CTgeo

Try the Stream Table Outdoors!

Many teachers avoid the stream table because of the seemingly unavoidable migration of sand from the stream table via wet hands to the classroom sink and floor. There are techniques to avoid gritty floors (such as asking students to keep their hands over the stream table and to rinse their hands in the drainage bucket to remove sand), but another idea that can work very well is to take the lab outdoors.

In order to use the stream table outdoors, one needs access to water, either a faucet, or, ideally, a sandy beach or shallow stream with a sandy bottom. A sandy beach or stream provides water and sand-the two most cumbersome materials used in stream table studies. At the end of the activity, the sand can be easily returned to it original location and the table can be rinsed clean with water.

Plastic stream tables are lightweight when empty and can be stacked for easy carrying. Equipment for each table can be carried in the same bucket that is used as a water source. Plastic tubing, a clamp, a plastic beaker or container (such as a delicatessan container, a plastic trowel (or 1" by 4" scrap wood), and food coloring are some of the more commonly used materials; many substitutions can be made depending on what the topic of investigation is. Aluminum cookie sheets with sides (jelly roll pans) also work.

Stream tables can be used in all sorts of investigations as models of surface and groundwater movement. Following are ideas for some more commonly used activities: 

  • Gradient vs. velocity 

  • Development of youthful, mature, and old age stream features 

  • Groundwater studies: How pollution moves below the surface 

  • Groundwater levels: Depressions, swamps, lakes 

  • Measuring headward erosion 

  • Stream piracy 

  • Deflection of stream direction by more resistant rock

Why Are Some Streams Faster than Others? 

and 

Can Streams Speed Up or Slow Down?

It is relatively easy to measure how fast the water in a stream is traveling or to measure the discharge of a stream. But what might happen to a stream’s velocity if a stream’s gradient changes? This happens to real streams as they flow and erode away large amounts of earth materials from their beds and sides. It also happens if plate tectonics becomes active and produces uplift of the land below a section of the stream. Or if humans or beavers build a dam.

Does the amount of water the stream discharges have anything to do with its speed? During a drought, when the level of the stream is down, does its velocity change? After heavy rains, when the level of water in the stream is high, does the stream's velocity change?

In this activity you will answer these questions by using a model of a stream. The model will make it possible to make progressive changes in the stream's gradient and discharge. This will have to be done in two separate experiments. Why?

Problem Statement: How Does Gradient Affect Stream Velocity?:

Materials 

  1. Bucket of water 

  2. Metric ruler 

  3. One piece of plastic tubing 

  4. One meter long plastic trough 

  5. Small object that floats such as a part of a leaf

  6. Level

Procedure: 

1.  Position a bucket of water near the closed end of the trough

2.  Use a level to bring the trough to a horizontal position. (Adjust the trough until the bubble in the level is centered.) Then take a stick and push it into the ground at the closed end of the trough. With a pen, mark the height of the upper closed end of the trough on the stick and label this mark as the zero point. Also draw a line for the ground level and mark this "G".

3.  Remove the stick and, starting at the zero mark, draw a line every two centimeters above the zero mark; label each line: 2 cm, 4 cm, 6 cm, 8 cm, etc. Push the stick back into the ground so that the "G" point is at the ground surface.

4.  Raise the trough on the closed end so that its upper surface is at the height of the 2 cm mark on the stick. The gradient of the trough is now 2 cm/m.                  

5.  Fill one piece of plastic tubing with water so that the water will siphon from the bucket. Introduce water into the upper end of the trough to produce a stream in the trough. Drop a small object that will float into the upper end of the stream at the same time that a lab partner begins timing the object’s movement with a stop watch.  When the object reaches the end of the one meter trough, record the time it traveled. The object’s velocity is approximately the same as the stream’s velocity. Why?

6.   Velocity should be recorded as a certain number of meters/second.        

        Suppose the object traveled one meter in 4 seconds. How far did it get in one second? You can figure this out by dividing  four into one. The answer is .25 m/s.

        Suppose the object traveled one meter in two seconds? Divide two into one. The answer is .5 m/s which means the object traveled one-half of a meter in one second.

7.  Raise the trough to the four centimeter mark and repeat the above process. Continue in this       manner until you have enough data to graph your results. Record the velocities in meters/second.  

Results:

8.  Organize your recorded data in a table.

9.  Construct a graph that shows the relationship between a stream's gradient and its velocity. What should be the independent variable? (What did you control?) Be sure to label each axis with appropriate units and to give the graph a title.

Analysis and Conclusion:

10. Write a well-organized paragraph relating your data to your problem statement. Think of your graph as an organized picture of your data that shows relationships between variables.

Validity:

11. Discuss the validity of this exercise. Did the activity give you sufficient information to answer the problem statement accurately? Were there any equipment problems that were beyond your control to correct? (Remember validity is not about what "might have happened" or about careless measurements. You are expected to measure carefully.)

Problem Statement: Does a Stream's Discharge Affect Its Velocity?

Discharge refers to the volume of stream water that passes a certain point in a set amount of time. In this activity, you will determine whether a change in the stream's discharge will produce a change in the stream's speed.

