NCERT Solutions for Class 5th The World Around Us Chapter 2 Two pictures of a river — Activity 4

Book page 30 Updated on2026-09-19

Q1.

Look at the two images given below. Which river looks healthy and why?

Answer

The river in the first picture — the one labelled ‘River with clean water’ — is the healthy one.

Compare the two pictures point by point.

What to look at	Picture 1 — clean river	Picture 2 — ‘green blanket’
The water surface	Open blue water, and you can see ripples on it	Almost fully covered with a thick green mat; only a few small patches of water show through
Colour of the water	Deep blue and clear	Green all over, like a lawn laid on the river
Can sunlight reach inside?	Yes	No — the green cover blocks it
Can air reach the water?	Yes, the whole surface is open to the air	Hardly at all — the surface is sealed by plants
The banks	Green trees on both sides	Trees are there, but the river itself has disappeared under green
Healthy?	Yes	No

Why open water means a healthy river: Fish and other water animals breathe air that is dissolved in the water. That air enters through the open surface. Water plants inside the river also need sunlight. When the surface is open, air and sunlight both get in and life below stays healthy. When a thick green mat covers everything, both are shut out — so the second river is in trouble even though green usually looks like a healthy colour.

Q2.

How do so many plants grow in water?

Answer

Because fertiliser from the fields gets washed into the river, and fertiliser is plant food. With so much extra food, the water plants grow much faster than normal and multiply until they cover the surface.

Step by step, this is how it happens:

A farmer spreads fertiliser on the field to help the crop grow.
Rain falls, or the field is watered.
Some of the fertiliser dissolves in that water and runs off the field into a drain or a stream.
The stream carries it into the river.
The tiny plants already living in the river now get an enormous amount of food.
They grow very fast and spread until they cover the whole surface — the 'green blanket' the chapter talks about.

Why extra food is a problem, not a gift: A little fertiliser is good for a crop, because the crop is harvested and taken away. In a river nothing takes the plants away. They just keep growing on top of each other. Dirty drain water and soapy water do the same thing, because they also carry plant food into the river.

How to stop it: use only as much fertiliser as the crop really needs; leave a strip of grass or trees between a field and a river to catch the run-off; and clean drain water before it enters a river.

Q3.

What do you think happens when too many plants grow in a river?

Answer

The green cover slowly chokes all the life inside the river. Fish find it hard to breathe, and the water becomes unsafe for people to drink. That is exactly what the chapter says on page 30.

Here is the chain of events:

The plants spread over the whole surface like a thick blanket.
Sunlight can no longer reach the water below, so the plants growing under the water die.
Air from above cannot mix into the water, because the surface is sealed.
The dead plants rot, and rotting uses up whatever air is left in the water.
Fish and other water animals cannot breathe. Many die.
The water starts to smell bad and turns unfit to drink.
Boats cannot move easily, and mosquitoes breed in the still water under the mat.

The same river with an open surface and with a green blanket over it.

Why “choking” is the right word: Everything alive in the water needs air, just as you do. The plants on top act like a lid on a box. Nothing can come in, so everything below slowly runs out of air. The river is not killed by a poison here — it is smothered.

What can be done: stop the fertiliser and drain water from entering; remove the weed mat regularly; and keep some part of the surface always open, so air can get in.

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