DEPOSIT A SLICE OF PIE
Deltas and alluvial fans are like identical twins: they look just alike! However, they are the result of similar depositional processes in very different environments. We take a closer look at these fan-shaped features in this activity.
Instructional Method: Experiment
Goal: Students will create pie shaped landforms and understand how and where they form.
Objectives: Students will be able to:
- Draw a sketch of a delta and alluvial fan
- Describe the environments that form each landform
- Distinguish alluvial fans and deltas by their depositional environments
Activity: 30 minutes
- Flat large container
- Jar or small bucket
Alluvial fans and deltas are sedimentary landforms deposited by flowing water. From above you can easily see their fan, or pie wedge shape.
Alluvial (stream) deposits contain intermixed gravel, sand and cobble sized rocks. The size of sediment moved by a stream depends on the speed of the flowing water. The faster water flows, the larger sediment size the stream can carry. Sediment settles out of flow when stream flow slows down, meaning the energy decreases in flow. This happens as flow reaches a broad, flat surface or a standing body of water (like a lake or ocean).
Alluvial fans are often found at the base of arid / semiarid mountain ranges where intermittent streams flow. For a stream to be intermittent it flows only went it rains or snow melts. Sediment drops out of stream flow as energy decreases due to widening of the channel. Over time sediment builds up in the channels at the base of mountains and forces the stream to carve another channel. When channels fill in the process starts again. Streams filling in and moving to the side form fan shaped deposits. Alluvial fan examples can be found in Death Valley National Park and along the sides of the Colorado River at Grand Canyon National Park.
If intermittent activity continues over a long time the fan can grow very large, sometimes interconnecting with nearby fans forming a bajada. Death Valley National Park has well exposed examples of bajadas. In the west many people like to build on alluvial fan these deposits because they offer beautiful views of valleys and farmers like to farm them because of their fertile soil.
Deltas are similar to alluvial fans in shape, but are deposited in a different environment. Sediments that form deltas are transported in continuous flowing water. Delta deposits are found at the mouths of streams or rivers as they empty into lakes and oceans. As water flows into standing bodies of water, it looses energy and can no longer carry the sediment it once did. The sediment settles to the bottom and forms an under water fan. Delta's get their name from of their similarity to the triangular shape of the Greek letter "Delta". Examples of deltaic deposits are found at the end of the Mississippi River in the Gulf of Mexico and various smaller deltas located on the southern end of Florida.
River channels cross delta and alluvial fan deposits. As deposits thicken the channels shift side to side and continue forming new deltaic and alluvial fan structures. As old deposits erode away, the river channel eventually migrates back to the original site and redeposits another fan or delta. The process continues until the river entirely changes course.
The following activities allow students to create alluvial fans and deltas.
1. Alluvial Fan
- Pour sand into a large plastic container with at least 2-3 inch high sides. You may want to experiment with both wet and dry sand.
- Create adjoining mountains in the sand.
- Pour a cup of water in the valley between the upper peaks of the adjoining mountains.
- Make sure you pour the water at the same speed and in the same place.
- Look for the resulting deposit at the base of the mountains. You may have to keep adding water until you see an alluvial fan form.
- Create a mound of sand adjacent to each other and next to a standing body of water (i.e. add water to the pan so there is a "lake" at the base of the mound).
- Repeat the same processes found in the Alluvial Fan instructions.
- Look for the resulting deposit to form in the standing water.
- Note: You may have to wait a few minutes to see the newly created delta because of all the floating sediment in the water.
Have students draw a sketch of the resulting deposit. Have them describe what happened to the sand as water was poured onto it. Will the same triangle shape be formed if a lot of water is used? If a little water is used? Do the shapes look similar in and out of the water?
In the place of sand, food is always a good substitute. Potatoes and gravy may be the most commonly played with foods, but did you know that you can do geologic experiments with them? When gravy is allowed to escape the pool created in a pile of potatoes it will form a fan or triangle shape. This is one to try at home.
Debris flows and mudflows can also be demonstrated with the same materials. The following information is background for debris and mudflows. A debris flow forms when the side of the mountain becomes oversaturated and collapses. Water, mud and rock slide downhill and form fan-like deposits. To create a debris flow, place small pebbles in with the sand. Pour sand and the pebbles into a bucket or jar and mix with a lot of water until it is runny. After stirring the mixture pour it onto the valley between the mountains and observe. A mud flow is made of runny mud. It forms when water washes mud down from the side of a mountain. The mud becomes as runny as gravy. When the mud stops flowing it will form a rounded triangle shape like the above deposits. To make a mud flow use the same materials but remove all large objects and increase the amount of water. Notice how much farther the mud flow will travel.
Included National Parks and other sites:
Utah Science Core:
4th Grade Standard 4 Objective 1,2
5th Grade Standard 2 Objective 1,2,3