Unformatted text preview: © 2009 Allan Ludman and Stephen Marshak W.W. Norton & Company CHAPTER 10 LANDSCAPES FORMED BY STREAMS
PURPOSE • To learn how streams erode and deposit material • To become familiar with landforms formed by stream erosion and deposition • To interpret active and ancient stream processes from landscape features. MATERIALS NEEDED • Thin string • Ruler with divisions in mm or tenths of an inch • Graph paper for constructing topographic profiles • Colored pencils • A magnifying glass or hand lens to help read close-spaced contour lines 10.1 INTRODUCTION Water flowing in a channel is called a stream whether it is as large as the Amazon River or as small as the smallest creek, run, rill, or brook. Streams are highly effective agents of erosion and may move more material after one storm in an arid region than wind does all year. This chapter explores why not all streams behave the same way and how streams can produce very different landscapes. 10.2 How do streams work? All streams operate according to a few simple principles regardless of their size:
• Water in streams flows downhill because of gravity. • Streams normally flow in a well-defined channel, except during floods when the water overflows the channel and spill out across the surrounding land. • The motion of water gives a stream kinetic energy, enabling it to do the geologic work of erosion and deposition. The amount of energy depends on the amount of water and its velocity (remember, Kinetic energy = ½ mv2), so big, fast-flowing streams will erode more than small, slow-flowing streams. • Its kinetic energy allows streams to transport sediment, from the finest mud-sized to small boulders. These particles slide or roll on the bed of the stream, bounce along, or are carried in suspension within the water. • The flow of water erodes unconsolidated sediment from the walls and bed of the channel and streams use this sediment to abrade solid rock. • Streams deposit sediment when they lose kinetic energy by slowing down (or evaporating). The heaviest particles are deposited first, then the smaller grains as the energy wanes. 1 EXERCISE 10.1 DIFFERENCES BETWEEN STREAMS The basic rules are the same for all streams, but the rules can be applied differently, resulting in streams that look very different from one another (Figure 10.1). Figure 10.1: A tale of two streams a)Yellowstone River, Wyoming b)Cascapedia River, Quebec a)In your own words, describe how these streams differ. Now for a few geologic terms: A stream channel is the area within which the water is actually flowing. A stream valley is the region within which the stream has eroded the land. In some cases the channel completely fills the bottom of the valley; in others, it is much narrower than the broad valley floor. Some valley walls are steep, others gentle .
b)Which stream in Figure 10.1 has the wider channel? __________________________ c)Which stream has the broader valley? __________________________________ d) Which stream has the most clearly developed valley walls? _________________ e)Describe the relationship between valley width and channel width for both streams. f)Which stream has the straighter channel? Which has a more sinuous (meandering) channel? g)Which stream appears to be flowing faster? What evidence did you use to determine this? h)Which stream appears to be flowing more steeply downhill? Note: the steepness of a stream channel is called its gradient and is a major factor in stream behavior. __________________ 2 i)Look carefully at the valley of the Cascapedia River. What evidence is there that the river had more energy at one time than it does now? Where could that energy have come from? These questions begin to look at factors that control stream activity. Streams are complex dynamic systems in which changes in one factor bring about changes in others, affecting the way the stream looks and behaves. For example, changes in a stream’s gradient can completely change the nature of erosion and deposition, the width of the valley, and the degree to which the channel meanders. 10.2.1 Stream Erosion: Downward Or Sideways A brief lesson in stream anatomy helps to understand stream erosion and deposition. A stream begins at its headwaters (or head) and the point at which it ends – by flowing into another stream, the ocean, or a topographic low – is called its mouth. The headwaters of the Mississippi River are in Lake Itasca, Minnesota and its mouth is the Gulf of Mexico in Louisiana. The longitudinal profile of a stream from headwaters to mouth is generally a smooth, concave-upward curve (Figure 10.2). The gradient (steepness) may vary from a few inches to hundreds of feet of vertical drop per mile and is typically steeper at the head than at the mouth (Figure 10.2). A stream can erode its channel only as low as the elevation at its mouth because if it cut deeper it would have to flow uphill to get to the mouth. The elevation at the mouth thus controls erosion along the entire stream and is called the base level. Sea level is the ultimate base level for streams that flow into the ocean; base level for a tributary that flows into another stream is the elevation where the tributary joins the larger stream. 3 Figure 10.2: Longitudinal stream profiles showing different gradients Headwaters
