Blue question mark

Why are so many lakes, streams, and ponds polluted these days?
— Ian Y., Grade 3

Ichthyologist Ian Harrison answers this question:

Hi Ian,

This is a good question. Although we have easy access to freshwater in most of the US, water is still a very important and very valuable resource. We should use it wisely, and manage it very carefully. 

Many freshwater lakes, streams, and ponds are polluted. The causes of pollution depend on many different factors. These factors vary in different parts of the world.

For example, in parts of the US, fertilizers and pesticides are two major sources of pollution. In many farms, chemicals are sprayed on crops to help them grow and to prevent bugs. When it rains, the extra fertilizers and pesticides flow into streams.

The pollutants can cause algae  to grow quickly. These 'blooms' of algae may produce toxins that harm other life in the river. When the algae die and decay, this uses up much of the oxygen in the stream. Without enough oxygen in the water, invertebrates  and fish will suffocate .

Algae bloom pond

Farming chemicals can cause algae to bloom. These microscopic organisms can take over a freshwater habitat like this pond.

Also, soil washed off from the fields into rivers can cover stream bottoms where invertebrates live. The soil can also clog the gills of fishes swimming in the river.

Further downstream, more and more pollutants get washed into the river and their harmful effects are even greater. In the Gulf of Mexico, there is a large area of water called the "Dead Zone." It is so low in oxygen that it can't support life at all! 

In some developing countries, a major problem is the building of large towns close to freshwater sources. The polluted water produced by homes and industries does not get properly treated. And this dirty water gets cycled back into the water system.

Gulf of Mexico

Sediments carrying chemical runoff from midwestern US flow into the Gulf of Mexico. This is one cause of the Dead Zone.

Water  isn't just used locally. We extract a lot of water from rivers and lakes around the world for our own needs. We use water for washing and drinking, to grow food, to generate energy , and for many other useful purposes. Since this leaves a smaller amount of water in these ecosystems, pollution will affect the ecosystem more.

Scientists are studying those areas that are at greatest threat. We hope to make the public more aware of the problems of water pollution so that local and national governments will develop better policies for water management.

girl drinking from water fountain

We all need clean water to survive!

As an individual, there are many things that you can do to help keep our water safe and clean. Check out Be a Water Saver below for ideas. 

I hope this answers your question. And I hope you'll continue to investigate how water is used in your local region. We can all help keep rivers and lakes clean, so that we can keep enjoying them for a long time to come.

You Can Make a Difference!

  • Be a Water Saver! You can help protect freshwater and the living things that depend on it.
  • Be an Ocean Helper! Find out what you can do to protect marine and freshwater organisms.


Explore More:

Ian Harrison

Ian Harrison

Job Title:
Curriculum Material Project Coordinator, Center for Biodiversity & Conservation

Known For:
Ian is an ichthyologist. He studies why freshwater fishes are threatened. He also researches how different groups of coastal fishes are related to each other.

Cool Fact:
Ian's research has taken him to several parts of the world — from a lake high in the Andes Mountains of South America (in pursuit of the legendary fat catfish) to the coral reefs of the remote Palmyra atoll in the middle of the Pacific Ocean!