Making an alluvial fan

Alluvial fans form where swift streams flowing from hills or mountains encounter a flatter terrain causing the water velocity to decrease. When water slows down, some of the sediment gets deposited. The deposit is wedge shaped in cross-sectional view and fan-shaped in map view.

You can illustrate this effect with a simple experiment. Get a short vinyl adjust-a-spout downspout extender and slope it into a small stream table basin. Put a mixture of gravel and sand in the downspout and run water through it. Adjust the slope until the sediment begins to wash downstream. Notice the accumulation of sediment at the change in slope at the bottom!

If your stream table doesn't have a fairly large outlet, water will begin to accumulate into a lake at the base of the slope during this experiment. This will alter the characteristics of your deposit.

An alluvial fan forms where the sediment accumulates at a decrease in slope. A delta accumulates where a stream flows into a standing body of water. In both cases, sediment is deposited where the water slows down. But the characteristics of the deposits differ.

Students often struggle to understand the difference between an alluvial fan and a delta. You might have your students invent an experimental design to show how the two are different. After creating both types of deposit through experiment, they could describe the features they see, identifying similarities and differences between the two. Beware! Your students might want to cut a big hole in the side of your stream table so that water doesn't accumulate. If you love science more than lab equipment, you might have to let them!

You might want to put a bucket under the outlet of the stream table so as not to make a terrible mess—or better yet, make a terrible mess! Students love that! But custodians don't, so we recommend the bucket or a sink as used in the experiments shown below.

Playing 20 questions with deltas and alluvial fans

It's great if teachers can incorporate local geological observations into their classes. Here's an activity that Mary Colson invented to help students notice and interpret features in NW Minnesota. You can use this activity in your classes as-is, or you can find something similar from your own area!

In June of 2000, ten inches of rain fell in a 24-hour period on our farming community in western Minnesota. Many of the low areas in the farm fields around our house filled with several feet of water. A few weeks after the rainfall event, we discovered this tiny pond (see picture below), with an exposed delta and wave-cut shorelines along with the gully from which the delta’s sediment had been eroded. Mary and husband Russ clambered down onto the little delta and pieced together what must have happened, based on the clues we could see left behind in the farm field.

The tiny delta and miniature shorelines have long-since been plowed under, but Mary uses that landscape as a geologic puzzle for her students to practice making sense of what’s visible in the landscape, to practice applying what they know about erosion and deposition, and to develop a model that explains how this little landscape formed.

Overview of the 20-questions game called Earth Stories.

After a Brief Introduction to the landscape, I divide the students into groups of four. Each group has time to examine the photos, discuss what they see, and come up with two yes/no questions.

During Round One, each group gets to ask me one of their two questions and everyone listens to my ‘yes’ or ‘no’ or ‘that’s not a yes/no question’. Students jot down new information as they hear it. Round One concludes when each group has had the chance to ask one question. Students then have time to discuss, in their small groups, what they think they know. They use the new ideas to formulate two more questions.

During Round Two of questioning, each group can ask one more question. Their question can be one of their new questions, or it can be their unasked question from Round One. Again, students jot down new ideas and information as they hear them

The End Game consists of each group writing a paragraph that tells a plausible story for how this landscape developed. The instructions for this part of the game include the idea that the story is based on observation, not outlandish make-believe: “Your story should describe, from first to last, the events and the results that occurred. Your story should be based on your observations of the features in the photographs and your explanations for how those features formed.” That evening, I choose a winning story – the one that is most correct and complete - for each class.

The Next Day, I give the students 12 event cards that formed the backbone of the story read in the landscape that summer day in 2000. Students put the event cards in order from first (the oldest event) to last (the youngest event) using logical reasoning and cross-cutting relationships. We then discuss the evidence for each event and the reasoning for the ordering of the events. By the time I announce the winning team, students already know if their own story was plausible and complete.

Have students apply the mental model that they've developed to a new question: Using your understanding of how the delta formed, can you explain why it has a flat top?

Click here for a PDF of the game instructions, student worksheets, event cards, and photos of the landscape.

Connecting to the Next Generation Science Standards

The experimental activity and game address a number of NGSS performance expectations, including the following: MS-ESS 2-1 and MS-ESS 2-2; HS-ESS 2-5. Students can exercise skills in the practices of 1) planning and implementing investigations, 2) developing and using models, 3) asking questions, 4) constructing explanations, and 5) arguing from evidence. Students to experience the cross-cutting concepts of 1) patterns, 2) cause and effect and 3) the flow of matter in the Earth system.

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The Teacher Resources for The Thieving Rain are written by Russ and Mary Colson.

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The Thieving Rain by Anne E. Johnson

Return to “Eww, There’s Some Geology in My Fiction.”

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