Key concepts
Water
Gravity
Erosion

From National Science Education Standards: Structure of the Earth system           

Introduction
How do babbling brooks and mighty rivers come to be? They both depend on two things: water and gravity. Although they might look flat, rivers and streams are always flowing slightly downhill. Eventually, most of the water dumps into lakes, seas or the ocean.

But what determines the size and shape of a river? Can a small stream turn into a rushing river over time? Get ready to watch this amazing and powerful process happen in record time.

Background
As water flows downhill, it often washes dirt, rock and other material away with it. This process is called erosion, and it can happen when soil gets washed into a stream after a rainstorm—and over millions of years as water slowly wears down rock to create canyons. Even the tremendous Grand Canyon was formed in large part by the Colorado River wearing down the land over millions of years.

But we can watch the process happen in just a few minutes.

Materials
•    Large, shallow rectangular pan or tray with edges (the plastic lid of a storage container will work, too)
•    Disposable plastic water bottle
•    Scissors
•    Flour or another light powder
•    Different color powder, such as cocoa powder, to represent "topsoil"
•    Small round snacks, such as seeds, to represent "rocks"
•    Medium-sized round snacks, such as small strawberries or dried figs, to be "boulders"
•    Longer snacks, such as carrot sticks, to represent obstacles
•    Cereal boxes (or other thin objects that can be used to prop up one side of the tray)
•    Sink (or other larger container to catch any excess water runoff)

Preparation
•    Pour an inch or two of flour or other powder evenly into the pan or tray.
•    Place your pan or tray with a short edge in the sink or container (to catch any extra water or flour runoff).
•    Prop up the other side by laying a cereal box on its side so that the bed of flour is slanted slightly toward the sink. Here you have a model of a hill!
•    Carefully poke a small hole in the full water bottle near the top (so you don't spill any water), and keep the cap on. This will be your water source, acting like a natural spring or rain.
•    What do you think is going to happen when you start dripping water on the top of the powdery hill?
•    Note: If at any point your powdery hill gets too mucky from the water, you can always toss the powder and start the activity again where you left off.

Procedure
•    Hold the water bottle at the center of the top end of the flour slope. Carefully tilt it so that water starts to drip out of the small hole you made.
•    Where is the water going? How is it getting there? Is it changing the surface on the way down?
•    Stop pouring the water.
•    Sprinkle some of the cocoa powder in and around the water's path—this represents topsoil.
•    Start pouring water into the stream again. What happens to the "soil" near the stream?
•    Now scatter a few small "rocks" (seeds) along the water's path, and pour some more water down the hill. What happens to the seeds? Do they affect the water?
•    Now place some big "boulders" (dried figs, strawberries, and so on) in the water's path. What happens to the flow of the water?
•    Stop pouring the water. What does the surface of the flour look like?
•    Finally, try placing some larger obstructions, such as carrot sticks, in the way of the stream. How do you think these will change the water flow?
•    Start pouring the water again. What happened to the stream?
•    If you have a big-enough tray, try propping it up at a steeper angle (to make it steeper, add more objects under the "uphill" side). How do you think the angle of the hill will affect the stream?
•    How do you think more water would affect the shape and behavior of the stream? If you are careful to make sure any extra water will go into the sink (or other larger container), you can try uncapping the water bottle and pouring slightly more water into your stream. How does this extra water change the shape and behavior of the stream?
•    Stop pouring the water. What does the streambed look like?

Read on for observations, results and more resources.


Observations and results
How did your stream change over time? What influenced its path, size and shape?

The study of water is called hydrology. Scientists study the ways in which different types of materials (such as dirt, sand and rock) affect the formation and shapes of streams and rivers. Some researchers have even found structures that look like riverbeds and canyons on Mars, which they say is evidence that Mars once had liquid water.

But water is not the only substance that changes the surface of Earth. Can you think of other forces that change Earth's surface? Big blocks of frozen water in the form of glaciers—such as those that covered much of North America during previous ice ages—can slowly scrape surfaces clear or push dirt and rock to make hills as they advance and recede. Even wind can blow lighter material, such as sand, around, shifting the position of sand dunes. The study of these different processes is called geomorphology.

Share your stream erosion observations and results! Leave a comment below or share your photos and feedback on Scientific American's Facebook page.

Cleanup
Recycle the water bottle and be careful when cleaning up the flour and water mixture.

More to explore
 "Data Deluge: Texas Flood Canyon Offers Test of Hydrology Theories for Earth and Mars" from Scientific American
 "Planting Trees Can Shift Water Flow" from Scientific American
 "Investigate and Report on Erosion" from Dirtmeister's Science Reporters
"Erosion" overview from BrainPOP
 Erosion: Changing Earth's Surface by Robin Koontz and Matthew Harrad, ages 4–8
 Janice VanCleave's Earth Science for Every Kid: 101 Easy Experiments that Really Work by Janice VanCleave, ages 9–12

Up next…
Big Space: The Scale of the Solar System

What you'll need
•    Nine small, round objects about the size of a peppercorn (0.1 inch, or 2.5 millimeters, across)
•    Measuring tape
•    Nine small pieces of paper labeled for the sun and planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune)
•    A clear, 21.5-foot- (6.6-meter-) long path on the floor inside or ground outdoors
•    A grapefruit (optional)
•    Two grains of table salt (optional)
•    Two grains of sea salt (optional)
•    35 feet (10.7 meters) of space in at least one direction (optional)