frost, the ice in a crack pushes a little; after many winters, the crack is wide enough for a piece to break away.*

Below a cliff of bare [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) high in the [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), the pieces broken off by the frost slide and tumble down, and pile up in a slope of sharp stones. They are not rounded: no water has rolled them yet.

![Below a mountain cliff, a slope of sharp-edged stones broken off by frost, seen from across the valley.](https://one-course.com/images/onecourse/chapters/earth-1/g5-water-shapes-the-land/img-ff6e53f1cc53.jpg)

*Below a [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) cliff, a slope of sharp-edged stones broken off by frost, seen from across the [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform).*

Plants help too. A tree root slips into a crack and, as it grows thicker year after year, it splits the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) like a slow wedge. [Rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) water itself slowly softens some [rocks](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) and turns them into clay and [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand): that is how the soil gets its mineral grains.

## 10.2 Water carries the pieces away

Once the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) is broken, the pieces can move. Something must carry them.

**Definition 10.2 (Erosion).**

*Erosion* is the wearing-away of the land and the carrying-away of the pieces of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock), by running water above all, but also by the wind, by ice and by the waves of the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean).

Running water does most of the work. The [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) washes soil and grains down the slopes into the streams; the streams tear more from their own banks, the way Tom and Lena’s stream did after the storm. The faster the water flows, the more it can carry, and the bigger the pieces it can move: a calm stream carries only fine [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand); a stream in [flood](#def-g5-water-shapes-the-land-flood) rolls [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) and even boulders along its bed.

On their way, the stones knock against each other and against the bed. Their corners break off, and they become rounded [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble), then smaller and smaller grains: [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand).

The wind does the same work in dry places, where nothing holds the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand): it blows it into dunes. Ice, in the [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) and near the [poles](https://one-course.com/books/earth/1/en/chapter/6-the-globe-continents-and-oceans#def-g3-continents-and-oceans-poles), drags stones along under its great weight. On the [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), waves pound the cliffs and roll the [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) of the beach back and forth.

**Remark 10.3 (How much?).**

Together, all the [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of the world bring more than 12 billion tonnes of [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) to the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) every year. That is about a billion tonnes every month.

## 10.3 Water lays the pieces down

A [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) cannot carry its load for ever. Where it slows down, it no longer has the strength to move its biggest grains, and they settle on the bed.

**Definition 10.4 (Deposition).**

*Deposition* is the laying-down of the pieces of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) carried by water, wind or ice, where they slow down and stop.

The heaviest pieces settle first: [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) drop where the current slackens a little, [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) where it slows more, and the fine [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) only where the water is almost still, in a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) or in the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean). That is why a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water)’s bed is pebbly in the [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) and sandy or muddy near the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean).

[Weathering](#def-g5-water-shapes-the-land-weathering), [erosion](#def-g5-water-shapes-the-land-erosion) and [deposition](#def-g5-water-shapes-the-land-deposition) are three steps of one long journey: the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) is broken where it stands, its pieces are carried away, and they are laid down somewhere else. Slowly, water moves the [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), grain by grain, down to the [plains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) and into the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean).

**Method 10.5 (Naming what water does).**

To name what is happening to [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) in a picture or a story, ask three questions:

1. Is the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) being broken, but the pieces stay where they are? That is *[weathering](#def-g5-water-shapes-the-land-weathering)* .
2. Are the pieces being worn away and carried off? That is *[erosion](#def-g5-water-shapes-the-land-erosion)* .
3. Are the pieces settling down and piling up? That is *[deposition](#def-g5-water-shapes-the-land-deposition)* .

**Example 10.6 (The stream after the storm).**

The brown water of Tom and Lena’s stream carries [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) torn from its banks: that is [erosion](#def-g5-water-shapes-the-land-erosion). A week later, the stream is clear again, and a thin layer of [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) covers the bottom of the calm pool at the end of the playground: that is [deposition](#def-g5-water-shapes-the-land-deposition).

## 10.4 A river from its source to its mouth

Every [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) has a beginning and an end.

