In the dark about physics? Perhaps the shadow knows

Dr. Wendy Sue Universe

Dr. Universe,

Why does water in ponds not get soaked up by the dirt at the bottom?

— Rocky, 11, New Jersey

That’s a great observation. If we investigated the bottom of a pond, we might find a few different things. Besides a few fishes and frogs swimming around, we might observe mud, algae, rocks and soil at the bottom.

My friend Joan Wu, a hydrologist at Washington State University, is really curious about the water on our planet. She told me a few different earth materials help keep pond water from seeping down into the ground.

Let’s imagine we filled a jar with one of these earth materials: rocks. Inside the jar, we would see some gaps between the rocks. If we poured water into the jar, the water will be able to move into those empty spaces. But now let’s say we had a jar of rocks, and we poured in some sand.

This time, the grains of sand would fill spaces between the rocks. Next, we could add particles of earth called silt that are so small they could fill in any spaces between the grains of sand.

Finally, we could add some even smaller particles of clay. In the jar — or the bottom of a pond — these materials are packed together. The material isn’t very permeable, which means it can keep the liquid from passing through it.

“Over a long, long time, the bottom of the pond itself evolves and changes,” Wu said. “The materials settle and the little particles, or sediments, fill in the large pores.”

As water, wind, gravity and even animals break down rocks, the rocks become smaller and smaller particles that sink to the bottom of the water. When water runs across the Earth’s surface during a storm or as snow melts, these fine materials also can end up in a pond.

For the most part, these materials keep the pond from losing too much water, but sometimes a little does escape into the ground. Meanwhile, a little water also can escape into the air.

“Eventually, you will lose water from the top and from the bottom of a pond,” Wu said.

We lose the water from the top of a pond because of something called evaporation. You may know about evaporation if you’ve ever seen a puddle on a sidewalk that was there one day and gone the next.

When the sun heats up the surface of water, the water can turn from a liquid into teeny tiny drops called vapor. The vapor rises up into the atmosphere, where it can eventually become clouds. Those clouds help produce rain and snow that fall back into lakes, rivers and ponds.

When we take the time to look, we can find a lot of connections between our atmosphere, water and earth. These systems shape many habitats for life on our planet.

The next time you visit a pond, see what kinds of living things call it home. Who knows, maybe one day you’ll be a scientist who can help us learn more about the world of water.

Have a science question? Ask Dr. Wendy Sue Universe, WSU’s resident science cat and writer, by email at

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