Steep gradient Mouth
Moderate gradient Elevation Gentle gradient One difference between the Yellowstone and Cascapedia rivers is the straightness of their channels: the Yellowstone has a relatively straight channel, whereas the Cascapedia channel meanders across a wide valley floor. The sinuosity of a stream measures how much it meanders, as shown in the following formula. Because sinuosity is a ratio of the two lengths, it has no units. An absolutely straight stream would have a sinuosity of 1.00 (if such a stream existed), whereas streams with many meanders have high values for sinuosity (Figure 10.3). Length of stream channel (meanders and all) Sinuosity = Straight-line distance between the same points
Figure 10.3 Stream sinuosity Low Sinuosity High Sinuosity A B C 4 EXERCISE 10.2: WHY SOME STREAMS MEANDER BUT OTHERS ARE STRAIGHT. A portion of the Bighorn River in Wyoming can help answer this question (Figure 10.4). a. Compare the course of the Big Horn River between points A and B with that of its tributary between points C and D. Fill in the table below. Table 10.1 Relationship between stream sinuosity and gradient Big Horn River Unnamed Tributary Channel length Straight line length Sinuosity (a/b) Highest elevation Lowest elevation Vertical drop (d-e) Gradient
b. What is the apparent relationship between a stream’s gradient and whether it has a straight or meandering channel? c. Test this hypothesis on the Genesee River, New York (Figure 10.5) and the Casino Lakes area, Idaho (Figure 10.6). Complete the following table and describe how the Genesee River differs from the Idaho streams. Table 10.2 Testing the relationship between gradient and sinuosity Genessee River, NY Casino Lakes area, Idaho Stream A-B Valley shape
(V-shaped or broad with flat bottom) Stream C-D Gradient Valley width Channel width Valley width/ channel width Sinuosity 5 A Figure 10.4: Part of the Bighorn River, Wyoming (Manderson and Orchard Bench 7.5’ quadrangles) D C B 6
Contour Interval = 20’ Figure 10.5: The Genesee River, south of Rochester, New York Contour Interval = 10 feet A B 7 Figure 10.6: Casino Lakes Idaho 7.5’ quadrangle Contour Interval = 40’
0 0.5 miles 1.0 A C
A B 8 D
A d. Did these maps support your hypothesis about the relationship between meandering (sinuosity) and gradient? Explain. e. What is the apparent relationship between sinuosity and valley width/channel width ratio? f. What is the apparent relationship between stream gradient and shape of a stream valley? Now apply what you’ve learned to the streams in Figures 10.1 and 10.3. g. Which probably has the steeper gradient – the Cascapedia River or Yellowstone River? Explain your reasoning. h. Which of the streams sketched in Figure 10.3 probably has the steepest gradient? The gentlest gradient? Explain your reasoning. 10.3. STREAM VALLEY TYPES AND FEATURES The Yellowstone and Cascapedia rivers in Figure 10.1 illustrate the two most common types of stream valleys – steep-walled V-shaped valleys whose bottoms are occupied fully by the channel and broad, flat-bottomed valleys much wider than the channel and within which the stream meanders widely between the valley walls. Figure 10.7a and 10.7b show how these valleys form.
Figure 10.7a Evolution of a V-shaped stream valley
Initial steep valley walls Channel V-shaped valley: Vertical erosion carves the channel and valley downward vertically (large arrow), producing steep valley walls. Mass wasting (slump, creep, landslides, and rock falls) reduces slope steepness to the angle of repose (curved arrows) and widens the top of the valley (dashed arrows). 9 Oxbow Valley Oxbow Figure 10.7b: Broad, flatbottomed valley As the stream meanders, it widens the valley (arrows). Mass wasting gentles the slopes of the valley walls as in (a). Oxbows mark the position of former meanders. Channel When water is added to a stream in V-shaped valley, the channel expands and fills more of the valley. When more water enters a stream with a broad, flat valley and a relatively small channel (Figure 10.8), it spills out of the channel onto the broad valley floor in a flood. Sediment carried by the floodwater is deposited on the floodplain and other depositional and erosional features can be recognized easily on topographic maps or photographs (Figure 10.8).
