sign in sign up
<<
Home , Dissolved load, Stream load

HSES_1eTE_C06.qxd 9/23/04 9:49 PM Page 164

Section 6.2 6.2 The Work of

1 FOCUS

Section Objectives 6.6 Explain how streams erode Key Concepts Vocabulary Reading Strategy their channels and transport How do streams erode ◆ Monitoring Your Understanding Preview . their channels and ◆ capacity the Key Concepts, topic headings, vocabulary, transport ? ◆ and figures in this section. List two things you 6.7 Explain how expect to learn about each. After reading, state How does stream ◆ delta occurs. what you learned about each item you listed. deposition occur? ◆ natural 6.8 Identify the two general types What are the two types ◆ floodplain What I Expect to Learn What I Learned of stream valleys. of stream valleys? ◆ flood 6.9 Predict the causes of What causes floods, and ◆ what are the major flood and describe major ◆ divide control measures? control measures. What is the relationship 6.10 Explain the relationship between a stream and between streams and drainage a drainage basin? basins.

Reading Focus Streams are Earth’s most important agents of . They can downcut or erode their channels. They can also transport enormous L2 Build Vocabulary amounts of sediment. Most of the sediment a stream carries comes Paraphrase Ask students to write the from . Weathering produces huge amounts of material that vocabulary terms on a sheet of paper. are delivered to the stream by sheet flow, mass movements, and Instruct students to write a definition, in groundwater. Eventually, streams drop much of this material to create their own words, for each term as they many different depositional features. encounter the term while going through the chapter. After writing their own Erosion definition, encourage students to write Streams generally erode their channels lifting loose particles by the term in a complete sentence. , grinding, and by dissolving soluble material. When the L2 flow of water is turbulent enough, it can dislodge loose particles from Reading Strategy the and lift them into the moving water. In this manner, the Answers will vary. Sample answer: force of running water rapidly erodes some streambeds and banks. What I Expect to Learn—how erosion The stronger the is, the more erosional power it has and the happens and what features it can form more effectively the water will pick up particles. What I Learned—streams erode by Sand and gravel carried in a stream can erode solid rock channels lifting loose particles and by abrasion like sandpaper grinds down wood. Moreover, pebbles caught in swirling stream currents can act like cutting tools and bore circular “potholes” into the channel floor. 2 INSTRUCT What are three ways that streams erode their channels? Erosion Build Reading Literacy L1 Refer to p. 334D in Chapter 12, which 164 Chapter 6 provides guidelines for outlining. Outline Have students outline the section, leaving room for notes. Then have students scan through each heading and find the main idea. Allow students to refer to their outlines when answering the questions in Section 6.2 Assessment. Logical, Verbal

164 Chapter 6 HSES_1eTE_C06.qxd 5/16/04 9:09 AM Page 165

Sediment Transport Integrate Chemistry L2 Streams transport sediment in three ways. and Suspensions Streams 1. in () transport sediment in solution and in 2. in suspension () suspension. In chemistry, a solution is 3. scooting or rolling along the bottom (bed load) a homogeneous mixture of dissolved substances. A suspension is a Dissolved Load Most of the dissolved load enters streams heterogeneous mixture that separates through groundwater. Some of this load also enters by dissolving rock into layers over time. Ask students to along the stream’s course. The amount of material the stream carries write a paragraph explaining how in solution changes depending on and the geologic setting. streams are both solutions and Usually the dissolved load is expressed as parts of dissolved material suspensions. (Streams are composed, per million parts of water (parts per million, or ppm). Some may in part, of groundwater that contains have a dissolved load of 1000 ppm or more. However, the average dissolved substances, and in this way figure for the world’s rivers is estimated at 115 to 120 ppm. Streams streams are solutions. Streams also supply almost 4 billion metric tons of dissolved substances to the transport fine sand, , and clay that are oceans each year. not dissolved, but rather suspended in moving water. In this way, streams are Suspended Load Most streams carry suspensions.) the largest part of their load in suspension. Verbal The visible cloud of sediment suspended in the water is the most obvious portion of a Build Science Skills L2 stream’s load. Streams usually carry only sand, silt, and clay this way. However, streams Using Models Some students will have also transport larger particles during a flood difficulty comprehending the tiny scale because water velocity increases. The total of one part per million. To help build amount of material a stream carries in sus- number sense and awareness of how pension increases dramatically during floods, small a part per million is, ask students as shown in Figure 8. to imagine they have a budget of one million dollars, and have them think of Bed Load Bed load is that part of a the cost of their lunch in terms of parts stream’s load of solid material that is made up per million. A four-dollar lunch would be of sediment too large to be carried in suspen- equal to 4 ppm. This represents only a sion. These larger, coarser particles move along the bottom, or bed, of Figure 8 During this 1997 flood, tiny fraction of their total budget. the suspended load in the muddy Logical the stream channel. The suspended and dissolved loads are always Ohio is clearly visible. The moving. But the bed load moves only when the force of the water is greatest erosion and sediment great enough to move the larger particles. The grinding action of the transport occur during floods. Applying Concepts What other bed load is very important in eroding the stream channel. types of load might account for the muddiness of the river? Competence and Capacity The ability of streams to carry a load is determined by two factors: the stream’s competence and its capacity. Competence of a stream measures the largest particles it can transport. A stream’s competence increases with its velocity. In fact, the competence of a stream increases four times when the velocity doubles. The capacity of a stream is the maximum load it can carry. Capacity is directly related to a stream’s . The greater the volume of water in a stream is, the greater its capacity is for carrying sediment.

