7. Erosion Patterns
This lesson is an extension of the GEMS guide“River Cutters” by the Lawrence Hall of Science (see note below). In the GEMS curriculum, students are introduced to erosion by modeling ...
This lesson is an extension of the GEMS guide: “River Cutters” by the Lawrence Hall of Science (see note below). In the GEMS curriculum, students are introduced to erosion by modeling the formation of rivers in tubs of diatomaceous earth, a silt-like substance into which meandering river channels and deltas form. This lesson builds off of the River Cutters activities by using a combination of sediment types in the models. They observe how gravel and large particles of sand remain in place whereas silt is washed downstream in fast flowing river channels. In contrast, where the water velocity slows as it reaches the newly forming bay, a beautiful silt-covered delta forms. These observations lead students to the conclusion that fast moving water picks up the smaller sediment particles and eventually deposits them in places where the water slows. Students can then take this theory to test it out in real world conditions at a local creek in the Sediment Study Project, observing sediments and water velocity at different sites along a creek’s length. The concept of how sediments are deposited becomes a core feature of subsequent geology lessons and investigations in which the environmental conditions surrounding the formation of large particled conglomerates may be differentiated from small particled shales and siltstones.
Special Note: This lesson plan is written with the assumption that students have some experience using the river models in the GEMS guide “River Cutters”, written by Cary Sneider and Katharine Barrett and produced by the Lawrence Hall of Science. In this guide, students make observations of rivers carved in just silt (diatomaceous earth), sequencing events in time, noticing patterns, recording information, and acquiring the terminology necessary to describe common erosion patterns. My students completed the first 5 sessions of River Cutters although completing the first 3 lessons is sufficient. So as not to infringe upon the copyright of the GEMS unit, only the extension activity is described here.
Can describe the major types of sediment.
Can identify common river features and erosion patterns such as a river’s source, channel, mouth, delta, etc.
Can model sedimentary deposition patterns.
Can explain how sediment size and current velocity affects deposition.
Can make observations and record data in a science lab notebook.
Can draw conclusions from data.
7. Erosion Patterns - Logistics
15 minutes – Set stage for the experiment
20 minutes – Run single sediment and mixed sediment rivers side by side
15 minutes – Teams make individual conclusions and clean up
15-20 minutes the following class period – Combine all data and make summative conclusions
Teams of 3-4 students. Half the teams will set up tubs with a single sediment (diatomaceous earth). The other half will set up tubs with a mixture of sediments (diatomaceous earth, playground sand, and aquarium gravel).
For Soil Separation Test Demonstration
- 1 small jar alum (available at supermarkets for pickling)
- 1 clear Mason jar with lid
For River Models
For each team of 3 students:
- 1 wide-mouthed, 9 oz plastic cup with a V-shaped notch cut into the edge
- 1 plastic stir stick
- 1 straightened paper clip , approximately the same length as the stir stick and thin enough to be inserted into one of the 2 holes in the stir stick
- 1 plastic food service tub, approximately 20”x15”x7”
- 1 piece of wood (1”x2” stakes are cheap and easy to find although they do have splinters) or other prop to raise one end of the tub off the table
For the class to share:
- 20 lbs of diatomaceous earth (see Sources for more information)
- 10 lbs aquarium gravel
- 10 lbs playground sand
- measuring cup
- several pitchers of water
- blue food coloring
- paper towels (for clean up)
7. Erosion Patterns - Background
Erosion is the process by which sediments are transported by wind, water, ice, or gravity. Often people mistake erosion for weathering, the process through which rocks are gradually chipped away by abrasion, water, and ice into sediments, a topic that is studied in greater detail in the Geology Box. On the other hand, erosion is the movement of these sediments from one place to another. Erosion is a very natural and essential natural process upon which many ecosystems depend including beaches, deltas and wetlands.
The central aspect of this activity is the observation of differences in sedimentation patterns based on the velocity of water movement. The central principle is as water moves faster, it can carry larger sediments and more of them. The smallest sediment particles (silt and clay) are picked up first, followed by sand then gravel. Thus, a rain-swollen river will carry a great deal of sediments of all sizes while a slow, meandering stream will carry very little and only of the smallest sizes. As the current velocity slows, sediments are deposited in reverse order, gravel is dropped first, then sand, then silt and clay.
In most watersheds, rivers begin in the high mountains with steep slopes, and thus, with fast running water. The smallest sediments are quickly borne away, leaving behind the gravel and larger rocks. As the slope of the landscape lessens in the foothills, the river slows and sandbars may accumulate along the curves and twists of the riverbed. Finally, as the slope becomes nearly flat, either upon reaching the valley floor or as it reaches a body of water such as a lake or bay, the current velocity is reduced to a crawl and even the smallest sediments are deposited, leading to silt-covered deltas and clay-like mud covering the bottom of lakes and bays.
All of these processes may be observed in the river models in which silt from the faster flowing river channels is picked up by the water and deposited when the water slows in the bay at the base of the model, eventually forming a delta. The sand and gravel in the upper regions of the model are gradually exposed. If your students do the silt only river models described in the GEMS guide “River Cutters” they will soon discover that the rivers formed in the mixed sediment rivers are slower to form, straighter and shallower, more similar to a young river, owing to the higher percentage of gravel and sand which are not as readily carried away by the water.
In my classes, I spend 1 class period introducing the activity, conducting the experiment, and allowing students time to make observations and draw individual conclusions. I spend the second class period in a group discussion, leading students towards a clear theory to explain their observations.
Students need hands-on experience with soil separation tests in order to see how the smallest particles of sediment stay suspended in agitated water longer than larger sediments (see the Soil Analysis lesson). Although this is used as a demonstration before beginning this lesson, students recognize the patterns and their implications best when they make the discovery themselves.
