RIVERS

Think about the last time you looked at a river — really looked at it. Was it straight, or did it bend and curve, and maybe even wind its way through town? The more you look at rivers, the more you might wonder why they aren't straight. They start in the mountains and run to the sea. It sounds like a simple path, one that they should have carved out early in their lives and never deviated from. However, the paths of rivers have surprised and intrigued people for thousands of years. When a time of flooding comes to an area, the path of the river becomes a topic for even more discussion. It's almost like a river has a mind of its own!

QUESTION

Why aren't rivers straight?

EXPERIMENT OVERVIEW

In this experiment, you'll first get to build a mountain. To do that, you're going to need some mountain supplies and a pretty open space for the water to flow out. You're also going to need plenty of water, so plan ahead. You'll have two options to choose from, in terms of the “rain” that falls: steady or occasional. Each pattern will result in a different set of rivers, so you might even want to try both.

SCIENCE CONCEPT

When water flows down a mountain, it finds the quickest path to the bottom, even if that path isn't straight. Trees, rocks, and hills cause it to change direction and the speed it travels. When water moves slowly, it tends to dig away at its boundaries (the riverbank), and sometimes will cut out a piece of the bank, which makes the river a little wider at that point. Every time the river changes, the water flow changes and that causes even more variation in the path of the river. So over time, a river can carve out all kinds of interesting paths to the sea. But that's not all. When a new piece of the river is carved out, the current carries rocks and dirt farther downstream. Where this material lands, the river gets shallower. That is why the mouths of rivers, especially where they empty into the ocean, tend to be really wide, open, and flat, with water that moves slowly into the ocean.

MATERIALS

A large mountain of rocks, dirt, sand, mud, and so forth, at least 3 feet high

An open place where the rivers can flow and deposit the mud they accumulate as they flow down the mountain

Plenty of water, either through a hose, a sprinkler, or a watering can

Camera

PROCEDURE

• Make sure the mountain is not the same all over. There should be obstacles all over it that will encourage the water to flow in interesting paths.

• Predict where the rivers will form.

• Choose a method of watering:

  Steady rain — Use a sprinkler on top of the mountain, or very near the edge of it to produce rainfall that will be steady throughout the experiment. A helpful assistant could simply hold a hose above the mountain or spray the water onto the top of the mountain. You may need to experiment to find what works best.

  Occasional rain — Use a watering can, a pitcher of water, or a short interval of rain from a hose. If you choose this method, you will need to return every hour or so to add more water. This allows the mountain to absorb some of the water and will result in a different set of rivers.

• Begin watering the mountain.

• If you are using a steady rain, take a picture before you begin, and then take a picture every 5 to 10 minutes until the rivers are no longer changing. Your goal is to observe the changes in the mountain over time. It's better to have too many photos than to have too few.

  If you choose the occasional rain method, take a picture before you begin and then during each rainfall. Apply the water for as long as you have chosen and then let the mountain sit until the next session. Repeat applications until the rivers stop changing. This method will likely take quite a bit longer than the other, but might give a more accurate depiction of true rainfall.

• Keep a record of your method and the photos you took.

• When you get the photos developed, you should have a record of the progress of the rivers you produced. If you find that the photos don't show enough change, use every other picture.

QUESTIONS FOR THE SCIENTIST

• Did the rivers form in the places you predicted?

• How much material was carried off the mountain to the surrounding area?

• Did you see any smaller rivers merge into larger rivers?

• Was there one river that changed its path more than the rest? If so, what were some of the characteristics of that river that made it change so much?

CONCLUSION

Each time you repeat this experiment, you will get a different result. That's part of the fun of science! Now that you've produced your own rivers, see if you can visit a local river and identify places where it veered over time, where it moves faster or slower than other places, and any obstacles that might affect the flow of water. Also, see if you can trace the source of the river, although it might be many miles away in the mountains.