Running Water and Groundwater 165

Customize for English Language Learners

Students who are learning English can benefit news. Have them discuss the type of damage from real-life examples that relate to science done by the flood and some of the amazing content. Encourage students to think of actual pictures of rescues and houses floating flooding events that may have occurred in downstream. Encourage students to share Answer to . . . your area or that they have heard about on the their knowledge and examples with the class. Figure 8 Dissolved load might account for the muddiness of the river. Streams erode their channels lifting loose particles by abrasion, grinding, and by dissolving soluble material.

Running Water and Groundwater 165 HSES_1eTE_C06.qxd 9/23/04 11:04 PM Page 166

Section 6.2 (continued) Mississippi Delta Region

A

t Deposition c Figure 9 h a Baton Rouge fa la M Movement This map y a is shows the growth of the R s i is v s Mississippi River delta over e ippi Riv r er the past 5,000 to 6,000 years. New Orleans Alluvium L2 As you can see, the river has 2 built a series of sub-deltas, Purpose Students learn how one after the other. The deposition occurs in a stream. numbers indicate the order in 4 Materials mixed sediment (pebbles, which they were deposited. 6 Locating In which overall sand, silt, and clay-sized particles), clear direction has the Mississippi 1 3 glass jar with lid, water River built its delta over the 5 7 past few thousand years? Procedure Fill the jar about one-third Gulf of Mexico full with the mixed sediment. Pour water Locating How has the growth of the delta changed into the jar and tightly close the lid. the location of the mouth of Shake the jar and allow sediment to the Mississippi River in settle out. relation to New Orleans? Expected Outcome Sediment should settle out in layers, with pebbles on the bottom and clay-sized particles at the Deposition top. The water sorts the material as the energy from the shaking dissipates, the Whenever a stream slows down, the situation reverses. As a stream’s heavier material drops out sequentially. velocity decreases, its competence decreases and sediment begins to drop If this is not illustrated, try using a larger out, largest particles first. Each particle size has a critical settling veloc- jar, which will allow more room for the ity. Deposition occurs as streamflow drops below the critical materials to settle out. settling velocity of a certain particle size. The sediment in that cate- gory begins to settle out. Stream transport separates solid particles of various sizes, large to small. This process is called sorting. It explains why particles of similar size are deposited together. The sorted material deposited by as stream is called alluvium. Many different depositional features are made of alluvium. Some occur within stream channels. Some occur on the floor next to the Answers channel. And others occur at the mouth of a stream. Locating southeast Locating New Orleans gets farther Deltas When a stream enters the relatively still waters of an ocean from the mouth as the delta is built. or lake, its velocity drops. As a result, the stream deposits sediment and forms a delta. A delta is an accumulation of sediment formed where a stream enters a lake or ocean. As a delta grows outward, the stream’s gradient lessens and the water slows down. The channel becomes choked with sediment settling out of the slow-moving water. As a result, the river changes direction as it seeks a shorter route to base level. The main channel often divides into several smaller channels called as shown in sub-delta 7 in Figure 9. These shifting channels act in the opposite way of .