Figure 10.8: Floodplain and associated features Black arrow indicates direction of stream flow; white arrows show movement of meander loops Sand bar Point Point bar Oxbow lake Floodplain 10
Natural levees 10.3.1 Features of flood plains: Natural levees form when a stream overflows its banks and deposits its coarsest sediment next to the channel. The levees are ridges of sediment that outline the channel when flooding took place. Several generations of natural levees are visible in Figure 10.8, showing how the meanders changed position with time. Point bars form when water on the inside of a meander loop slows down, causing sediment to be deposited. At the same time, erosion occurs on the outside of the meander loop because water there moves faster. The result is that meanders migrate with time, moving outward (toward their convex side) and downstream. Sometimes, a stream cuts off meanders and straightens itself. The levees that formerly flanked the meanders help outline the former position of the river, leaving meander scars. Meander scars that have filled with water are called oxbow lakes 10.4. CHANGES IN STREAMS OVER TIME Streams erode vertically by leveling the longitudinal profile to the elevation of the mouth. Streams also erode laterally, broadening their valleys by meandering. Most streams erode both laterally and vertically at the same time, but the balance between vertical and lateral erosion commonly changes as the stream evolves. Headwaters of a high-gradient stream are much higher than its mouth and stream energy is used largely in vertical erosion, lowering channel elevation all along its profile. Over time, the gradient lessens as erosion lowers the headwater area. The stream still has energy with which to alter the landscape and uses some of it to erode laterally. The valley then widens by a combination of mass wasting and meandering. Even when headwaters are lowered to nearly the same elevation as the mouth, a stream still has energy for geologic work but because it cannot cut vertically below its base level, most energy at this stage must be used for lateral erosion. 11 At first a stream meanders within narrow valley walls but with time it erodes those walls farther and farther, eventually carving a very wide valley. As its gradient decreases, a stream redistributes sediment that it had deposited, moving it back and forth across the floodplain. It is important to understand that some streams have very gentle gradients and meander widely from the moment they begin. Streams on the Atlantic and Gulf coastal plains are good examples of this kind of behavior.
EXERCISE 10.3: INTERPRETING STREAM BEHAVIOR Figure 10.9 shows three meandering streams each of which balances energy use differently between vertical and lateral erosion a. From their valley width/channel width ratios alone, which stream would you expect to have the steepest gradient? The gentlest gradient? Explain your reasoning. b. Which stream do you think is doing the most vertical erosion? The least? Explain. c. What indicates former positions of the meandering rivers? For all three? Explain. EXERCISE 10.4: RECOGNIZING FLUVIAL LANDFORMS Label as many erosional and depositional fluvial landforms as you can in Figures 10.4, 10.5, 10.6, and 10.9. What do these features tell about the active and ancient stream processes in their areas? 10.5 STREAM NETWORKS Streams are particularly effective agents of erosion because they form networks that cover much of Earth’s surface. Rain falling on an area runs off into tiny channels that carry water into bigger streams and eventually into large rivers. Each stream – from tiniest to largest – expands headward over time as water washes into its channel, increasing the amount of land affected by stream erosion. Understanding the geometric patterns of stream networks and the way they affect the areas they drain is the key to understanding how to prevent or remedy stream pollution, soil erosion, and flood damage. 12 Figure 10.9: Valley width vs channel width variations for three meandering streams
0 miles 10 a)Mississippi River, Arkansas and Mississippi c)Arkansas River near Tulsa, Oklahoma b)Meadow River, West Virginia 13
1.0 0 1.0 0 miles miles 10.5.1 Drainage Basins The area drained by a stream is its drainage basin, and is separated from adjacent drainage basins by highlands called drainage divides. The drainage basin of a small tributary may cover a few square miles, but that of the master stream may be hundreds of thousands of square miles. Figure 10.10 shows large-scale drainage basins whose waters flow into the Pacific, Arctic, and Atlantic oceans, Hudson’s Bay, and Gulf of Mexico. The Mississippi drainage basin (heavy black line) is the largest and drains much of the interior of the United States. The Continental Divide separates streams that flow into the Atlantic Ocean from those that flow to the Pacific. The Appalachian Mountains are the divide separating Gulf of Mexico and direct Atlantic Ocean drainage; the Rocky Mountains separate Gulf of Mexico and Pacific drainage. A favorite tourist stop in Alberta, Canada is a triple divide that separates waters flowing north to the Arctic Ocean, west to the Pacific, and south to the Gulf of Mexico.