**Definition 10.7 (Source, mouth and tributary).**

The *source* of a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) is the place where it begins: a spring, a melting glacier, or a high, wet land where many tiny streams start. Its *mouth* is the place where it ends, flowing into the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) or a bigger [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). A *tributary* is a smaller [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) that flows into a bigger one.

Along its way, a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) changes. We split its length into three parts, its upper, middle and lower course.

![The long profile of a river, from its source to its mouth (the heights are much stretched). Below: what lies on its bed.](https://one-course.com/images/onecourse/chapters/earth-1/g5-water-shapes-the-land/fig-ab6d2b826dea.svg)

*The long profile of a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), from its [source](#def-g5-water-shapes-the-land-source-mouth) to its [mouth](#def-g5-water-shapes-the-land-source-mouth) (the heights are much stretched). Below: what lies on its bed.*

In its *upper course*, in the [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) is young and steep. Its water rushes down, rolls boulders along its bed and cuts downwards into the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock). It digs a narrow [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) with steep sides, shaped like the letter V.

In its *middle course*, the slope is gentler. The [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) swings from side to side in great loops.

**Definition 10.8 (Meander).**

A *meander* is a loop of a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) that winds across a flat [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) floor.

A [meander](#def-g5-water-shapes-the-land-meander) does not stay still: it moves. On the outside of each bend, the water flows fast and deep: it eats into the bank, which becomes a small cliff. On the inside of the bend, the water flows slowly: it drops [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble), which build a gentle beach. Year after year, the outer bank is worn back and the inner beach grows, and the loop swings wider and wider.

![A meander seen from above, and cut across along the dashed line A–B. The water wears away the outer bank of each bend and drops sand on the inner bank: the loop slowly moves.](https://one-course.com/images/onecourse/chapters/earth-1/g5-water-shapes-the-land/fig-53a59355d89d.svg)

*A [meander](#def-g5-water-shapes-the-land-meander) seen from above, and cut across along the dashed line A–B. The water wears away the outer bank of each bend and drops [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) on the inner bank: the loop slowly moves.*

Sometimes two loops grow so close that, during a [flood](#def-g5-water-shapes-the-land-flood), the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) breaks through the narrow neck of land between them and takes the short cut. The old loop is left behind as a curved [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water).

![The Mamoré River, in the lowlands of Bolivia in South America, seen from the International Space Station. The brown river winds in meanders through the forest. Photograph: NASA (public domain).](https://one-course.com/images/onecourse/chapters/earth-1/g5-water-shapes-the-land/img-54aaf225b2a1.jpg)

*The Mamoré [River](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), in the lowlands of Bolivia in South America, seen from the International Space Station. The brown [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) winds in [meanders](#def-g5-water-shapes-the-land-meander) through the forest. Photograph: NASA (public domain).*

In its *lower course*, the land is almost flat. The [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) flows slowly across a wide, flat [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) floor, made of the [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) it has laid down itself over a very long time.

![The valley cut across, in the mountains and near the sea (not to the same scale). In flood, the river spreads over its whole floodplain.](https://one-course.com/images/onecourse/chapters/earth-1/g5-water-shapes-the-land/fig-6cca1eff1016.svg)

*The [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) cut across, in the [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) and near the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) (not to the same scale). In [flood](#def-g5-water-shapes-the-land-flood), the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) spreads over its whole [floodplain](#def-g5-water-shapes-the-land-flood).*

At last the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) reaches its [mouth](#def-g5-water-shapes-the-land-source-mouth). There the water slows down almost completely, and the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) drops the last of its load. Where the waves and the tides of the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) are too weak to carry all of it away, the [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) pile up, and new land grows out into the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean).

**Definition 10.9 (Delta).**

A *delta* is new, flat land built by a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) at its [mouth](#def-g5-water-shapes-the-land-source-mouth) from the [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) it lays down. The [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) splits into several branches that spread across it, often in the shape of a fan.

The name comes from the Greek letter $\Delta$ ([delta](#def-g5-water-shapes-the-land-delta)), a triangle, because the [delta](#def-g5-water-shapes-the-land-delta) of the Nile, in Egypt, has that shape.

Some [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) end without a [delta](#def-g5-water-shapes-the-land-delta). Where strong waves and tides sweep the [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) away, the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) ends in a wide, funnel-shaped [mouth](#def-g5-water-shapes-the-land-source-mouth) where the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) comes in and out with the tides: an *estuary*.