166 Chapter 6

Facts and Figures

The city of New Orleans, Louisiana, is built on If they dug down only a few feet, the grave a delta at the mouth of the Mississippi River. filled with water and caused the casket to As is expected, the water table in this area is float. very high due to the fact that the delta is built Finally, settlers adopted another method right into the ocean. This high water table of burial. They built above-ground vaults. leaves New Orleans with a troubling Today many of the cemeteries in New Orleans problem—how do they bury their dead? have tombs arranged in a street-like fashion. Early settlers were forced to bury their dead In fact, the cemeteries are often referred to in shallow graves due to the high water table. as “cities of the dead.”

166 Chapter 6 HSES_1eTE_C06.qxd 5/16/04 9:10 AM Page 167

Stream Valleys Rather than carrying water into the main channel like tributaries, L2 distributaries carry water away. After many shifts of the channel, a delta Integrate Language Arts may grow into a triangular shape, like the Greek letter delta (⌬). Prefixes Remind students that a However, not all deltas have this idealized shape. Differences in the is a stream that empties into shapes of shorelines and variations in the strength of waves and cur- another stream (p. 162). The text rents result in different shapes of deltas. provides a contextual definition of , but tell students that even Natural Some rivers occupy valleys with broad, flat floors. without this context, they could deduce Successive floods over many years can build natural levees along them. that a distributary is the opposite of a A natural levee is a landform that parallels some streams. They form tributary by knowing the prefix dis- when a stream overflows its banks. When it overflows, its velocity rap- means, among other things, “opposite idly decreases and leaves coarse sediment deposits in strips that border of.” Encourage students to make a list of the channel. As the water spreads out over the valley, less sediment is common word parts as they read. Have deposited. This uneven distribution of material produces the gentle Figure 10 The Yellowstone River them look up each one in a dictionary. is an example of a V-shaped Doing so will help them with new slope of the natural levee. valley. The and show that the river is vigorously vocabulary and verbal portions of Stream Valleys the channel. standardized tests. Narrow Valleys The Yellowstone River, shown in Verbal Figure 10, is an excellent example of a narrow valley. A L2 narrow V-shaped valley shows that the stream’s primary Build Science Skills work has been downcutting toward base level. Rapids and Designing are the most prominent features of a narrow valley. Experiments Divide Both rapids and waterfalls occur where the stream profile students into groups drops rapidly. The variations in the erosion of the underlying and ask them to model bedrock cause these rapid drops. a delta using a sloped paint tray (the type used with rollers), sand, and a Wide Valleys Once a stream has cut its channel closer to constant supply of water (such as from a base level, downward erosion becomes less dominant. More hose or faucet). (First, dampen the sand. of the stream’s energy is directed from side to side. The result Then distribute the sand evenly in is a widening of the valley as the river cuts away first at one a thin layer over the sloped part of the and then at the other. tray. Supply a gentle but constant flow of The side-to-side cutting of a stream eventually produces a water to the top of the slope, and observe flat valley floor, or floodplain. A floodplain is appropriately the channel the water makes in the sand. named because during a flood the river overflows its banks Next, observe how some sand is eroded and floods the plain. and transported to the mouth of the Streams that flow on floodplains move in meanders. Once channel, where it settles into the pool of a bend in a channel begins to form, it grows larger. Most of water at the flat part of the tray.) the erosion occurs on the outside of the —often Kinesthetic, Visual called the —where velocity and turbulence are great- est. Much of the debris the stream removes at the cut bank moves downstream where it is deposited as point bars. Point bars form in zones of decreased velocity on the insides of meanders. In this way, meanders move side to side by eroding the outside of bends and depositing on the inside.

