CHANGING CHANNELS (LANDFORMS) IN MIDSTREAM
Rivers are not all created equal. River channels form by flowing water and differ in their shape. The channel shape depends on a number of factors, which we will explore in this activity.
Instructional Method: Experiment
Goal: Students will perform experiments to see how each channel shape is formed.
Objectives: Students will be able to:
- Describe three river channel shapes
- List what forms each channel shape
- Identify the channels in nature or photos.
Preparation: 30 min.
Demonstration: 45 min.
Discussion: 15 min.
- Broad, flat container
- Bucket or jar
- Blocks or bricks
In nature, many landforms are created and destroyed by stream flow processes. Streams create landforms, but water in a stream is not a landform. The sediment or rock forming a stream channel is the landform. This activity discusses stream channels as landforms.
Not all stream channels look alike. Different factors affect stream shape:
Channel bedrock (i.e. solid rock vs. sediment): Harder rock does not erode as easily as soft rock or loose sediment. Water velocity (water speed): As water velocity increases, so does the rate of erosion. Faster water moves abundant sediment and larger sized particles. When fast-moving water slows down, large particles fall out of flow and deposited on the channel bottom. When stream gradient increases, so does water velocity and so does the streams erosiveness. Stream gradient (the amount of downhill angle the river channel has): Steep streams are straight and narrow. Flatter streams wriggle more and are broader. Water flow consistency: If flow is constant, streams continuously incise the rock. If flow is intermittent, downcutting and flow deposition can be the same.
Three river channels types are:
- Meandering (wriggling)
Straight streams have river channels that are relatively straight. Often they form in channels with a steep gradient. Water velocity in these rivers is fast. Most erosive action takes place along the river channel bottom rather than along the sides of the channel. This process is known as downcutting, or incision. An example of a river where downcutting dominates the erosional direction is found at Black Canyon of the Gunnison National Park.
Meandering streams are characterized by wandering, or wriggling, rivers that cross a valley bottom and form a river plain. The channel can be wide or narrow with valleys that are usually broad and relatively flat. Side to side erosion dominates meandering river systems. That is why these rivers change their course so often. Buffalo National River is a broad meandering stream that covers a large plain. Some meandering streams have cut so deeply into the rock that they no longer change course as easily as they did in the past. This is the case at Canyonlands National Park.
Braided streams are characterized by a broad channel with multiple subchannels and streams. The small streams weave back and forth, like the meandering streams, connecting randomly as they flow. When viewed from above it appears as if the smaller channels have been braided forming the stream system. Braided patterns tend to form in streams that are carrying and depositing an excessive amount of sediment. As sediment is carried and deposited in other portions of the smaller channel, it changes to form another small channel where it can cut and deposit again. Areas that flow occasionally due to storms, runoff or melt are common places to find braided streams (desert and glacial regions). These channel systems are constantly changing. Braided stream can be seen and Death Valley National Park and at Denali National Park.
The following activity allows students to experiment with different amounts of water flow and slope gradient to make the three discussed river channels.
- Use a large flat container that can be angled and keep its shape.
- Pour sand in container and spread it out leaving a space void of sand at one end (to catch the water and sediment).
- Raise the sandy end of container 2 inches with blocks.
- Pour water on raised end and observe. Keep pouring water until channel pattern is obvious. You may want to experiment with wet vs. dry sand. Repeat a few times and then change the angle by placing a few more blocks beneath it. Draw sketches of the different results.
- Now smooth out the sand and our water with a lot of sand mixed with it. This will form the braided stream types.
- Repeat steps 1 - 4 a few times. Remember to use sandy water this time.
- It may be helpful to smooth out sand between each attempt.
- Have students note on paper what occurs at each elevation and with the different rates of water flow.
What stream patterns were formed each time? Did the rate at which the water was poured affect the channel? Did the angle change the river channel? Did the compactness of sand affect the river channel?
Try pouring water at different rates and observing the resulting landforms. Vary the compactness of sand and observe!
Included National Parks and other sites:
Buffalo National River
Black Canyon of the Gunnison National Park
Canyonlands National Park
Capitol Reef National Park
Death Valley National Park
Denali National Park
Grand Canyon National Park
Zion National Park
Utah Science Core:
4th Grade Standard 5 Objective 1,2
5th Grade Standard 2 Objective 1,2,3