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How Do Wetlands Filter Water? See for Yourself

Wetlands don’t necessarily look like the cleanest places. The water can look mucky with mud, plants and debris. Stuff is floating in them, and you can’t see the bottom like in a swimming pool. But all those layers of plants, roots, different types of soil, sand and more are acting like a filter, sifting out harmful things before they can get to our rivers, lakes and oceans.

Heavy rains can be very cleansing, but things that are washed off, like pesticides in lawns, roadway chemicals, garbage and other debris, have to go somewhere. The natural path is for them to eventually end up in waterways, but wetlands can help slow the flow.


  • Empty flowerpot

  • Deep dish or pan, large enough to hold the amount water in the pitcher

  • Coffee filter

  • Charcoal

  • Sand

  • Gravel or small stones

  • Pitcher of pond water


1. Set the flowerpot in the dish.

2. Fit the coffee filter into the flowerpot.

3. Fill the bottom third with sand.

4. Add about the same amount of charcoal and press it onto the sand.

5. Scoop gravel on top of the charcoal, leaving a half-inch to 1 inch empty on top.

6. Hold the colander over the flowerpot.

7. Pour the pond water through the colander and into the flowerpot.

8. Check the water in the pan. Does it look different?

You may notice that the water comes out through the bottom of our “wetland” slower than it’s poured in. That’s true in a real wetland too. Wetlands act like sponges, holding water.

Doesn’t the bad stuff it filters out hurt the wetland? Maybe, especially if there is an overabundance of harmful materials. But there are plants and other organisms living in wetlands that are adapted to survive in spite of those things, or even break them down into useful matter over time.

Try putting the materials in a different order. Does it change the flow or water quality?

What happens if you add some plants? Wait a few weeks so the roots have time to grow and try again.


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