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164 Chapter 6

6.2 The Work of Streams

Reading StrategyMonitoring Your Understanding Previewthe Key Concepts, topic headings, vocabulary,and figures in this section. List two things youexpect to learn about each. After reading, statewhat you learned about each item you listed.

Key ConceptsHow do streams erodetheir channels andtransport sediment?

How does streamdeposition occur?

What are the two typesof stream valleys?

What causes floods, andwhat are the major floodcontrol measures?

What is the relationshipbetween a stream anda drainage basin?

Vocabulary◆ bed load◆ capacity◆ alluvium◆ delta◆ natural levee◆ floodplain◆ flood◆ drainage basin◆ divide

Streams are Earth’s most important agents of erosion. They candowncut or erode their channels. They can also transport enormousamounts of sediment. Most of the sediment a stream carries comesfrom weathering. Weathering produces huge amounts of material thatare delivered to the stream by sheet flow, mass movements, andgroundwater. Eventually, streams drop much of this material to createmany different depositional features.

ErosionStreams generally erode their channels lifting loose particles by

abrasion, grinding, and by dissolving soluble material. When theflow of water is turbulent enough, it can dislodge loose particles fromthe channel and lift them into the moving water. In this manner, theforce of running water rapidly erodes some streambeds and banks.The stronger the current is, the more erosional power it has and themore effectively the water will pick up particles.

Sand and gravel carried in a stream can erode solid rock channelslike sandpaper grinds down wood. Moreover, pebbles caught inswirling stream currents can act like cutting tools and bore circular“potholes” into the channel floor.

What are three ways that streams erode theirchannels?

What I Expect to Learn What I Learned

164 Chapter 6

FOCUS

Section Objectives6.6 Explain how streams erode

their channels and transportsediments.

6.7 Explain how stream depositionoccurs.

6.8 Identify the two general typesof stream valleys.

6.9 Predict the causes of floodsand describe major floodcontrol measures.

6.10 Explain the relationshipbetween streams and drainagebasins.

Build VocabularyParaphrase Ask students to write thevocabulary terms on a sheet of paper.Instruct students to write a definition, intheir own words, for each term as theyencounter the term while going throughthe chapter. After writing their owndefinition, encourage students to writethe term in a complete sentence.

Reading StrategyAnswers will vary. Sample answer:What I Expect to Learn—how erosionhappens and what features it can formWhat I Learned—streams erode bylifting loose particles and by abrasionmeanders

INSTRUCT

ErosionBuild Reading LiteracyRefer to p. 334D in Chapter 12, whichprovides guidelines for outlining.

Outline Have students outline thesection, leaving room for notes. Thenhave students scan through each headingand find the main idea. Allow studentsto refer to their outlines when answeringthe questions in Section 6.2 Assessment.Logical, Verbal

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Reading Focus

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Section 6.2

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Sediment TransportStreams transport sediment in three ways.

1. in solution (dissolved load)2. in suspension (suspended load)3. scooting or rolling along the bottom (bed load)

Dissolved Load Most of the dissolved load enters streamsthrough groundwater. Some of this load also enters by dissolving rockalong the stream’s course. The amount of material the stream carriesin solution changes depending on climate and the geologic setting.Usually the dissolved load is expressed as parts of dissolved materialper million parts of water (parts per million, or ppm). Some rivers mayhave a dissolved load of 1000 ppm or more. However, the averagefigure for the world’s rivers is estimated at 115 to 120 ppm. Streamssupply almost 4 billion metric tons of dissolved substances to theoceans each year.

Suspended Load Most streams carrythe largest part of their load in suspension.The visible cloud of sediment suspended inthe water is the most obvious portion of astream’s load. Streams usually carry onlysand, silt, and clay this way. However, streamsalso transport larger particles during a floodbecause water velocity increases. The totalamount of material a stream carries in sus-pension increases dramatically during floods,as shown in Figure 8.

Bed Load Bed load is that part of astream’s load of solid material that is made upof sediment too large to be carried in suspen-sion. These larger, coarser particles move along the bottom, or bed, ofthe stream channel. The suspended and dissolved loads are alwaysmoving. But the bed load moves only when the force of the water isgreat enough to move the larger particles. The grinding action of thebed load is very important in eroding the stream channel.