![Left: a delta seen from above; the mud and sand laid down by the branches build new land beyond the old coast. Right: the Nile delta, in Egypt, from a satellite (sea at the top): a green fan of fields on the river’s mud, in the middle of the desert. Photograph: NASA (public domain).](https://one-course.com/images/onecourse/chapters/earth-1/g5-water-shapes-the-land/fig-0b178878e579.svg)

![Left: a delta seen from above; the mud and sand laid down by the branches build new land beyond the old coast. Right: the Nile delta, in Egypt, from a satellite (sea at the top): a green fan of fields on the river’s mud, in the middle of the desert. Photograph: NASA (public domain).](https://one-course.com/images/onecourse/chapters/earth-1/g5-water-shapes-the-land/img-2920e40f2178.jpg)

*Left: a [delta](#def-g5-water-shapes-the-land-delta) seen from above; the [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) laid down by the branches build new land beyond the old [coast](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). Right: the Nile [delta](#def-g5-water-shapes-the-land-delta), in Egypt, from a satellite ([sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) at the top): a green fan of fields on the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water)’s [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), in the middle of the desert. Photograph: NASA (public domain).*

## 10.5 Floods

After long days of [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation), or when the [snow](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation) melts fast in spring, a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) receives more water than its channel can hold.

**Definition 10.10 (Flood and floodplain).**

A *flood* happens when water covers land that is usually dry. A [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) floods when it rises above its banks and spreads over the flat land on either side of it. That flat land, which the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) covers from time to time, is its *floodplain*.

![A meadow under flood water after days of rain. The line of trees marks the river’s usual bank; the farm stands on higher ground.](https://one-course.com/images/onecourse/chapters/earth-1/g5-water-shapes-the-land/img-c687f72aec22.jpg)

*A meadow under [flood](#def-g5-water-shapes-the-land-flood) water after days of [rain](https://one-course.com/books/earth/1/en/chapter/7-clouds-rain-and-snow#def-g4-clouds-rain-snow-precipitation). The line of trees marks the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water)’s usual bank; the farm stands on higher ground.*

A [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) that [floods](#def-g5-water-shapes-the-land-flood) is not doing anything strange: it is doing what [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) have always done. The [floodplain](#def-g5-water-shapes-the-land-flood) is its territory. In fact the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) built it: each [flood](#def-g5-water-shapes-the-land-flood) leaves behind a thin layer of [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) on the fields, which is why [floodplains](#def-g5-water-shapes-the-land-flood) have such rich soil. Long ago, farmers in Egypt waited for the yearly [flood](#def-g5-water-shapes-the-land-flood) of the Nile, which watered their fields and brought them new [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand).

But a [flood](#def-g5-water-shapes-the-land-flood) is dangerous for people who live on the [floodplain](#def-g5-water-shapes-the-land-flood). Houses built on it can be filled with water, and fast [flood](#def-g5-water-shapes-the-land-flood) water can sweep away cars and people. A wise town builds on higher ground and leaves the [floodplain](#def-g5-water-shapes-the-land-flood) to fields, meadows and parks. In [Chapter 12](https://one-course.com/books/earth/1/en/chapter/12-natural-hazards#ch-g5-natural-hazards) we will see how people prepare for [floods](#def-g5-water-shapes-the-land-flood) and stay safe.

**Safety.**

Never walk, cycle or play in [flood](#def-g5-water-shapes-the-land-flood) water, and never go near a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) in [flood](#def-g5-water-shapes-the-land-flood): the water is stronger and deeper than it looks, and it can hide holes and open drains. Keep away until the grown-ups in charge say it is safe.

## 10.6 Exercises

**Exercise 10.1 ★.**

Name the process: [weathering](#def-g5-water-shapes-the-land-weathering), [erosion](#def-g5-water-shapes-the-land-erosion) or [deposition](#def-g5-water-shapes-the-land-deposition)?

1. [(a)] Ice in a crack splits a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) on a [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) .
2. [(b)] A [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) in [flood](#def-g5-water-shapes-the-land-flood) carries [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) away from its banks.
3. [(c)] A layer of [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) settles at the bottom of a calm [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) .
4. [(d)] A tree root splits a stone wall.

