The Active River. Rivers: Agents of Erosion. Key Concept Water running downhill is the dominant process in shaping the landscape.

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Chad Strickland
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1 What You Will Learn Moving water shapes the surface of Earth by the process of erosion. The sun is the major source of energy that drives the water cycle.

Why It Matters Factors that affect how water moves over the surface of Earth influence the shape of Earth s landscape.

1 1 What You Will Learn Moving water shapes the surface of Earth by the process of erosion. The sun is the major source of energy that drives the water cycle. Three factors that affect the rate of stream erosion are gradient, discharge, and load. Why It Matters Factors that affect how water moves over the surface of Earth influence the shape of Earth s landscape. Vocabulary erosion divide water cycle channel tributary load watershed Graphic Organizer In your Science Journal, create Combination Notes that express information about the stages of river development both in words and in pictures or diagrams. The Active River Key Concept Water running downhill is the dominant process in shaping the landscape. Imagine that you fell asleep with your toes dangling in the Colorado River 6 million years ago. Today, you wake up to find that your toes are hanging about 1.6 km (about 1 mi) above the river! The Colorado River carved the Grand Canyon, shown in Figure 1, by washing billions of tons of rock and sediment from its riverbed. This process can take millions of years. Rivers: Agents of Erosion Six million years ago, the area now known as the Grand Canyon was nearly as flat as a pancake. Tectonic uplift raised the land, and water running downhill became the Colorado River. The Colorado River cut down into the rock and formed the Grand Canyon over millions of years through the process of erosion. Erosion is the process by which wind, water, ice, and gravity move soil and sediment from one place to another. Rivers, such as the Colorado River, are agents of erosion that shape Earth s landscape. Because of erosion caused by water, the Grand Canyon is now about 1.6 km deep and 446 km long. In this section, you will learn about stream development and river systems. You will also learn about factors that affect the rate of stream erosion. landscape? What is erosion, and how does it shape Earth s 6.2.a, Figure 1 The Grand Canyon is located in northwestern Arizona. The canyon formed over millions of years as running water eroded the rock layers. 6.2.a Students know water running downhill is the dominant process in shaping the landscape, including California s landscape. Students know rivers and streams are dynamic systems that erode, transport sediment, change course, and flood their banks in natural and recurring patterns. 6.4.a Students know the sun is the major source of energy for phenomena on Earth s surface; it powers winds, ocean currents, and the water cycle. 366 Chapter 11 Rivers and Groundwater

2 The Water Cycle Have you ever wondered where the water in rivers comes from? This water is part of the water cycle. The water cycle, shown in Figure 2, is the continuous movement of water between the atmosphere, the land, and the oceans. The major source of energy that drives the water cycle is the sun. Describe the water cycle. 6.4.a erosion (ee ROH zhuhn) the process by which wind, water, ice, or gravity transports soil and sediment from one location to another water cycle (WAWT uhr SIE kuhl) the continuous movement of water between the atmosphere, the land, and the oceans Figure 2 The Water Cycle Condensation takes place when water vapor cools and changes into water droplets that form clouds in the atmosphere. Water loses energy during condensation. Precipitation is rain, snow, sleet, or hail that falls from clouds onto Earth s land and oceans. Evaporation takes place when water from the oceans and Earth s surface changes into water vapor. Energy from the sun causes evaporation. Water gains energy during evaporation. Percolation is the downward movement of water through pores and other spaces in soil and is due to gravity. Runoff is water that flows over land into streams and rivers. This water later enters oceans. Section 1 The Active River 367

Erosion Disasters What are the differences between youthful, mature, and old rivers? Create a brochure on the recreational activities that you would recommend at each type of river. Go to go.hrw.

3 Erosion Disasters What are the differences between youthful, mature, and old rivers? Create a brochure on the recreational activities that you would recommend at each type of river. Go to go.hrw.com, and type in the keyword HY7DEPW. tributary (TRIB yoo TER ee) a stream that flows into a lake or into a larger stream watershed (WAWT uhr SHED) the area of land that is drained by a river system divide (duh VIED) the boundary between drainage areas that have streams that flow in opposite directions channel (CHAN uhl) the path that a stream follows River Systems The next time you take a shower, notice that individual drops of water join to become small streams. These streams join other small streams and form larger streams. Eventually, all of the water flows down the drain. Every time you shower, you make a model of a river system. A river system is a network of streams and rivers that drain an area of its runoff. A stream that flows into a lake or into a larger stream is called a tributary. a river system. Watersheds and Divides Describe the difference between a tributary and River systems are divided into regions called watersheds. A watershed is the area of land that is drained by a river system. Watersheds vary in size. The largest watershed in the United States is the Mississippi River watershed. This watershed has hundreds of tributaries. These tributaries extend from the Rocky Mountains, in the West, to the Appalachian Mountains, in the East. A main stream that drains a large watershed and has many tributaries is called a river. The image in Figure 3 shows that the Mississippi River watershed covers more than one-third of the United States. Other major watersheds are the Columbia River, the Rio Grande, and the Colorado River watersheds. Watersheds are separated from each other by an area of higher ground called a divide. Figure 3 The Continental Divide runs through the Rocky Mountains. It separates the watersheds that flow into the Atlantic Ocean and the Gulf of Mexico from those that flow into the Pacific Ocean. The Mississippi River watershed is shown in light green. Continental Divide Missouri R. Arkansas Red R. R. Mississippi R. Ohio R. Tennessee R. Gulf of Mexico 368 Chapter 11 Rivers and Groundwater

Figure 4 California Rivers That Have Different Gradients This stream in John Muir Wilderness has a high gradient. Therefore, the stream flows swiftly downhill over rocks.