Competence and Capacity The ability of streams to carry aload is determined by two factors: the stream’s competence and itscapacity. Competence of a stream measures the largest particles it cantransport. A stream’s competence increases with its velocity. In fact, thecompetence of a stream increases four times when the velocity doubles.

The capacity of a stream is the maximum load it can carry. Capacityis directly related to a stream’s discharge. The greater the volume ofwater in a stream is, the greater its capacity is for carrying sediment.

Running Water and Groundwater 165

Figure 8 During this 1997 flood,the suspended load in the muddyOhio River is clearly visible. Thegreatest erosion and sedimenttransport occur during floods. Applying Concepts What othertypes of load might account forthe muddiness of the river?

Sediment TransportIntegrate ChemistrySolutions and Suspensions Streamstransport sediment in solution and insuspension. In chemistry, a solution isa homogeneous mixture of dissolvedsubstances. A suspension is aheterogeneous mixture that separatesinto layers over time. Ask students towrite a paragraph explaining howstreams are both solutions andsuspensions. (Streams are composed,in part, of groundwater that containsdissolved substances, and in this waystreams are solutions. Streams alsotransport fine sand, silt, and clay that arenot dissolved, but rather suspended inmoving water. In this way, streams aresuspensions.)Verbal

Build Science SkillsUsing Models Some students will havedifficulty comprehending the tiny scaleof one part per million. To help buildnumber sense and awareness of howsmall a part per million is, ask studentsto imagine they have a budget of onemillion dollars, and have them think ofthe cost of their lunch in terms of partsper million. A four-dollar lunch would beequal to 4 ppm. This represents only atiny fraction of their total budget.Logical

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Customize for English Language Learners

Students who are learning English can benefitfrom real-life examples that relate to sciencecontent. Encourage students to think of actualflooding events that may have occurred inyour area or that they have heard about on the

news. Have them discuss the type of damagedone by the flood and some of the amazingpictures of rescues and houses floatingdownstream. Encourage students to sharetheir knowledge and examples with the class.

Answer to . . .

Figure 8 Dissolved load mightaccount for the muddiness of the river.

Streams erode theirchannels lifting loose

particles by abrasion, grinding, andby dissolving soluble material.

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Baton Rouge

New Orleans

Gulf of Mexico

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Atchafalaya River

Mississippi River

166 Chapter 6

DepositionWhenever a stream slows down, the situation reverses. As a stream’svelocity decreases, its competence decreases and sediment begins to dropout, largest particles first. Each particle size has a critical settling veloc-ity. Deposition occurs as streamflow drops below the criticalsettling velocity of a certain particle size. The sediment in that cate-gory begins to settle out. Stream transport separates solid particles ofvarious sizes, large to small. This process is called sorting. It explains whyparticles 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 occurwithin stream channels. Some occur on the valley floor next to thechannel. And others occur at the mouth of a stream.

Deltas When a stream enters the relatively still waters of an oceanor lake, its velocity drops. As a result, the stream deposits sediment andforms a delta. A delta is an accumulation of sediment formed where astream enters a lake or ocean. As a delta grows outward, the stream’sgradient lessens and the water slows down. The channel becomeschoked with sediment settling out of the slow-moving water. As aresult, the river changes direction as it seeks a shorter route to baselevel. The main channel often divides into several smaller channelscalled distributaries as shown in sub-delta 7 in Figure 9. These shiftingchannels act in the opposite way of tributaries.

Mississippi Delta Region

Figure 9

Movement This mapshows the growth of theMississippi River delta overthe past 5,000 to 6,000 years.As you can see, the river hasbuilt a series of sub-deltas,one after the other. Thenumbers indicate the order inwhich they were deposited.Locating In which overalldirection has the MississippiRiver built its delta over thepast few thousand years?Locating How has thegrowth of the delta changedthe location of the mouth ofthe Mississippi River inrelation to New Orleans?

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Deposition

AlluviumPurpose Students learn howdeposition occurs in a stream.

Materials mixed sediment (pebbles,sand, silt, and clay-sized particles), clearglass jar with lid, water

Procedure Fill the jar about one-thirdfull with the mixed sediment. Pour waterinto the jar and tightly close the lid.Shake the jar and allow sediment tosettle out.

Expected Outcome Sediment shouldsettle out in layers, with pebbles on thebottom and clay-sized particles at thetop. The water sorts the material as theenergy from the shaking dissipates, theheavier material drops out sequentially.If this is not illustrated, try using a largerjar, which will allow more room for thematerials to settle out.