**Solution of Exercise 10.1.**

(a) [Weathering](#def-g5-water-shapes-the-land-weathering): the [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) is broken where it stands. (b) [Erosion](#def-g5-water-shapes-the-land-erosion): the [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) is worn away and carried off. (c) [Deposition](#def-g5-water-shapes-the-land-deposition): the [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) settles. (d) [Weathering](#def-g5-water-shapes-the-land-weathering): the root breaks the stone where it stands.

**Exercise 10.2 ★.**

What is the source of a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water)? What is its [mouth](#def-g5-water-shapes-the-land-source-mouth)? What is a [tributary](#def-g5-water-shapes-the-land-source-mouth)?

**Solution of Exercise 10.2.**

The [source](#def-g5-water-shapes-the-land-source-mouth) is the place where the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) begins (a spring, a melting glacier, a high wet land). The [mouth](#def-g5-water-shapes-the-land-source-mouth) is the place where it ends, in the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) or a bigger [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). A [tributary](#def-g5-water-shapes-the-land-source-mouth) is a smaller [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) that flows into a bigger one.

**Exercise 10.3 ★.**

Put these parts of a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) in order, from the [source](#def-g5-water-shapes-the-land-source-mouth) to the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean): the [meanders](#def-g5-water-shapes-the-land-meander); the [delta](#def-g5-water-shapes-the-land-delta); the narrow V-shaped [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform); the [source](#def-g5-water-shapes-the-land-source-mouth).

**Solution of Exercise 10.3.**

The [source](#def-g5-water-shapes-the-land-source-mouth); the narrow V-shaped [valley](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform); the [meanders](#def-g5-water-shapes-the-land-meander); the [delta](#def-g5-water-shapes-the-land-delta).

**Exercise 10.4 ★.**

True or false? Explain.

1. [(a)] Hard [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) never changes.
2. [(b)] A fast [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) can carry bigger stones than a slow [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) .
3. [(c)] The finest [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) settles first, near the [source](#def-g5-water-shapes-the-land-source-mouth) .

**Solution of Exercise 10.4.**

(a) False: frost, water, roots, heat and cold slowly break even hard [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) ([weathering](#def-g5-water-shapes-the-land-weathering)). (b) True: the faster the water, the bigger the pieces it can move. (c) False: the heaviest pieces settle first; the finest [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) settles last, where the water is almost still, in a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) or in the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean).

**Exercise 10.5 ★.**

Here is a bend of a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) seen from above.

![](https://one-course.com/images/onecourse/chapters/earth-1/g5-water-shapes-the-land/fig-093583096f09.svg)

On which bank, A or B, does the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) wear the land away? On which bank does it drop [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand)?

**Solution of Exercise 10.5.**

A is on the outside of the bend: the water there is fast and deep, and it wears the bank away ([erosion](#def-g5-water-shapes-the-land-erosion)). B is on the inside of the bend: the water there is slow, and it drops [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) ([deposition](#def-g5-water-shapes-the-land-deposition)).

**Exercise 10.6 ★.**

In the [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), the bed of a [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) is covered with large stones. Near the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean), it is covered with [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand). Why?

**Solution of Exercise 10.6.**

In the [mountains](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform) the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) is steep and fast: it can roll large stones along, and it carries the small grains away. Near the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) is slow: it can no longer move large stones, only [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand), and it drops them there. On the way, too, the stones have been broken and worn into smaller grains.

**Exercise 10.7 ★★.**

A [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) is 600 kilometres long. Its upper course is the first $\frac{1}{6}$ of its length, and its middle course the next $\frac{1}{2}$.

1. [(a)] How long is the upper course? the middle course?
2. [(b)] How long is the lower course? What fraction of the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) is it?

**Solution of Exercise 10.7.**

(a) Upper course: $600 \div 6 = 100$ kilometres. Middle course: $600 \div 2 = 300$ kilometres. (b) Lower course: $600 - 100 - 300 = 200$ kilometres. Since $3 \times 200 = 600$, it is $\frac{1}{3}$ of the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water). (Check: $\frac{1}{6} + \frac{1}{2} + \frac{1}{3} = \frac{1}{6} +
\frac{3}{6} + \frac{2}{6} = \frac{6}{6}$.)