Running Water and Groundwater 167

Running Water and Groundwater 167 HSES_1eTE_C06.qxd 5/16/04 9:10 AM Page 168

Oxbow lake Section 6.2 (continued) Figure 11 A One Plugs with meander has overtaken silt and clay the next, forming a Build Science Skills L2 ring of water on the floodplain. B After Inferring Draw on the board a cross deposits of sediment section of the river valley. (a broad cut off the ring, an A B relatively flat-bottomed shape, similar to a forms. horizontal bracket) Then have students draw cross sections representing at least Erosion is more effective on the downstream side of a meander three earlier stages in the development because of the slope of the channel. The bends gradually travel down the of the valley. Tell them the first stage valley. Sometimes the downstream movement of a meander slows when should be of a time when waterfalls and it reaches a more resistant portion of the floodplain. This resistance allows rapids were common along the river’s the next meander upstream to overtake it, as shown in Figure 11. course. (Cross sections should show a Gradually the neck of land between the meanders is narrowed. narrowing valley with the earliest cross Eventually the river may erode through the narrow neck of land to the section showing a V-shaped valley.) next loop. The new, shorter channel segment is called a cutoff and, Logical because of its shape, the abandoned bend is called an oxbow lake. Such Use Visuals L1 a situation is shown in the bottom portion of Figure 6 on page 163. Figure 11 Have students study the Floods and streams shown in the figures. Ask: Are A flood occurs when the discharge of a stream becomes so great that it other oxbow lakes shown on these exceeds the capacity of its channel and overflows its banks. Floods are diagrams? (Yes, the remnants of two the most common and most destructive of all natural geologic hazards. other crescent-shaped lakes are shown.) Most floods are caused by rapid snow melt or storms What does this show you about the that bring heavy rains over a large region. Heavy rains caused the river? (The oxbow lakes indicate previous devastating floods in the upper Mississippi River Valley during the positions of the river.) Where might the next oxbow lake form along this summer of 1993, as shown in Figure 12. section of river? (It will most likely form Unlike far-reaching regional floods, flash floods are more limited along the course at the lower part of the in extent. However, flash floods occur with little warning, and they can diagrams, where a neck is forming at the be deadly as walls of water sweep through river valleys. Several factors base of a meander that loops to the left. Some students might think the course of the river at the top of the diagrams represents an even narrower neck that is Mississippi River Flooding July 18, 1993 not shown but would pinch off the large Mississippi Figure 12 meander to the right, forming an oxbow July 4, 1988 lake.) Region These satellite Mississippi River Visual, Logical images show the confluence of the Missouri and Mississippi rivers. The River Floods and Flood first photo shows the rivers Control during normal flow. Interpreting Missouri River Photographs What does Missouri River the second satellite image show? How do you know?

Answer Interpreting Photographs The 168 Chapter 6 second satellite image shows the rivers during flood stage. You can tell the rivers are at flood stage because the area covered by water is much wider. Facts and Figures

On May 31, 1889, residents of Johnstown, million tons of water crashed down the Pennsylvania, heard what sounded like a roar river valley made narrower by the growing of thunder. Their worst fears were realized. community. Over 2200 people died as a result The South Fork , located 22.4 km of the flood and the aftermath of fires that upstream along the Little Conemaugh River, followed. broke after a night of heavy rain. Twenty

168 Chapter 6 HSES_1eTE_C06.qxd 5/16/04 9:11 AM Page 169

influence flash floods: rainfall intensity and duration, surface condi- tions, and topography. As you have learned, many urban areas are L2 susceptible to flash floods. Mountainous areas are also susceptible Some students may have heard the term because steep slopes can send runoff into narrow . Q Sometimes a major flood is Human interference with the stream system can worsen or even cause described as a 100-year flood. “100-year flood” and assume that this floods. A prime example is the failure of a dam or an artificial levee. These What does that mean? means that floods only occur in the area structures are designed to contain floods of a certain size. If that size is A The phrase “100-year flood” every 100 years. This is not true and is exceeded, water can then spill over or break through a dam or levee and is misleading because it makes often misleading. A major flood can occur rush downstream causing a disastrous flood. people believe that such an along a river during any year. What the event happens only once every There are several flood control strategies. Measures to control term does suggest, however, is that 100 years. In truth, a huge statistically speaking there is a 1 in 100 flooding include artificial levees, flood control , and placing flood can happen any year. The limits on floodplain development. phrase “100-year flood” is really chance that in any given year, a major a statistical designation. It indi- flood will occur. The chances of flooding Artificial Levees Artificial levees are earthen mounds built on cates that there is a 1-in-100 along any stream are reevaluated chance that a flood this size will periodically. It has been found that the the banks of a river. These levees increase the volume of water a happen during any year. channel can hold. When levees confine a river during periods of high Perhaps a better term would be flooding likelihood changes and that water, the river often deposits material in its channel as the discharge the “1-in-100 chance flood.” major flooding may occur more diminishes. This discharge is sediment that would have been dropped frequently than every 100 years. Ask on the floodplain. Because the stream cannot deposit material outside students to predict what sort of human of its channel the bottom of the channel is gradually built up. When activity may increase the likelihood of the channel is built up, it takes less water to overflow the levee. As a flooding. (Urban development and the building of dams may increase the amount result, people may have to raise the height of the levee periodically to of serious flooding in an area.) protect the floodplain behind it. Moreover, many artificial levees are Logical not built to withstand periods of extreme flooding. For example, there were many levee failures in the Midwest during the summer of 1993 when the upper Mississippi experienced record flooding.