AnswersLocating southeastLocating New Orleans gets fartherfrom the mouth as the delta is built.

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Section 6.2 (continued)

The city of New Orleans, Louisiana, is built ona delta at the mouth of the Mississippi River.As is expected, the water table in this area isvery high due to the fact that the delta is builtright into the ocean. This high water tableleaves New Orleans with a troublingproblem—how do they bury their dead?

Early settlers were forced to bury their deadin shallow graves due to the high water table.

If they dug down only a few feet, the gravefilled with water and caused the casket tofloat.

Finally, settlers adopted another methodof burial. They built above-ground vaults.Today many of the cemeteries in New Orleanshave tombs arranged in a street-like fashion.In fact, the cemeteries are often referred toas “cities of the dead.”

Facts and Figures

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Rather than carrying water into the main channel like tributaries,distributaries carry water away. After many shifts of the channel, a deltamay grow into a triangular shape, like the Greek letter delta (�).However, not all deltas have this idealized shape. Differences in theshapes of shorelines and variations in the strength of waves and cur-rents result in different shapes of deltas.

Natural Levees Some rivers occupy valleys with broad, flat floors.Successive floods over many years can build natural levees along them.A natural levee is a landform that parallels some streams. They formwhen a stream overflows its banks. When it overflows, its velocity rap-idly decreases and leaves coarse sediment deposits in strips that borderthe channel. As the water spreads out over the valley, less sediment isdeposited. This uneven distribution of material produces the gentleslope of the natural levee.

Stream ValleysNarrow Valleys The Yellowstone River, shown in Figure 10, is an excellent example of a narrow valley. Anarrow V-shaped valley shows that the stream’s primarywork has been downcutting toward base level. Rapids andwaterfalls are the most prominent features of a narrow valley.Both rapids and waterfalls occur where the stream profiledrops rapidly. The variations in the erosion of the underlyingbedrock cause these rapid drops.

Wide Valleys Once a stream has cut its channel closer tobase level, downward erosion becomes less dominant. Moreof the stream’s energy is directed from side to side. The resultis a widening of the valley as the river cuts away first at onebank and then at the other.

The side-to-side cutting of a stream eventually produces aflat valley floor, or floodplain. A floodplain is appropriatelynamed because during a flood the river overflows its banksand floods the plain.

Streams that flow on floodplains move in meanders. Oncea bend in a channel begins to form, it grows larger. Most ofthe erosion occurs on the outside of the meander—oftencalled the cut bank—where velocity and turbulence are great-est. Much of the debris the stream removes at the cut bankmoves downstream where it is deposited as point bars. Pointbars form in zones of decreased velocity on the insides ofmeanders. In this way, meanders move side to side by erodingthe outside of bends and depositing on the inside.

Figure 10 The Yellowstone Riveris an example of a V-shapedvalley. The rapids and waterfallshow that the river is vigorouslydowncutting the channel.


Stream ValleysIntegrate Language ArtsPrefixes Remind students that atributary is a stream that empties intoanother stream (p. 162). The textprovides a contextual definition ofdistributary, but tell students that evenwithout this context, they could deducethat a distributary is the opposite of atributary by knowing the prefix dis-means, among other things, “oppositeof.” Encourage students to make a list ofcommon word parts as they read. Havethem look up each one in a dictionary.Doing so will help them with newvocabulary and verbal portions ofstandardized tests.Verbal

Build Science SkillsDesigningExperiments Dividestudents into groupsand ask them to modela delta using a sloped paint tray (thetype used with rollers), sand, and aconstant supply of water (such as from ahose or faucet). (First, dampen the sand.Then distribute the sand evenly ina thin layer over the sloped part of thetray. Supply a gentle but constant flow ofwater to the top of the slope, and observethe channel the water makes in the sand.Next, observe how some sand is erodedand transported to the mouth of thechannel, where it settles into the pool ofwater at the flat part of the tray.)Kinesthetic, Visual

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Missouri River

July 4, 1988July 4, 1988

Mississippi River

Mississippi River

Missouri River

July 18, 1993July 18, 1993

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Erosion is more effective on the downstream side of a meanderbecause of the slope of the channel. The bends gradually travel down thevalley. Sometimes the downstream movement of a meander slows whenit reaches a more resistant portion of the floodplain. This resistance allowsthe next meander upstream to overtake it, as shown in Figure 11.Gradually the neck of land between the meanders is narrowed.Eventually the river may erode through the narrow neck of land to thenext loop. The new, shorter channel segment is called a cutoff and,because of its shape, the abandoned bend is called an oxbow lake. Sucha situation is shown in the bottom portion of Figure 6 on page 163.

