Chapter 12 Lesson 2, "Erosion Caused by Water"
Materials:
Objective:
Anticipatory Set:* Show students several pictures of before and after situations. Ask the students what they think caused the change in the area.
| Before and After 1 | Before and After 2 | Before and After 3 | Before and After 4 |
| Before and After 5 | Before and After 6 | Before and After 7 | Before and After 8 |
| Before and After 9 | Before and After 10 | ||
Teacher Input:* Weathering was the breaking apart of rocks into smaller and smaller pieces so if a mountain weathers why don't we end up with a huge pile of sand sized particles? What carries the small pieces away? (rivers, waves, glaciers, wind, and gravity are the main culprits) Today we are going to talk about what happens when water carries away these small particles. What is the term for wearing away and moving of weathered rock and soil? By understanding erosion we can anticipate it and hopefully avoid being hurt by it.
What causes the most erosion? (rivers and water running downhill) The water erosion starts as gullies or small washes. The small particles that are carried by the water are known as sediment. The sediment is then carried to larger rivers. The larger rivers flow to larger and larger rivers dropping larger particles as the water currents slow and even eroding the river banks as the river moves along. Then finally dropping most of the sediment in the ocean. A river goes through a cycle when developing. A young river has a very deep canyon and is relatively straight. Waters in a young river are usually very swift. This gives them a lot of energy to cut a deep canyon and push rocks along the river's path. Rapids and waterfalls are common in a young river. As the river gets older it turns into a mature river. The boulders and rocky edges have been warn away. The slope of a river is less steep and the river does not flow as fast. A mature river is can move pebbles, sand, and mud but not large rocks. The valley is much wider than the river itself. A mature river has some curves but as it ages and turns into an old river it gets much more. The old river is identified by these twisty curves. What are the twisty curves called? An old river also has a very large flat flood plain. Flood waters deposit sediments on these flood plains leaving them very fertile. The old river valleys are wide and flat. Another characteristic of an old river is an oxbow lake.
When a river reaches the ocean or a big lake it begins dropping sediments that it was carrying. What is that process called? (deposition) The heavy stuff drops first and then the smaller stuff travels further and further out into the ocean or lake. This place where a river flows into a larger body of water is known as ?(mouth) As the sediments drop they build up the land near the mouth what is the name of that build up? (delta) In fact a delta is a lot like the fan shaped soil weathered eroded, and deposited at the base of mountain. In this case it is known as a ? (alluvial fan)
Can you think of any other way water erodes soil other than with rain and stream flow? (wave erosion) Waves are hurling not only water at the shore but also small bits of rock. As the rock batters the rocks it breaks off pieces of the rocks along the shore.
The pieces that get broken off become deposition along the shoreline known as ? (beaches) This deposition can change shapes and positions depending on wave movement.
Guided Practice:* Show the students an animation of how a river ages and oxbow lakes form. http://www.school-portal.co.uk/GroupDownloadFile.asp?file=21606
Students will complete the questions on Page 291.
Independent Practice:* Students will complete the worksheet, "Erosion Caused by Water"
Check for Understanding:* Students will complete the quiz on Chapter 12 Lesson 2.
Duration:
30 minutes + 10-15 for Independent Practice (Could be longer if taken for homework).
Alaska Content Standards Addressed in this lesson:
