Pre Activities - (Let the River Go - What's Your Ambition? - A Home For Many)

INSTRUCTOR NOTES - STUDENT ACTIVITY

LET THE RIVER GO!
Instructor Notes

Topic/Keyword
science, water, discharge, transport, watershed, river channel

Grade Levels
6-8

Time Considerations
Instructor Preparation: 15 minutes
Student Activity: one 50-minute period

Objectives
After completing this lesson, students should be able to:
· understand and discuss
--- how and why rivers change course
--- artificial measures designed to prevent this change
--- long range effects of artificial barriers
· make predictions based on accepted physical principles
· perform routine calculations accurately
· formulate logical questions
· assess cause and effect relationships based on scientific principles

Teacher Notes for Discussion
A river is a system of matter and energy that is in a state of constant change. Changes are not unpredictable or random. They are normal responses to increases or decreases in the amount of water flowing in a channel (discharge); increases or decreases in downstream velocity; and the erosion (transport) and deposition (sedimentation) of the river channel.

For rivers flowing in alluvium-lined channels, the downstream slope of the channel (gradient) is naturally adjusted over time to accommodate average discharge and velocity. When the discharge and velocity increase, the result is erosion of channel alluvium, which causes the slope to increase. A decrease in discharge and velocity results in deposition of stream sediment. This causes the channel to become built-up and the gradient to decrease, thus maintaining flow conditions.

A complete analysis of river geomorphology is quite complex and includes a consideration of the quantity and grain-size of sediment supplied to the river from the drainage basin (watershed) and the shape of the channel. Man-made structures (bridges, dams, etc.) further complicate the picture.

Many streams in the upper Midwest have developed meandering channels with relatively low gradients, velocity, and discharge. Such meandering channels are subject to frequent changes in course when floods occur. Rising discharge and velocity result in erosion and the creation of new channel segments. This often happens at narrow places (necks) between meandering channels. The former meander may be abandoned in favor of the new channel, thus producing an oxbow or an oxbow lake.

Oxbows are important, although temporary, features of many meandering river systems. They constitute wetland environments for wildlife and storage basins for flood waters.

LET THE RIVER GO!
Student Activity

Understand Your Mission
In this activity you will discover why variable water flow will change the course and shape of the river. By understanding how rivers are formed and beginning to recognize the different parts, you should also be able to predict changes in the course of a river over time.

Study the Facts
A river makes its own bed, but not like you make yours every day. A river carves its bed out of the surrounding landscape. How deep or how wide it makes its bed depends on the type of material in the bed and the amount of water and discharge flowing through the bed.

River systems are not static entities. They are systems of matter and energy in a state of constant change. Matter includes the water, sediment, riverbed, and the riverbanks. The flow of water and its interaction with matter create energy

The flow of a river, called the discharge, plays a large role in determining the rate of erosion and deposition in a river system. Erosion happens when the water picks up and moves sediment. Deposition is the opposite of erosion and occurs when the sediment settles onto the riverbed. Erosion increases when the water is moving faster (an increase in discharge). Deposition increases when the water is moving slowly (a decrease in discharge).

A river flows in the path of least resistance. Using the power of gravity, this is naturally the area with the greatest slope, or gradient. Rivers flowing down a steep gradient are often straight and narrow. A river often develops into a broadly curving channel when the area has a low gradient. When a river encounters new obstacles, it has to make quick detours that sometimes cause a meandering river. When the water flow of a river increases or decreases, due to floods or drought, it leaves behind parts of the river creating U-shaped bends in the river called "oxbows." Although temporary, oxbows create wetland environments for wildlife and storage basins for food and water.

Learn the Lingo
deposition discharge erosion
floods gradient levee
meandering necks oxbow

Gather Your Supplies
topographic maps of meandering rivers
ruler
calculator

Chart a Course for Exploration
1) Using a map, identify parts of the river, including oxbows and necks.
2) Determine the gradient for the entire stretch of a channel identified on your map. (Gradient is determined by dividing the elevation change, in feet, by the channel distance, in miles.)
3) Suppose the discharge increases enough that water spills out of the main channel (floods) and begins to flow across the necks in a downstream direction.
· In which area will erosion most likely form a new channel?
· Why?
4) Sketch or create a model of the river with its new channel. Remember to show where an oxbow lake will be located.
5) Determine the new gradient for the river.
· Is it higher or lower, on average?
· Is the gradient the same over the entire length of the river?
· Where is the gradient the highest?
· If the discharge returns to pre-flood level, how will the river adjust its channel so as to return to an average gradient equal to pre-flood conditions?

Go Beyond
Compare old and recent maps of the Mississippi River. Has the river changed its course? Where have control structures been installed? How have they impacted the river?