Floods and Flood ControlA flood occurs when the discharge of a stream becomes so great that itexceeds the capacity of its channel and overflows its banks. Floods arethe most common and most destructive of all natural geologic hazards.

Most floods are caused by rapid spring snow melt or stormsthat bring heavy rains over a large region. Heavy rains caused thedevastating floods in the upper Mississippi River Valley during thesummer of 1993, as shown in Figure 12.

Unlike far-reaching regional floods, flash floods are more limitedin extent. However, flash floods occur with little warning, and they canbe deadly as walls of water sweep through river valleys. Several factors

Oxbow lakePlugs withsilt and clay

Figure 11 A Onemeander has overtakenthe next, forming a ring of water on thefloodplain. B Afterdeposits of sedimentcut off the ring, anoxbow lake forms.

A B

Figure 12

Region These satelliteimages show theconfluence of the Missouriand Mississippi rivers. Thefirst photo shows the riversduring normal flow.InterpretingPhotographs What doesthe second satellite imageshow? How do you know?

Mississippi River Flooding

168 Chapter 6

Build Science SkillsInferring Draw on the board a crosssection of the river valley. (a broadrelatively flat-bottomed shape, similar to ahorizontal bracket) Then have studentsdraw cross sections representing at leastthree earlier stages in the developmentof the valley. Tell them the first stageshould be of a time when waterfalls andrapids were common along the river’scourse. (Cross sections should show anarrowing valley with the earliest crosssection showing a V-shaped valley.)Logical

Use VisualsFigure 11 Have students study thestreams shown in the figures. Ask: Areother oxbow lakes shown on thesediagrams? (Yes, the remnants of twoother crescent-shaped lakes are shown.)What does this show you about theriver? (The oxbow lakes indicate previouspositions of the river.) Where might thenext oxbow lake form along thissection of river? (It will most likely formalong the course at the lower part of thediagrams, where a neck is forming at thebase of a meander that loops to the left.Some students might think the course ofthe river at the top of the diagramsrepresents an even narrower neck that isnot shown but would pinch off the largemeander to the right, forming an oxbowlake.)Visual, Logical

Floods and FloodControl

AnswerInterpreting Photographs Thesecond satellite image shows the riversduring flood stage. You can tell therivers are at flood stage because thearea covered by water is much wider.

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On May 31, 1889, residents of Johnstown,Pennsylvania, heard what sounded like a roarof thunder. Their worst fears were realized.The South Fork Dam, located 22.4 kmupstream along the Little Conemaugh River,broke after a night of heavy rain. Twenty

million tons of water crashed down theriver valley made narrower by the growingcommunity. Over 2200 people died as a resultof the flood and the aftermath of fires thatfollowed.

Facts and Figures



influence flash floods: rainfall intensity and duration, surface condi-tions, and topography. As you have learned, many urban areas aresusceptible to flash floods. Mountainous areas are also susceptiblebecause steep slopes can send runoff into narrow canyons.

Human interference with the stream system can worsen or even causefloods.A prime example is the failure of a dam or an artificial levee. Thesestructures are designed to contain floods of a certain size. If that size isexceeded, water can then spill over or break through a dam or levee andrush downstream causing a disastrous flood.

There are several flood control strategies. Measures to controlflooding include artificial levees, flood control dams, and placinglimits on floodplain development.

Artificial Levees Artificial levees are earthen mounds built onthe banks of a river. These levees increase the volume of water achannel can hold. When levees confine a river during periods of highwater, the river often deposits material in its channel as the dischargediminishes. This discharge is sediment that would have been droppedon the floodplain. Because the stream cannot deposit material outsideof its channel the bottom of the channel is gradually built up. Whenthe channel is built up, it takes less water to overflow the levee. As aresult, people may have to raise the height of the levee periodically toprotect the floodplain behind it. Moreover, many artificial levees arenot built to withstand periods of extreme flooding. For example, therewere many levee failures in the Midwest during the summer of 1993when the upper Mississippi experienced record flooding.

