CHAPTER 3 LESSON 2 Erosion and Deposition Landforms Shaped by Water and Wind Key Concepts What are the stages of stream development? How do water erosion and deposition change Earth s surface? How do wind erosion and deposition change Earth s surface? What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After 3. A beach is a landform that does not change over time. 4. Windblown sediment can cut and polish exposed rock surfaces. Make Vocabulary Cards Make a vocabulary card for each boldface term in this lesson. Write each term on one side of the card. On the other side, write the definition and draw a picture that represents or explains the term. Use these cards to review the vocabulary for the lesson. 1. Identify What are the two most important agents of weathering, erosion, and deposition? Shaping the Land with Water and Wind Earth s surface is constantly changing. Recall that weathering and erosion are destructive processes that shape Earth s surface. These destructive processes often produce tall, jagged landforms. Deposition is a constructive process that also shapes Earth s surface. Constructive processes often produce flat, low-lying landforms. What causes these processes that continually tear down and build up Earth s surface? In this lesson, you will read that water and wind are two of the most important causes of weathering, erosion, and deposition. Water and wind can change the shape of landforms. Fast-moving water can change the shape of rocks, and strong winds can move sediment great distances. In the next lesson, you will read about the changes to Earth s surface by the downhill movement of rocks and soil. You will also learn how the movement of glaciers changes Earth s surface. 36 Erosion and Deposition Reading Essentials
Water Erosion and Deposition Moving water can cause great changes both on and below Earth s surface. The shape of the landforms created by water erosion and deposition depends on the speed of water movement and the depositional environment. Water Erosion Have you ever gone swimming in an ocean? If so, then you know that moving water can have great energy. Moving water causes erosion along streams and rivers, at beaches, and underground. Stream Erosion Streams are active systems that erode land and transport sediment. Erosion by a stream depends on the stream s energy. Young streams in mountainous areas usually have the greatest energy. Water flows rapidly downhill where the mountain slopes are steep. The rushing water often carves steep, V-shaped valleys. Waterfalls and river rapids are common in steep mountain streams. Water in a young stream slows as it reaches gentler slopes near the base of mountains. The stream is then called a mature stream. The shape and speed of a mature stream are different from those of a young stream. Mature streams flow more slowly through gently sloping valleys. Slower-moving water erodes the sides of the stream channel more than it erodes the bottom of the channel. Curves called meanders develop in the channel. A meander is a broad, C-shaped curve in a stream. Meanders can make a mature stream into a large, snake-shaped feature in the land. A stream moves even slower when it reaches flat land. A stream flowing through flat land is called an old stream. Over time, meanders change shape. More erosion occurs on the outside of bends, where the water flows faster. More deposition occurs on the inside of bends, where the water flows slower. Over time, this increases the size of the meander. Coastal Erosion Like streams, coastlines are always changing. Waves and currents along a shore cause erosion of loose sand, gravel, and rock. A longshore current is a current that flows parallel to the shoreline. Longshore currents transport sediment from place to place, continually changing the size and shape of the beaches. Review how coastal erosion is caused by a longshore current in the figure on the next page. Make a two-tab book to organize information about erosion and deposition by water and wind. Erosion and Deposition Water Wind 2. Describe What is the shape of a valley formed by a young stream? (Circle the correct answer.) a. U-shaped b. V-shaped c. S-shaped Key Concept Check 3. Sequence Describe the stream development stages. Reading Essentials Erosion and Deposition 37
Erosion by Longshore Currents Visual Check 4. Locate Circle the arrows on the diagram that show where the longshore current flows. Shoreline Sediment transport Key Concept Check 5. Explain How does water erosion change Earth s surface? 6. Explain How does water erosion form a cave? Key Concept Check 7. Cause and Effect How does water deposition change Earth s surface? Longshore currents, shown above, are one cause of coastal erosion. Coastal erosion also occurs when waves cut away rocky shorelines. This type of erosion creates sea cliffs. Sea stacks, sea caves, and sea arches are features that form when waves erode less-resistant rocks along the shore. Sea stacks are tall pillars of rock just offshore. Sea arches are rock bridges that extend from the shore into the sea. Groundwater Erosion Water beneath Earth s surface is called groundwater. Flowing groundwater erodes rock and forms features such as caves. When carbon dioxide in the air mixes with rainwater, a weak acid forms. Some of this acidic rainwater seeps through rock and soil into the ground. As the acidic water passes through layers of limestone, it dissolves and washes away the limestone, forming a cave. Water Deposition Flowing water deposits sediment as the water slows. A loss of speed reduces the amount of energy that the water has to carry sediment. Deposition Along Streams Deposition by a stream takes place anywhere along its path where the water s speed decreases. Slower-moving water deposits sediment on the inside curves of meanders. A stream also deposits sediment when it reaches flat land or a body of water, such as a lake or an ocean. A delta is a large deposit of sediment that forms where a stream enters a large body of water. Deposition Along Coastlines Much of the sand on most ocean beaches was originally deposited by rivers. Longshore currents moved the sand along ocean coasts. The sand was eventually deposited where the currents were slower and had less energy. Sandy beaches often form at those locations. 38 Erosion and Deposition Reading Essentials