Figure 10.10: Major drainage basins of North America Hudson’s Bay Pacific Ocean
Great Basin Atlantic
Gulf of Mexico Boundary of Mississippi River drainage basin Ocean EXERCISE 10.5: DRAINAGE BASINS AND STREAM DIVIDES Figure 10.11 is a map showing several tributaries on the north and south sides of the Missouri River near Jefferson City, Missouri. One large tributary, the Osage River, joins the Missouri from the south near the eastern margin of the map, but most of the tributaries on the north are much smaller. 14 Figure 10.11: Drainage divides near the Missouri River, Jefferson City, Missouri * 15 Contour Interval = 20’ a. With a colored pencil, trace one of the tributary creeks feeding directly into the Missouri from the north. With the same pencil, trace tributaries which flow directly into that creek, and then the tributaries of these smaller streams. b. With a different color, trace an adjacent tributary of the Missouri, and its tributaries. Repeat for more streams and their tributaries on the north side of the Missouri River. c. You have just outlined most of the drainage on the north side of the Missouri. Now, with a red pencil, trace the divides that separate individual drainage basins for which the each creek is the master stream. This should be easy since you’ve already identified streams in each drainage basin with a different color. Note that some divides are defined sharply by narrow ridges, but others are more difficult to locate within broad upland areas where most of the headwaters are located. d. With dotted lines, suggest how headward erosion might extend each main stream in the future. e. What do you think will happen when the headwaters of two streams meet during headward erosion? f. Local residents are worried that a recent toxic spill at an electrical substation (asterisk in Figure 10.11) will work its way into the drainage system. Based on your drainage basin analysis, shade in areas that might be affected. Be conservative: if there is any doubt, err on the side of including areas rather than excluding them. 10.5.2 Drainage Patterns Master and tributary streams in a network typically form one of six geometric patterns (Figure 10.12). Dendritic patterns (from the Greek dendros for veins in a leaf), develop where surface materials are equally resistant to erosion. This may mean horizontal sedimentary or volcanic rocks; loose, unconsolidated sediment; or igneous and metamorphic areas where most rocks erode at the same rate. Trellis patterns form where ridges of resistant rock alternate with valleys underlain by weaker material. Rectangular patterns indicate zones of weakness (faults, fractures) perpendicular to one another. In radial patterns, streams flow either outward (centrifugal) from a high point (e.g. a volcano) or inward (centripetal) toward the center of a large basin. Annular drainage patterns occur where there are concentric rings of alternating resistant and weak rocks –commonly structures called domes and basins. 16 Figure 10.12: Common drainage patterns Dendritic Trellis Rectangular Radial: centrifugal Radial: centripetal Annular EXERCISE 10.6 RECOGNIZING DRAINAGE PATTERNS a. What drainage pattern is associated with the Mississippi River drainage basin? What does that tell about the materials that underlie the central part of the United States? b. What drainage patterns are associated with the areas shown in Figures 10.4, 10.5, 10.6, 10.11, 10.15, 10.16, and 10.17? What do these indicate about the rocks underlying those areas? 10.6 CHANGES IN STREAM-CARVED LANDSCAPES WITH TIME Just as a single stream or entire drainage network changes over time, so too do fluvial (streamcreated) landscapes. Consider a large block of land uplifted to form a plateau. Several things will change with time: the highest elevation, number of streams, stream gradients, amount of flat land vs amount of land that is part of valley walls. Figure 10.13 summarizes idealized changes. In the real world things rarely remain constant long enough for this cycle to reach its end: sea level may rise or fall due to glaciation or tectonic activity, the land may be uplifted tectonically, etc. Nevertheless, the three stages are typical of landscapes produced by stream erosion and can be recognized on maps and other images. 