Flood-Control Dam Flood-control dams store floodwater andthen let it out slowly. Since the 1920s, thousands of dams have beenbuilt on nearly every major river in the United States. Many dams haveother 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 longerreplenishes them during floods. Built up sediment behind a dam meansthe volume of the stored water will gradually diminish. This build-upreduces the effectiveness of the dam for flood control. Large dams alsocause ecological damage to river environments.

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

Q Sometimes a major flood isdescribed as a 100-year flood.What does that mean?

A The phrase “100-year flood”is misleading because it makespeople believe that such anevent happens only once every100 years. In truth, a hugeflood can happen any year. Thephrase “100-year flood” is reallya statistical designation. It indi-cates that there is a 1-in-100chance that a flood this size willhappen during any year.Perhaps a better term would bethe “1-in-100 chance flood.”

Some students may have heard the term“100-year flood” and assume that thismeans that floods only occur in the areaevery 100 years. This is not true and isoften misleading. A major flood can occuralong a river during any year. What theterm does suggest, however, is thatstatistically speaking there is a 1 in 100chance that in any given year, a majorflood will occur. The chances of floodingalong any stream are reevaluatedperiodically. It has been found that theflooding likelihood changes and thatmajor flooding may occur morefrequently than every 100 years. Askstudents to predict what sort of humanactivity may increase the likelihood offlooding. (Urban development and thebuilding of dams may increase the amountof serious flooding in an area.)Logical

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Section 6.2 Assessment

Reviewing Concepts1. How do streams erode their channels?

2. What causes floods?

3. What is the relationship between a streamand a drainage basin?

4. How do streams transport sediments?

Critical Thinking5. Analyzing Concepts How does urban

development interfere with the naturalfunction of floodplains?

6. Summarizing Explain the formation of oneof the landforms that streams create bydeposition.

Drainage BasinsEvery stream has a drainage basin. A drainage basin is the landarea that contributes water to a stream. An imaginary line called adivide separates the drainage basins of one stream from another.Divides range in scale from a ridge separating two small gullies on ahillside to a continental divide, which splits continents into enormousdrainage basins. The Mississippi River has the largest drainage basin inNorth America. See Figure 13. The river and its tributaries collect waterfrom more than 3.2 million square kilometers of the continent.

Figure 13 Mississippi RiverDrainage Basin Divides are theboundaries that separatedrainage basins from each other.

Descriptive Paragraph Use librarysources or the Internet to research thecauses of a recent major flood. Write aparagraph that tells the name of the flood,when it happened, where it happened, andthe conditions that led to the flood itself.

Mississippi RiverDrainage Basin

Divide

Mississippi RiverDrainage Basin

Divide

Divide

Missouri River

Ohio RiverM

ississipp

iRiver

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Drainage BasinsUse VisualsFigure 13 Point out the Mississippi Riverdrainage basin. Ask: Do other drainagebasins exist within this one? (Yes, everystream, regardless of size, has its owndrainage basin. A larger river, such as theMississippi, will have a drainage basin thatincludes those of all of its tributaries.)Where is the divide that is commonlycalled the Continental Divide? (This isthe western portion of the Mississippi Riverdrainage basin divide that runs throughthe Rocky Mountains.)

Use CommunityResourcesDrainage Basins Invite a hydrologistto speak to the class about a drainagebasin in your area. Have students traceout the drainage basin of a local streamor river and discuss their findings withthe scientist.Interpersonal

ASSESSEvaluateUnderstandingTo assess students’ knowledge of sectioncontent, have them create a visualshowing a narrow and a wide streamvalley.

ReteachHave students make a chart summarizingthe differences between erosion anddeposition.

Student paragraphs should describeaccurately researched floods and theircauses and effects.

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4. Streams transport sediment in solution, insuspension, and by rolling along the bottom.5. Urban development can decrease the effec-tiveness of floodplains by replacing water-absorbing vegetation with concrete andasphalt and increasing flooding.6. Sample answer: Deltas are formed as accu-mulating sediment is deposited where astream or river enters a lake or ocean.

Section 6.2 Assessment

1. Streams erode their channels by liftingloose particles by abrasion, grinding, and bydissolving soluble material.2. Floods occur when the discharge of astream exceeds the capacity of the channel.Most floods are caused by rapid spring snowmelt and storms.3. A drainage basin is the land area thatcontributes water to a stream.


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