Flood-Control Dam Flood-control dams store floodwater and then let it out slowly. Since the 1920s, thousands of dams have been built on nearly every major river in the United States. Many dams have other non-flood related functions, such as providing water for irriga- tion and for hydroelectric power generation. Although dams may reduce flooding and provide other benefits, building dams has consequences. For example, dams trap sediment. Deltas and floodplains downstream can erode because silt no longer replenishes them during floods. Built up sediment behind a dam means the volume of the stored water will gradually diminish. This build-up reduces the effectiveness of the dam for flood control. Large dams also cause ecological damage to river environments.

Limiting Development Today many scientists and engineers advocate sound floodplain management instead of building structures. That often means preserving in their natural state. Minimizing development on floodplains allows them to absorb flood- waters with little harm to homes and businesses.

Running Water and Groundwater 169

Running Water and Groundwater 169 HSES_1eTE_C06.qxd 5/16/04 9:12 AM Page 170

Section 6.2 (continued) Drainage Basins Every stream has a drainage basin. A drainage basin is the land Drainage Basins area that contributes water to a stream. An imaginary line called a L1 Use Visuals divide separates the drainage basins of one stream from another. Figure 13 Point out the Mississippi River Divides range in scale from a ridge separating two small on a drainage basin. Ask: Do other drainage hillside to a continental divide, which splits continents into enormous basins exist within this one? (Yes, every Figure 13 Mississippi River drainage basins. The Mississippi River has the largest drainage basin in Drainage Basin Divides are the stream, regardless of size, has its own boundaries that separate North America. See Figure 13. The river and its tributaries collect water drainage basin. A larger river, such as the drainage basins from each other. from more than 3.2 million square kilometers of the continent. Mississippi, will have a drainage basin that includes those of all of its tributaries.) Where is the divide that is commonly called the Continental Divide? (This is Misso ur the western portion of the Mississippi River i Divide R i v drainage basin divide that runs through e r the Rocky Mountains.) Mississippi River Use Community Drainage Basin L2 M Resources is s ver is i s R

i io

Drainage Basins Invite a hydrologist p Oh

p i

to speak to the class about a drainage R i

v e Divide

basin in your area. Have students trace r out the drainage basin of a local stream

or river and discuss their findings with Divide the scientist. Interpersonal

3 ASSESS Evaluate Understanding L2 Section 6.2 Assessment To assess students’ knowledge of section content, have them create a visual showing a narrow and a wide stream Reviewing Concepts 6. Summarizing Explain the formation of one valley. 1. How do streams erode their channels? of the landforms that streams create by deposition. 2. What causes floods? Reteach L1 3. What is the relationship between a stream and a drainage basin? Have students make a chart summarizing 4. How do streams transport sediments? the differences between erosion and deposition. Critical Thinking Descriptive Paragraph Use library sources or the Internet to research the 5. Analyzing Concepts How does urban causes of a recent major flood. Write a development interfere with the natural paragraph that tells the name of the flood, function of floodplains? when it happened, where it happened, and Student paragraphs should describe the conditions that led to the flood itself. accurately researched floods and their causes and effects. 170 Chapter 6

Section 6.2 Assessment 4. Streams transport sediment in solution, in suspension, and by rolling along the bottom. 1. Streams erode their channels by lifting 5. Urban development can decrease the effec- loose particles by abrasion, grinding, and by tiveness of floodplains by replacing water- dissolving soluble material. absorbing vegetation with concrete and 2. Floods occur when the discharge of a asphalt and increasing flooding. stream exceeds the capacity of the channel. 6. Sample answer: Deltas are formed as accu- Most floods are caused by rapid spring snow mulating sediment is deposited where a melt and storms. stream or river enters a lake or ocean. 3. A drainage basin is the land area that contributes water to a stream.

170 Chapter 6