Stream Erosion A stream forms as water erodes soil and rock to make a channel. A channel is the path that a stream follows.

The increased water flow from the tributaries causes the stream to become longer and wider. Gradient Figure 4 shows two rivers that have very different gradients.

4 Figure 4 California Rivers That Have Different Gradients This stream in John Muir Wilderness has a high gradient. Therefore, the stream flows swiftly downhill over rocks. Mad River Slough near Eureka, has a low gradient. It flows slowly and has less erosive energy than streams with a high gradient. Stream Erosion A stream forms as water erodes soil and rock to make a channel. A channel is the path that a stream follows. As the stream continues to erode rock and soil, the channel gets wider and deeper. Over time, tributaries join the main channel. The increased water flow from the tributaries causes the stream to become longer and wider. Gradient Figure 4 shows two rivers that have very different gradients. Gradient is the measure of the change in elevation over a certain distance. The water in a stream or river that has a high gradient moves very rapidly. This rapid water flow gives the stream or river a lot of energy to erode rock and soil. A river or stream that has a low gradient has less energy for erosion. channels? How does gradient affect the erosion of stream Discharge The amount of water that a stream or river carries in a given amount of time is called discharge. The discharge of a stream increases when a major storm occurs or when warm weather rapidly melts snow. As the stream s discharge increases, the water s speed and erosive energy increase. The amount of solid material that the stream can carry also increases. Floating down the River At home with a parent or guardian, study a map of the United States. Find the Mississippi River. Imagine that you are planning a rafting trip down the river. On the map, trace the route of your trip from Lake Itasca, Minnesota, to the mouth of the river in Louisiana. If you were floating down the Mississippi River on a raft, what major tributaries would you pass? What cities would you pass? Mark them on the map. How many kilometers would you travel on this trip? Section 1 The Active River 369

load (LOHD) the materials carried by a stream Load The materials carried by a stream are called the stream s load.

Fast-moving streams can carry large particles. Rocks and pebbles bounce and scrape along the bottom and sides of the stream bed.

Streams that move more slowly carry smaller particles and have less erosive energy. How does load affect rates of erosion?

These rocks are called the bed load. A stream can carry sand in suspension.

5 load (LOHD) the materials carried by a stream Load The materials carried by a stream are called the stream s load. The three types of load are shown in Figure 5. The size of the particles in a stream s load is affected by the stream s speed. Fast-moving streams can carry large particles. Rocks and pebbles bounce and scrape along the bottom and sides of the stream bed. Thus, a stream that has a load of large particles has a high rate of erosion. Streams that move more slowly carry smaller particles and have less erosive energy. How does load affect rates of erosion? Figure 5 Three Types of Load A stream can bounce large materials, such as pebbles and boulders, along the stream bed. These rocks are called the bed load. A stream can carry sand in suspension. These materials, called the suspended load, make the river look muddy. The dissolved load is material carried in solution, which means that the material is dissolved in the water. Sodium and calcium are some of the materials in the dissolved load. 370 Chapter 11 Rivers and Groundwater

Quick Lab River s Load As a fast-moving river slows down, some of its load will settle. Use the following procedure to model the ways in which sediments in a river s load may settle. Try It! 1.

6 Quick Lab River s Load As a fast-moving river slows down, some of its load will settle. Use the following procedure to model the ways in which sediments in a river s load may settle. Try It! 1. Add water to a plastic soda bottle until the bottle is two-thirds full. 2. Add 30 g (1/4 cup) of soil, a large handful of pebbles, and 40 g (3 Tbsp) of salt to the bottle. 3. Twist the cap onto the bottle. Shake the bottle vigorously until all of the sediment is mixed in the rapidly moving water. 4. Set the bottle on the table. Draw a diagram of what you observe. Think About It! 5. Which materials best represent a river s bed load, suspended load, and dissolved load? Label each type of load on your diagram. 6. Which sediment settled first? Which sediment settled last? 7. What can you conclude about the size of the sedi ment and the rate at which the sediment settles? 20 min Describing Rivers William Morris Davis was a geomorphologist who studied how rivers shape the landscape. His studies of rivers in the Appalachian Mountains helped him develop a model to describe the stages of river development. In his model, rivers went through a series of stages. Davis described rivers as evolving from a youthful stage to an old-age stage. He thought that all rivers eroded in the same way and at the same rate. Today, scientists support a model that differs from Davis s model. Because materials erode at different rates, one river may develop more quickly than another river does. Many factors, such as climate, local geology, gradient, and load, influence the development of a river. Scientists no longer use Davis s model to explain river development. But they still use many of his terms to describe a river. These terms describe a river s general features, not a river s actual age. Youthful Rivers A youthful river erodes its channel deeper rather than wider. Its channel is narrow and straight. The river tumbles over rocks in rapids and waterfalls. Youthful rivers have very few tributaries. These rivers flow quickly because they have steep gradients. Many youthful rivers have steep gradients because the areas that these rivers drain have been tectonically uplifted. A youthful river in Wyoming is shown in Figure 6. Describe a youthful river. Figure 6 This youthful river is located in Yellowstone National Park in Wyoming. Rapids and falls exist where the river flows over hard, resistant rock. Section 1 The Active River 371