17 Figure 10.13: Idealized stages in the evolution of a stream-carved landscape Stage 1: Few streams, but all have relatively steep gradients. Divides between streams are broad, generally flat. Streams channels are relatively straight. Relief is low to modest (not much difference in elevation between tops of plateau-like divides and channels) Stage 2: Numerous streams dissect most of region. Streams typically have moderate gradients. Stream divides are narrow, sharp, and most of the area is in slope. Streams meander moderately. Relief is high and there is little, if any, land that is at the original elevation. Stage 3: Few streams once again, but all have gentle gradients. Divides between streams are once again broad and flat and relief is low. Streams have very sinuous channels. EXERCISE 10.7 RECOGNIZING STAGES OF LANDSCAPE EROSION a. Examine the topographic maps in Figures 10.14, 10.15, and 10.16. Fill in the following table and explain your reasoning for each decision: 18 Southeast Texas (Figure 10.14)
Greatest relief (distance between highest and lowest points) Greatest number of streams Greatest land area involved in valley slopes Steepest stream gradients Stream divides: flat vs broadly rounded vs angular Stage of stream dissection (Stage 1, 2, or 3) Colorado Plateau (Figure 10.15) Appalachian Plateau (Figure 10.16) b. With this practice, look at the other fluvial landscapes in this chapter and suggest which stages of erosion each map represents. c. Arrange the map areas in order, from earliest stage of erosion to most advanced. EARLIEST STAGE LATEST STAGE 19 Figure 10.14: Stream dissection in southeast Texas (Fred, Spurger, Magnolia Springs, and Potato Patch Lake quadrangles) Contour Interval = 5 feet 20 Figure 10.15: Dissected part of the Colorado Plateau (Del Muerto Quadrangle, Arizona) Contour Interval = 20’ 21 Figure 10.16: Stream dissected part of the Appalachian Plateau, Hamlin quadrangle, West Virginia Contour Interval = 20’ 22 10.7 WHEN STREAMS DON’T SEEM TO FOLLOW THE RULES Most stream erosion and deposition follow the principles you just deduced, but there are some notable exceptions. Actually, the streams aren’t violating any rules; they are following them to the letter but their situations are more complex than the basic ones we have examined. Consider, for example, the Susquehanna River as it flows across the Pennsylvania landscape shown in Figure 10.17. This area is part of the Valley and Ridge Province of the Appalachian Mountains, and is characterized by elongate ridges and valleys made of resistant and non-resistant rocks respectively. Streams are the dominant agent of erosion in the area.
EXERCISE 10.8 DEDUCING THE HISTORY OF THE SUSQUEHANNA RIVER a. Figure 10.17 shows a part of the Valley and Ridge province in Pennsylvania. What is unusual about the relationship between the Susquehanna and Juniata rivers and the ridge and valley topography? Most of the small streams flow in the elongate valleys but the Susquehanna and its tributary, the Juniata River, cut across the ridges at nearly right angles. It is tempting to think that the big streams had enough energy to cut through the ridges while the small ones couldn’t, but this is not the case. The answer lies in a multi-stage history of which only the last phase is visible today. b. Why do most of the smaller streams flow in the elongate valleys? c. Suggest as many hypotheses as you can to explain why the two larger rivers cut across the valley and ridge topography. Hint: how might the landscape have been different at an earlier time? Rivers with enough energy to cut through the ridges should certainly have been able to simply meander around them, but the Susquehanna and Juniata rivers didn’t take the easy way out. It’s almost as if they didn’t even know the ridges and valleys were there. As a matter of fact… but no more help. d. With that clue, suggest a series of events that explains the behavior of the Susquehanna and Juniata rivers. Hint: this type of stream is called a superposed stream. 