What happens to fish and plants when a river is polluted?

In one line

The rule

Pollution in rivers harms aquatic life by making the water unsafe for fish and plants to live, grow, and reproduce.

There is no single formula, but the main idea is: more pollution = more harm to living things in the river.

Let's understand it simply

Imagine you are living in a house where the air is full of smoke, the water is dirty, and there is garbage everywhere. You would find it hard to breathe, drink, or even eat. Fish and plants in a river feel the same way when the water is polluted. They need clean water to live, just like you need clean air and food. If the river gets dirty, it is like their home is being ruined. They can get sick, stop growing, or even die.

Think of a fish tank at home. If you keep dropping leftover food, soap, or oil into it, the water turns cloudy and smells bad. The fish stop swimming happily and may float upside down. Plants in the tank turn yellow or brown. This is what happens in a polluted river, but on a much bigger scale.

How it works

1. Factories, farms, and homes release waste into the river (chemicals, sewage, plastic, oil).
2. Pollutants (harmful substances) mix with the river water.
3. Fish and plants absorb these pollutants through their skin, gills, or roots.
4. Some pollutants use up the oxygen in the water, making it hard for fish to breathe.
5. Toxic chemicals can poison fish and plants, stopping them from growing or reproducing.
6. Over time, the river may lose its fish and plants, or only a few tough species survive.

The story behind it

People have used rivers for thousands of years for drinking, farming, and washing. But as cities and industries grew, more waste was dumped into rivers. Scientists noticed fish dying and plants disappearing in polluted rivers. In the 20th century, famous rivers like the Ganga and Yamuna in India, and the Thames in England, became so polluted that fish could not survive. This led to new rules and clean-up efforts.

One famous case is the Thames River in London. In the 1800s, it was so polluted that people called it "The Great Stink." Fish died, and the smell was unbearable. Only after cleaning efforts did fish and plants return.

Where you see it in real life

• Yamuna River, Delhi: The water is black and smells bad. Fish are rarely seen. Plants on the banks are covered in foam or die.
• Village pond after Holi: Colours and plastic waste make the water unsafe. Fish float dead, and plants rot.
• Ganga near Kanpur: Tannery waste (from leather factories) turns water brown. Fish populations drop, and water plants vanish.
• Monsoon drains: When drains carry sewage into rivers, the water becomes cloudy. Fish gasp for air at the surface.
• Plastic bags in rivers: Plants get covered and cannot get sunlight. Fish eat plastic bits and get sick.
• Algal bloom (green layer on water): Too much fertilizer causes algae to grow fast. They use up oxygen, and fish die.

Why it matters

• Food chain: Fish and plants are food for birds, animals, and humans. If they die, the whole food chain is broken.
• Clean water: Plants help clean water by absorbing waste. Without them, water gets dirtier.
• Jobs: Fishermen and farmers lose their income if rivers have no fish or healthy plants.
• Health: Polluted rivers spread diseases to people who use the water for drinking or bathing.

Worked example

Let’s say a factory releases waste with a chemical called mercury into a river:

Given: Mercury is toxic to fish at 0.002 mg/L
Step 1: Factory releases waste, river mercury level rises to 0.01 mg/L
Step 2: Fish absorb mercury through gills
Step 3: Mercury damages fish brains and organs
Step 4: Fish become weak, cannot swim or reproduce
Step 5: Fish die, plants may also absorb mercury and stop growing

Common mistakes

• Thinking only fish are affected: Plants, insects, birds, and even humans are harmed by river pollution.
• Believing polluted water looks clean: Some dangerous chemicals are invisible and have no smell.
• Assuming rivers "clean themselves" quickly: Some pollutants stay for years and keep harming life.
• Forgetting about small creatures: Tiny animals (like snails, insects) also die, breaking the food chain.

Remember it

If the river is dirty, fish and plants cannot live — clean water means healthy life.