My Science Box

Lesson Plan
Introducing the activity

1. Tell the students that they will be exploring erosion when there are multiple types of sediments, not just silt (diatomaceous earth) as in the River Cutter experiments. Ask the students “What do you know about erosion so far?” Review the idea that erosion is the movement of sediment from one place to another by water, wind, or other natural forces. Review the idea that erosion happens when sediments are carried in a river – the river’s load - and deposited elsewhere.
2. Show the students the soil separation demonstration jar. Point out the mixture of silt, sand and gravel within. Ask students to predict what will happen if you shake the jar then allow the contents to settle. Remind them of their own experiences with soil separation tests if they have done them before.
3. Shake the jar vigorously for 10 seconds.
4. Set the jar down on a countertop. Have students make observations of what they see and notice. The gravel should settle to the bottom, sand in the middle and silt on the top.
5. Now ask students WHY they think the sediments layered themselves in this way. In particular, emphasize the idea that the tiny particles of silt stay suspended in water longer while the largest particles of gravel are densest and quickly settle to the bottom.
6. Explain the set up for today’s experiment: students will cut two rivers, one in a tub of just silt and one in a tub of mixed sediments like those in the jar. Ask students to predict what they think will happen. Some more specific questions to consider include:
   • What differences do you think there will be between the single sediment river and the mixed sediment river? (If students completed session 4 or 5 of River Cutters, remind them of some of the differences they observed: the depth of the channel, the width of the channel, the amount of material moved, how much it meanders, the number of tributaries, the size of the delta, etc. Ask them how they think each of these variables might play out in this experiment and why.)
   • What do you think will happen to the silt in the mixed sediment river?
   • What do you think will happen to the gravel in the mixed sediment river?
   • What do you think will happen to the sand in the mixed sediment river?
   • Have students write down their own hypothesis in their lab notebook. Make sure they explain why they think their results will turn out they way they predict.
   • Explain that students will be running a single sediment and a mixed sediment river side by side. The two rivers will run for 5 minutes, students will make observations, then the rivers can be run for 5 minutes more before a final set of observations are made.

   Conducting the experiment
   

7. Divide the students into groups of 3 or 4. Assign each group to a condition, either single sediment or mixed sediment. Two groups should work at each table or cluster of desks so that a single sediment tub and a mixed sediment tub may be run side by side. Place the tub on the table, prop up one end with the wood, and place the dripper system inside at the top of he slope.
8. Have one student from each group gather the necessary materials: the appropriate tub, a piece of wood top prop up one end of the tub, a dripper system.
9. Distribute the pitchers of water around the room, ideally, one pitcher per pair of tubs.
10. Have students run five minute rivers in their tubs, starting the two rivers at the same time and adjusting the flow rates to be approximately equal. Circulate around the room to make sure everyone understands the directions and teams are working well together. Assist students as needed.
11. After five minutes, make sure that students have stopped the flow of the rivers. Check to see that students are making observations of their rivers in their lab notebook – drawing maps of the two rivers, labeling features, and writing 1-2 sentences describing their observations.
12. Have students run their rivers for 5 minutes more, again staring the rivers at the same time and adjusting the flow rates to be approximately equal. Circulate and help students as needed.
13. After the second five minutes has passed, make sure that students are making their second set of observations.
14. Finally, pose the following questions to the class:
    • What differences do you observe between the single sediment river and the mixed sediment river?
    • What do you think will happen to the silt in the mixed sediment river?
    • What happened to the gravel in the mixed sediment river?
    • What happened to the sand in the mixed sediment river?
15. Have students discuss these questions within their teams. When they feel like they have reached consensus, they should write down their conclusions in their notebooks.
16. When teams have finished their own conclusions, they may visit other teams’ rivers and compare them to their own.
17. If you are ending here for the day, have students put the materials back and wipe down their tables.

    Group discussion (This discussion may take place the same day if you have time or the following day.)
    

18. Have one member of each team report their conclusions to the class. Create a list of conclusions on the front board or on an overhead. Note any conclusions reported by multiple groups and note any discrepancies, but do not discuss them until all groups have reported.
19. Students will probably observe that mixed sediment rivers are shallower, straighter, and wider. The gravel and sand should be exposed near the source and in the river channel while the silt accumulates in the delta and bay. Encourage a discussion as to why this might be.
20. Return to the class’ observations of the soil separation test and begin to put the pieces together. Some questions you may want to consider using to shape the discussion are:
    • Do silt particles behave differently in fast moving water (either the river channel or the sediment jar) than gravel particles? How? Why?
    • Where is the water moving fastest in their rivers and where is it moving slowest?
    • How does the speed of the current affect the sediment load, in particular, the types of sediment that might be suspended?
    • How does the speed of the current affect deposition, in particular, when different types of sediment might be deposited?
    • How do these models compare to real rivers in the real world? Would real rivers be more like the single sediment rivers or the mixed sediment rivers?