**Exercise 10.8 ★★.**

Look again at the photograph of the Mamoré [River](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) seen from the space station, on page [64](#fig-g5-water-shapes-the-land-mamore). What are the pale patches on the inside of the bends? What are the dark curved [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) beside the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water)? How were they made?

**Solution of Exercise 10.8.**

The pale patches are [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [gravel](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) laid down on the inside of the bends, where the water is slow ([deposition](#def-g5-water-shapes-the-land-deposition)). The dark curved [lakes](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) are old loops of the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water): two loops grew so close that the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) broke through the neck of land between them, took the short cut, and left the old loop behind as a [lake](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water).

**Exercise 10.9 ★★.**

A town wants to build a new school in the flat green meadows beside its [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water), because the land there is flat and cheap. What would you tell the town council? Use the words *[flood](#def-g5-water-shapes-the-land-flood)* and *[floodplain](#def-g5-water-shapes-the-land-flood)*.

**Solution of Exercise 10.9.**

The flat meadows beside the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) are its [floodplain](#def-g5-water-shapes-the-land-flood): the land the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) covers when it [floods](#def-g5-water-shapes-the-land-flood). They are flat precisely because the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) built them from its own [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand). One day the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) will [flood](#def-g5-water-shapes-the-land-flood) again and fill the school with water, and fast [flood](#def-g5-water-shapes-the-land-flood) water is dangerous. The school should be built on higher ground, and the meadows kept as fields, meadows or a park.

**Exercise 10.10 ★★.**

The [rivers](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) of the world bring about 12 billion tonnes of [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) to the [sea](https://one-course.com/books/earth/1/en/chapter/3-water-on-earth#def-g2-water-on-earth-ocean) every year.

1. [(a)] How many tonnes is that every month, on average?
2. [(b)] A big lorry carries 10 tonnes. How many lorry-loads would it take to carry one month’s [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) ?

**Solution of Exercise 10.10.**

(a) A year has 12 months: $12$ billion $\div\, 12 = 1$ billion tonnes a month on average. (b) One billion is $1\,000\,000\,000$; divided by 10, it is $100\,000\,000$: a hundred million lorry-loads.

**Exercise 10.11 ★★★.**

Tell the story of a grain of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand). It starts as part of a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) high on a [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform), and it ends, many years later, in a [delta](#def-g5-water-shapes-the-land-delta). Use the words *[weathering](#def-g5-water-shapes-the-land-weathering)*, *[erosion](#def-g5-water-shapes-the-land-erosion)* and *[deposition](#def-g5-water-shapes-the-land-deposition)*, and say what happens to the grain in the upper, middle and lower course of the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water).

**Solution of Exercise 10.11.**

One possible story. The grain is part of a [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) high on a [mountain](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-landform). Frost opens a crack around it, night after night, until a piece of [rock](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-rock) breaks off: that is [weathering](#def-g5-water-shapes-the-land-weathering). The piece falls to the stream below. In the upper course the water is fast and steep: it rolls the piece along ([erosion](#def-g5-water-shapes-the-land-erosion)), and knocking against other stones it breaks into [pebbles](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-pebble) and then into grains of [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand). In the middle course the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) slows; the grain rests for a while on the sandy beach inside a [meander](#def-g5-water-shapes-the-land-meander) bend, until a [flood](#def-g5-water-shapes-the-land-flood) picks it up again. In the lower course the [river](https://one-course.com/books/earth/1/en/chapter/2-landscapes-around-us#def-g1-landscapes-around-us-water) is slow and wide; the grain travels with the water to the [mouth](#def-g5-water-shapes-the-land-source-mouth). There the water almost stops, and the grain settles with the [mud](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) and [sand](https://one-course.com/books/earth/1/en/chapter/5-stones-pebbles-and-sand#def-g3-stones-pebbles-sand-sand) of the [delta](#def-g5-water-shapes-the-land-delta): that is [deposition](#def-g5-water-shapes-the-land-deposition).