Dear, Finn, Emily, and friends:

When a raindrop falls from a cloud, there are quite a few places it could end up.

We might follow that raindrop into a stream, river or ocean. If it’s in the ocean and it gets warm enough, it might evaporate into even tinier droplets of water to form clouds. It’s part of the water cycle.

Clouds can only hold so much rain before they get saturated. Then it starts to rain again. Maybe this time the raindrop falls into soil and helps a plant grow. Or perhaps the raindrop falls onto the sidewalk, street, or highway. If it falls on the pavement, it could flow into a drain and back into the streams and oceans.

Lots of these raindrops make up what scientists call storm water. But it’s not just water. Along the way, water can pick up other things on the road. It might sweep away something like leaves. It also picks up things that aren’t very good for our planet, like oil, animal waste, metal from car brakes, or other kinds of chemicals and pollutants.

We can’t always see these pollutants with our eyes, but they can really threaten animals who call the streams, rivers, and ocean home. Pollution can create a toxic environment for fish. As a researcher at Washington State University, my friend Michelle Chow studies some of the fish that get sick from pollutants in storm water.

Helping save the salmon

Coho salmon that live in the Pacific Northwest of the U.S. will often die if they are in this polluted water for a couple of days. But Chow and other researchers at WSU are working on ways to help save the salmon.

It turns out soil is really great at filtering toxic stuff out of water. If you are curious how it works, check out this video, “Polluted Puddles.” One thing people can do to help clean up the environment and help save salmon is to plant a rain garden. When the rain comes down, it gets filtered through the garden’s soil instead of running off into the road. You can find out how to plant a rain garden in your community with the help of friends at WSU Extension.

Another way researchers are helping is by developing a kind of pavement that looks a bit like a Rice Krispies Treat. It’s called permeable pavement and is designed to let water go straight through the pavement down into the soil. That way, it doesn’t run off and carry pollutants to nearby bodies of water.

Chow said there are other ways we can also help keep pollutants out of our environment. Can you think of something you might be able to do? I might just walk to work or ride my bike. Helping find solutions is good for salmon who live in the water and good for animals who like to eat them, too. Together, we can help improve and restore healthy water habitats.

Dr. Universe



  • Ask Dr. Universe connects K-8 students with researchers at Washington State University through Q&A. Students can submit science questions on the ASK page.
  • Are you a teacher, parent, or curious grown-up? Follow along on Twitter or Facebook.
  • Do you want to reprint this Q&A? Just send a message to