Groundwater Deposition Weathering and erosion form caves. But many of the structures inside caves form as the result of deposition. Stalactites and stalagmites are cave features that form as dripping groundwater deposits minerals. Stalactites are icicle-like deposits that hang from cave ceilings. Stalagmites build up on the cave s floor. Land Use Practices Damage caused by water erosion can be affected by the ways people use land. Two areas of concern are beaches along coasts and surface areas within continental interiors. Beach Erosion Ocean waves can erode beaches by removing sediment. To reduce this erosion, people sometimes build structures such as retaining walls and groins. Retaining walls are structures that are built parallel to a beach. They help to reduce erosion by waves. Groins are structures that are built at right angles to a beach. As longshore currents flow along a beach, groins trap sediment and reduce the effects of longshore currents. Some ways people affect beaches are unintended. For example, people build dams along rivers. The dams are built to control flooding and for other reasons. Normally, when rivers reach the ocean, they drop sediment. Dams can stop sediment from reaching beaches. As a result, beach sand that is washed out to sea by waves might not be replaced. Surface Erosion Human activities affect surface erosion in several ways. Removing vegetation from the land increases surface erosion. Agricultural production, construction activities, and cutting trees for lumber and paper production are some of the reasons that people remove vegetation. A floodplain is a wide, flat area located next to a river. A floodplain is usually dry land. However, when a river overflows its banks, the dry land can become flooded. Heavy rain or rapid melting of snow can cause a river to flood. Building homes or businesses within a floodplain can be risky. However, flooding makes the soil rich, so it is ideal for farming. One way to reduce flooding on a floodplain is to build a levee. A levee is a long, low ridge of soil built along a river. Reducing flooding also reduces the supply of mineralrich soil. When levees break, such as in New Orleans during Hurricane Katrina in 2005, great flood damage can occur. 8. Describe How do human activities affect water erosion? 9. State How do levees affect erosion on a floodplain? Reading Essentials Erosion and Deposition 39
Key Concept Check 10. Determine How do wind erosion and deposition change Earth s surface? Wind Erosion and Deposition The gentle winds that blow leaves around in autumn are not likely to cause land erosion and deposition. However, strong or long-lasting winds can greatly change the land. Wind Erosion As wind blows, it carries sediment. This sediment cuts and polishes exposed rock. Abrasion is the grinding away of rock or other surfaces as particles carried by wind, water, or ice scrape against them. Wind Deposition Two common types of windblown deposits are dunes and loess (LUHS). A dune is a pile of windblown sand. Dunes form when wind loses energy and drops the sand it is carrying. Entire fields of dunes can move over land as wind blows the sand. The figure below shows how dunes form. Visual Check 11. State What can the wind cause the dune to do? Sand grains blow and bounce up the dune. Dune moves Sand grains drop down the steep face. 12. State What are some ways in which human activities affect wind erosion on farmland? Dune Formation Loess is a crumbly, windblown deposit of silt and clay. One type of loess forms from rock that was ground up and deposited by glaciers. When wind picks up this small, fine-grain sediment and drops it, thick layers of dust called loess form. Land Use Practices Human activity can contribute to wind erosion. One example of this takes place when wind blows across farmlands. Plowed fields and dry, overgrazed pastures leave topsoil exposed to the wind. Strong winds can remove topsoil that is not held in place by plants. One way to reduce the erosion of topsoil by wind is to leave fields unplowed after the crops are harvested. Farmers can also plant rows of trees that slow the wind and protect the farmland. 40 Erosion and Deposition Reading Essentials
Mini Glossary abrasion: the grinding away of rock or other surfaces as particles carried by wind, water, or ice scrape against them delta: a large deposit of sediment that forms where a stream enters a large body of water dune: a pile of windblown sand loess (LUHS): a crumbly, windblown deposit of silt and clay longshore current: a current that flows parallel to the shoreline meander: a broad, C-shaped curve in a stream 1. Review the terms and their definitions in the Mini Glossary. Write a sentence to compare and contrast a dune and loess. 2. Use the terms from the Mini Glossary to complete the table. Use each term only once. Agent of Change Erosion Deposition Water Old streams erode the land to form a. A erodes sediment along a beach. A forms where a river empties into the ocean. Wind What do you think happens when sand cuts and polishes rocks and other surfaces. Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? A is a large pile of sand deposited by the wind. is a layer of silt deposited by the wind. 3. Review your vocabulary cards. Write one term and its meaning on the lines below. ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. END OF LESSON Reading Essentials Erosion and Deposition 41