23 Figure 10.17: The Susquehanna River cutting across the Appalachian Valley and Ridge Provicnce , Pennsylvania 24 EXERCISE 10.9: ORIGIN OF INCISED MEANDERS: The Green River is in the Colorado Plateau, an area where meanders of many rivers cut deeply (are incised) into the bedrock (Figure 10.18). The most famous is the Colorado River itself, particularly where it flows through the Grand Canyon. This behavior is totally unlike that of the meandering streams encountered earlier in this chapter. The Green River seems to violate rules of stream behavior but, as with the Susquehanna, it is following them perfectly. Some geologic detective work will let you figure out the difference. a. Describe the path of the Green River as shown in Figure 10.18. b. What is the sinuosity of the Green River? ____________ c. When a river meanders with such sinuosity, how is it using most of its energy – in lateral or vertical erosion? d. What evidence is there that the Green River is eroding laterally? e. What evidence is there that the Green River is eroding vertically? f. What is probability that the Green River will straighten its path by cutting through the walls of the Bowknot Bend? Explain. g. The Green River flows into the Colorado which flows into the Gulf of Southern California. How far above base level is the river in this area? Is this what you expect for a meandering stream? Explain. h. Suggest an origin for the incised meanders. (Don’t forget possible effects of tectonic activity.) 25 Figure 10.18: Incised meanders of the Green River, southeastern Utah 26 10.8 WHEN THERE’S TOO MUCH WATER: FLOODS A flood occurs when more water enters a stream than its channel can hold. Many floods are seasonal, caused by heavy spring rains or melting of thick winter snow. Others, called flash floods, follow storms that can deliver a foot or more of rain in a few hours. Figure 10.19 is a map compiled by FEMA showing estimated flood potential in the United States, based on the number of square miles that would be inundated. It might appear that states lightly shaded in Figure 10.19 would have little flood damage but that would be an incorrect reading of the map because two of the worst river floods in U.S. history occurred in Rapid City, South Dakota, and Johnstown, Pennsylvania. Indeed, these two cities have been flooded many times. The area of potentially flooding may not be as large as in some other states, but the conditions for flooding may occur frequently. South Dakota’s Rapid Creek has flooded more than 30 times since the late 1800’s, including a disastrous flash flood on June 9, 1972 triggered by 15 inches of rain in six hours. The creek overflowed or destroyed several dams, ruined more than 1,300 homes, 5,000 cars, and killed more than 200 people in Rapid City (Figure 10.20).
Figure 10.19: Flood risk in the United States (after FEMA) 27 EXERCISE 10.10: ESTIMATING POTENTIAL FLOOD PROBLEMS Streams shown on maps earlier in the chapter are all subject to flooding, but the potential problems for cities along their banks are not the same. a. For which streams would flooding not be a problem? Explain. b. What would be the effect of a flash flood that raised the level of the Bighorn River (Figure 10.4) 20 feet above its banks? Shade the area that would be affected.
Figure 10.20: Effects of the June 9, 1972 flood, Rapid City, South Dakota a. Rapid Creek has a narrow flood plain where it flows through Dark Canyon, 1 mile upstream of Rapid City. Flood waters filled the entire flood plain. Concrete slabs (arrows) are all that are left of the homes built in the flood plain. b. The spillway of the Canyon Creek Dam (arrow) was clogged by debris carried by the floodwater causing water to flow over the dam which then failed completely. c. Not the usual line up of cars for gasoline d. This railroad trestle was washed away, along with highway bridges, slowing relief efforts. 28 c. Would students at the State University of New York campus at Geneseo on the east bank of the Genesee River (Figure 10.5) river have to be evacuated if water rose 20’? d. Shade the area that would be inundated if flood waters of the Neches River (Figure 10.14) crested 15 feet above normal level. e. Compare the map of the Missouri River in Figure 10.11with Figure 10.21, a satellite image of the same area. What information does the satellite image add? f. What things have been built in the floodplain that could be destroyed or made unusable in a major flood? How would the loss of these affect relief efforts? Figure 10.21: Aerial image of the flood plain of the Missouri River at Jefferson City, Missouri (compare with Figure 10.12) 29 ...
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