In addition, it is strongly recommended that students complete a minimum of the first 3, and preferably, the first 5 sessions in the GEMS guide so that students have experimented with time (how long a river has been running) and with slope before adding the variable of multiple sediments to the model.
7. Erosion Patterns - Getting Ready
Setting up the demonstration
- Fill the jar 1/4 full with gravel, another 1/4 full with sand and another 1/4 full with diatomaceous earth. Mix well.
- Add a pea sized amount of alum, about 1/2 a teaspoon.
- Fill the rest of the jar with water, leaving just a little air space at the top.
- Seal the jar tightly.
Preparing the tubs
- In HALF the tubs, place 13 cups of diatomaceous earth. Diatomaceous earth may be irritating to the lungs if it is inhaled so pour diatomaceous earth slowly. (These are identical to those used in River Cutters. If you have already conducted experiments from River Cutters, leave half the tubs the way they are.)
- In the other HALF of the tubs, add 10 cups of diatomaceous earth, 2 cups of playground sand, and 1 cup of aquarium gravel. Mix the sediments together thoroughly. (If you have already conducted experiments from River Cutters, you can adapt half the tubs to mixed sediment without needing to discard the whole thing. Remove 4 cups of the diatomaceous earth from the existing tubs. Add 2 cups of playground sand and 1 cup of aquarium gravel. Add 2 cups of water. Mix thoroughly then adjust the moisture level until you can cut a channel.)
- In ALL tubs, add 12 cups of water. Mix thoroughly. It should begin to resemble that fascinating non-Newtonian fluid, cornstarch in water. In the mixed sediments tub, the gravel and sand will likely rise to the surface – that’s OK.
- The consistency of the mixture is very important. Check to see if there is too much water by lifting one end of the tub. If the sediment slides down to the lower end of the tub, then it is too wet. Use the sponge to soak up some extra water. Check to see if here is too little water by slowly pouring some water onto the surface. If the water soaks in before it forms a rivulet, then it is too dry and needs more water. Ideally, the water should trickle across the surface and erode a gully as it flows downhill.
Preparing the dripper systems
- Cut a shallow V shaped notch in each cup.
- Insert a piece of paper clip wire into each stir stick.
- Gently curve the stir stick into U shape resembling the Saint Louis Gateway Arch or the end of an egg.
- In each pitcher, put 3 drops of blue food coloring then fill the rest of the pitcher with water.
Saint Louis Gateway Arch
Working the dripper systems
- Fill the cup with water
- Dunk the stirrer into the cup so that the entire straw fills with water (insert the straw middle first with the ends pointing upwards). You should see bubbles come out of the ends.
- Carefully pick the straw out of the cup. I find that holding a finger over one end of the straw helps.
- Quickly turn the straw over so that the ends point downward and simultaneously stick one end (the end with your finger over the tip) into the cup. Position the straw onto one of the notches.
- Hopefully, the straw drips slowly and steadily, around 2 drops per second. If there is no water at all, try again. If the water flows too quickly, bend the straw straighter. If the water flows too slowly, bend the straw into a deeper U shape.
7. Erosion Patterns - Lesson Plan
Introducing the activity
- 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.
- 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.
- Shake the jar vigorously for 10 seconds.
- 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.
- 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.
- 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
- 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.
- 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.
- Distribute the pitchers of water around the room, ideally, one pitcher per pair of tubs.
- 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.
- 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.
- 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.
- After the second five minutes has passed, make sure that students are making their second set of observations.
- 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?
- 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.
- When teams have finished their own conclusions, they may visit other teams’ rivers and compare them to their own.
- 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.)
- 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.
- 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.
- 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?
7. Erosion Patterns - Assessments
- Have students, either individually or in teams, propose a theory in answer to the questions: “How does the speed of the current affect the type of sediments that are carried and deposited by a river? When water moves quickly, what happens? When water moves slowly, what happens? Why?”
- Bring the conclusions of this experiment into the real world. How are different sediments distributed at a real creek? What causes the variance? See the Sediment Study Project for a detailed description.
7. Erosion Patterns - Sources and Standards
The primary inspiration for this unit is the GEMS guide “River Cutters”, written by Cary I. Sneider and Katharine Barrett and produced by the Lawrence Hall of Science. I strongly recommend this guide, even if you don’t intend to use it because they offer fabulous tips and recommendations for using river cutter tubs with students. In addition, the guide provides fabulous diagrams, handouts, homework assignments, suggestions, resources and more that can be used as a prelude or complement to this activity.
For more information about erosion and sediments:
- Wikipedia provides a great primer on the basics of erosion.
- Georgia Perimeter College has some excellent notes on erosion written for their teacher education program by Dr. Pamela Gore.
- Watersheds.org also provides a good overview about erosion, divided into Field and Slope Erosion versus Valley and Stream Erosion. Their website is useful for connecting common erosion patterns to a real world creek, Bryant Creek.
Shaping Earth’s Surface
2. Topography is reshaped by the weathering of rock and soil and by the transportation and deposition of sediment. As a basis for understanding this concept:
Students know water running downhill is the dominant process in shaping the landscape, including California’s landscape.
Students know rivers and streams are dynamic systems that erode, transport sediment, change course, and flood their banks in natural and recurring patterns.
Students know beaches are dynamic systems in which the sand is supplied by rivers and moved along the coast by the action of waves.
Investigation and Experimentation
7. Scientific progress is made by asking meaningful questions and conducting careful investigations. As a basis for understanding this concept and addressing the content in the other three strands, students should develop their own questions and perform investigations.