Materials: 

  1. One meter plastic trough 
  2. 4 plastic hoses 
  3. One bucket of water 
  4. One small floatable object

Procedure:

1. Set up the plastic trough so that the closed end is four centimeters higher than the open end.

2. Position a bucket of water next to the closed end of the trough and, using the plastic tubing, siphon water from the bucket into the upper end of the trough.

3. Time how long it takes for a small floatable object, such as a small piece of a leaf, to float from one end of the model stream to the other end. Do this several times. Record the individual trials and average the data.

4. Take a second piece of plastic tubing and siphon water from it as well as from the original tubing. The water flowing into the trough should now be double what it was in the first trial. The discharge of the model stream has doubled.

5. Time how long it takes for an object to float on this stream several times. Record the times for the individual trials and average them.

6. Repeat this procedure twice more, once with triple the original discharge (water flowing from three plastic tubes into the trough) and once with four times the original discharge (water flowing from four plastic tubes into the trough).

Results:

7. Organize data collected into a table.

8. Construct a graph showing the relationship between the model stream's discharge and its velocity. Remember the independent variable is recorded on the X-axis. Is discharge or velocity the independent variable?

Analysis and Conclusion:

9. Answer the problem statement by interpreting the relationship shown in the graph.

Validity:

10. Explain whether you think the experiment can really determine the relationship between discharge and velocity. Were there any problems in the lab that were uncontrollable and that affect your results?

What Kinds of Changes Occur when a Stream Slows Down…?

Background:

Over geologic time a steep (high gradient) stream wears away the earth materials that it flows over; eventually, the stream will have moved so much material that it will no longer be steep. An old stream has what geologists call a gentle or low gradient.

A stream with a gentle gradient does not behave in the same manner as a high gradient stream and it produces a unique set of features. In this investigation, you will observe and compare the kinds of features that form in young, steep streams and in old, low gradient streams. As you observe the model streams you produce, remember what you learned in the previous experiment about the relationship between the stream's gradient and its velocity. This relationship is an important clue in understanding what you observe.

Some of the features you may observe in this activity are listed at the end of this paragraph. The features are not listed in any kind of order. If you do not know what the features are, you should either look them up first (so you know what to watch for when observing the streams) or do the book research after doing the activities to determine the names of the features you observed: Waterfall, oxbow lake, cutoff, canyon, delta, undercutting, levee, back swamp, locations of erosion and deposition, headward erosion, stream piracy.

Procedure:

1. Set an empty stream table near the edge of a stream or beach so that you have access to sand and water.

2. Use a level to place the table in a horizontal position like a table top. The bubble should be centered between the lines in the window on the level.

3. Add sand to the stream table until it reaches to about two-thirds from the top of the stream table at one end and extends about two-thirds the length of the table. Use your hands or a block of wood to smooth the sand giving it a gentle slope in the direction of the empty part of the stream table.

Observing a YOUNG STREAM:

a. Using blocks of scrap wood, raise the sand-filled end of the stream table to a height of 15-20 cm.

b. Set a pail filled with water at the raised end of the stream table.

c. Using two fingers, or a block of wood of about the same width, dig a shallow, relatively straight channel from the highest end of the stream table to the opposite end.

d. Use rubber tubing to siphon water from the bucket into the empty stream channel.

e. Observe and record the water's activity. Describe how the water changes the landscape as it flows.

f. Illustrate and name any specific features the stream produces.

Observing an OLD STREAM:

g. Remove the blocks of wood from the raised end of the stream table so that it is in its original horizontal position.

h. Smooth out the sand, giving it a very gentle slope downward in the direction of the empty part of the stream table.

i. Using two fingers, or a block of wood about the same width, dig a shallow stream channel with meanders.

j. Siphon water into the upper end of the empty stream channel. If the water does not stay within the channel, the water may be traveling too fast or the stream channel may need to be deepened.

k. Observe the changes the stream produces in the surrounding landscape over time.

l. Add a drop of food coloring to the stream and observe where the water hits the stream's bank with the greatest force. Illustrate what you observe. What is happening to the stream's bank at this location? Is the stream's shape changing? If so, how?

m. Where does the water seem to be moving the slowest? What is the evidence you see for slow movement of water?

n. Observe, illustrate and label any new features you observe appearing in the stream as it continues to erode its surroundings.

Lab Write-Up:

Your lab report should follow the traditional format:

Problem Statement: What were you attempting to figure out?

Procedure: Explain what you did in third person. Write "Sand was added….." and not "I added sand…". Giving directions, such as "Add sand to …" is also not correct form.

Results: Illustrations and observations (third person)

Analysis and Conclusion: Try to make sense of what you observed. Why did the young and old streams produce different features? Why did particular features such as an oxbow lake, for example, form? In other words, think about what you observed, and ask whys. The conclusion of the report should include your thinking and ideas. It should also be written in third person. Do not say, "I think this was a really neat lab and I liked the way the stream traveled…"; instead, say, "The youthful stream observed in this study traveled a straight path….." Tell your teacher you liked the lab, but do not write about your feelings in your formal lab report.

Validity: Do you think this activity is a reliable way to learn about the development of streams over geologic time? Explain.

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