Figure 7 A mature river, such as this one in the Amazon basin of Peru, curves back and forth. Mature Rivers Unlike a youthful river, a mature river erodes its channel wider rather than deeper.

Because of its large watershed, a mature river has more discharge than a youthful river does. A mature river is shown in Figure 7. Notice that a mature river tends to curve back and forth.

Rejuvenated Rivers A rejuvenated (ri JOO vuh NAYT ed) river forms where the land has been raised by tectonic activity.

7 Figure 7 A mature river, such as this one in the Amazon basin of Peru, curves back and forth. Mature Rivers Unlike a youthful river, a mature river erodes its channel wider rather than deeper. The gradient of a mature river is not as steep as that of a youthful river. Therefore, a mature river has fewer falls and rapids. A mature river is fed by many tributaries. Because of its large watershed, a mature river has more discharge than a youthful river does. A mature river is shown in Figure 7. Notice that a mature river tends to curve back and forth. These curves or bends in the river s channel are called meanders. Describe the difference between youthful rivers and mature rivers in terms of how these rivers erode their channels. Rejuvenated Rivers A rejuvenated (ri JOO vuh NAYT ed) river forms where the land has been raised by tectonic activity. When land rises, the river s gradient becomes steeper and the river flows more quickly. This rapid water flow allows the river to cut more deeply into the valley floor. Steplike formations called terraces commonly form on both sides of a stream valley because of rejuvenation, as Figure 8 shows. Figure 8 The rejuvenated river above is located in Canyonlands National Park in Utah. Figure 9 This old river is located in New Zealand. Old Rivers An old river has a low gradient, so it has little erosive energy. Instead of widening and deepening its banks, the river deposits rock and soil in and along its channel. An old river has wide, flat floodplains, or river valleys, and many bends. An old river also has fewer tributaries than a mature river does. An old river has fewer tributaries because its smaller tributaries have joined together. An old river commonly forms an oxbow lake, such as the one shown in Figure 9. An oxbow lake forms when the strip of land that separates parts of a meander is eroded. This process shortens the river s length. 372 Chapter 11 Rivers and Groundwater

Review 6.2.a,, 6.4.a 7 Comparing Which type of river is characterized by flat floodplains? Summary Rivers shape Earth s landscape through the process of erosion.

A watershed is a region that collects runoff water that then becomes part of a river system that drains into a lake or the ocean. Gradient affects the speed at which water flows in a stream.

8 Review 6.2.a,, 6.4.a 7 Comparing Which type of river is characterized by flat floodplains? Summary Rivers shape Earth s landscape through the process of erosion. The sun is the major source of energy that drives the water cycle. A river system is made up of a network of streams and rivers. A watershed is a region that collects runoff water that then becomes part of a river system that drains into a lake or the ocean. Gradient affects the speed at which water flows in a stream. The higher the gradient, the faster the water flows. Water that flows rapidly has a lot of energy for eroding soil and rock. When a stream s discharge increases, the stream s erosive energy also increases. A stream can carry eroded particles as bed load, suspended load, or dissolved load. A stream that has a load of large particles has a high rate of erosion. A river can be described as youthful, mature, old, or rejuvenated based on its characteristics. 1 Use water cycle, tributary, channel, and load in separate sentences. 2 Write an original definition for watershed, erosion, and divide. 3 Evaluating Explain how gradient affects a stream s erosive energy. 4 Describing How did erosion by water running downhill form the Grand Canyon? 5 Analyzing Explain how the sun s energy drives the water cycle. 6 Summarizing What are three factors that affect the rate of stream erosion? 8 Making Inferences How are river systems part of the water cycle? 9 Making Comparisons How do youthful rivers, mature rivers, and old rivers differ? INTERPRETING GRAPHICS Use the pie graph below to answer the next two questions. 0 Evaluating Data Where is most of the water in the world located? q Evaluating Data In what form is the majority of the world s fresh water? w Analyzing Processes Explain how a river system is part of the water cycle. Then, describe how the Earth s water cycle and river systems may be affected if the sun s rays are significantly blocked by pollutants in the atmosphere. Internet Resources For a variety of links related to this chapter, go to Topic: Rivers and Streams SciLinks code: HY

Chapter 10. Running Water aka Rivers. BFRB Pages

Chapter 10. Running Water aka Rivers. BFRB Pages Chapter 10 Running Water aka Rivers BFRB Pages 101-116 Stream Erosion and Transportation Running water is all precipitation (rain, snow, etc) that falls on Earth and is pulled downhill by gravity. Running