Geography
River Landforms
River landforms are physical features created by the action of rivers. They include meanders, oxbow lakes, river deltas, and floodplains. Meanders are bends in the river, while oxbow lakes form when a meander is cut off from the main river. River deltas are triangular-shaped landforms at the mouth of a river, and floodplains are flat areas adjacent to the river that are prone to flooding.
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12 Key excerpts on "River Landforms"
eBook - PDF- Alan F. Arbogast(Author)
- 2017(Publication Date)
- Wiley(Publisher)
Thus, it is possible to classify landforms gen- erally as being either erosional or depositional in their nature. Erosional landforms are created when sediment, soil, or rock is stripped away by some geomorphic process. Depositional landforms, in contrast, form when sediment accumulates after being dropped. Figure 16.15 shows a simplified example of these cate- gories. Here, the mountain slopes have largely been shaped by erosion due to the high energy created by the steep relief. This process creates some distinctive landforms. The most prominent feature is a peak, which is the highest point on any given mountain. Peaks are typically separated by a lower land- form called a saddle. As streams cut into the mountain slopes, they first create a shallow gully, which can enlarge to become a ravine and then, if sufficient time and erosion later occur, a deep and broad canyon. These features are separated from one another by a relatively high ridge called a spur, which is, in effect, a drainage divide. Over time, the eroded hillslope sedi- ments are transported into the valley below, where they may be deposited on more level terrain within an alluvial fan or river floodplain. Here the relief lessens and geomorphic processes lose their power. These landforms will be described in more detail later in the chapter. Fluvial Erosion on Hillslopes The logical place to begin a discussion of stream erosion is by focusing on hill- slopes, which are the part of the landscape that is most intensely eroded by running water. Hillslopes are the most active zones of fluvial erosion because, as indicated before, FIGURE 16.14 Flooding along the Mississippi River in 1993. (a) Aerial photograph of flooding along the Mississippi River in 1993. (b) Landsat images of the confluence of the Missouri and Mississippi Rivers north of St. Louis during a normal year (left) and during 1993 (right). In these images, vegetation and urban areas appear in green and pink tones, respectively.
eBook - PDF- Alan H. Strahler(Author)
- 2013(Publication Date)
- Wiley(Publisher)
The sediment is finally deposited downstream, where it build ups into plains, levees, fans, and deltas. Waves, glacial ice, and wind also shape unique landforms, but these processes are restricted to certain areas on the globe, as we will see in later chapters. The landforms shaped by the progressive removal of bedrock are called erosional landforms . Fragments of soil, regolith, and bedrock that are removed from the parent rock mass are transported and deposited elsewhere, where they take shape as an entirely differ- ent set of surface features—the depositional landforms (Figure 15.1). SLOPE EROSION Fluvial erosion starts on the uplands as soil erosion. When raindrops hit bare soil, their force lifts soil particles, which fall back into new positions, causing splash ero- sion (Figure 15.2). A torrential rainstorm can disturb as much as 225 metric tons of soil per hectare (about 100 U.S. tons per acre). On a sloping ground surface, splash erosion shifts the soil slowly downhill. The soil surface also becomes much less capable of absorbing water. This important effect occurs because the natural soil openings ost of the landforms we see around us are sculpted by running water as it erodes, trans- ports, and deposits sediment. What causes slopes to erode, and what happens to eroded particles? How do streams build their beds and wear away their banks? How do stream valleys evolve over time? Under what conditions do streams form floodplains and meanders? These are some of the questions we will answer in this chapter. 500 Landforms Made by Running Water M AirPhoto-Jim Wark Steve Winter/NG Image Collection 15.1 Erosional and depositional landforms Erosion, Transportation, and Deposition 501 Alaska Stock Images/NG Image Collection become sealed by particles shifted by raindrop splash. Thus, water cannot infiltrate the soil as easily, so a much greater depth of overland flow can be triggered from a smaller amount of rain.
eBook - PDF- Mateo Gutierrez(Author)
- 2012(Publication Date)
- CRC Press(Publisher)
8 FLUVIAL GEOMORPHOLOGY I 1. Introduction 2. Brief history of fluvial geomorphology 3. The fluvial system—concept of a hydrosystem 4. Morphometry of a river basin 5. Hydraulics of fluid flow 6. Sediment transport 7. Fluvial erosion 8. Longitudinal profile, base level, and stream capture 9. Fluvial channel systems 10. Stability of fluvial channels 11. Metamorphosis of rivers 8.1 Introduction Fluvial geomorphology has many definitions; Gregory (2004a) has compiled many of them. According to Richards (1987), the fundamental goal of fluvial geomorphology is to explain the relationships among the physical processes of channel flow on a moving bed, the mechanism of flow-induced sedi- ment transport, and alluvial channel landforms created by sediment transport. Rivers are essentially erosional and transport agents that carry water and sediment from continents to the oceans. In spite of the fact that <0.005% of continental water is found in rivers at any given time, water flow is one of the most impor- tant forces at work on the Earth's surface (Knighton, 1998). Rivers transport about 19 billion tons of material every year, 80% as solids and 20% as dissolved material (Meybeck, 1979; Milliman and Meade, 1983; Walling, 1987). During floods, the amount of water and sediment carried by some rivers is even higher. For example, the highly destructive Mississippi River flood of 1973 reached Saint Louis with a discharge of 24,120 m 3 /sec but fell short of the maximum recorded flood on the Mississippi, which exceeded 56,640 m 3 /sec. Even so, this flood is less than the average flow of the Amazon, the largest river in the world (Fig. 8.1) (Chorley et al., 1984). A river is a body of water that flows in a channel. The characteristics of water flow are the domain of hydrau- lic engineering whereas the dimensions and channel systems are geomorphic subjects (Chorley et al., 1984).
eBook - PDFPhysical Geography
Great Systems and Global Environments
- William M. Marsh, Martin M. Kaufman(Authors)
- 2012(Publication Date)
- Cambridge University Press(Publisher)
Thus, stream systems stand, as it were, with one arm in the atmosphere and one arm in the lithosphere, and function in response to both. Stream Systems, Valley Formation, and Fluvial Landscapes 556 3 3 Streams are complex geomorphic systems comprised of three main phases. Solar radiation and gravity are the primary sources of energy that drive these systems, but the landscape influences the distribution and amount of work they accomplish. 3 3 Stream discharge, velocity, and water depth change with distance downstream. When water moves, potential energy is converted to kinetic energy, which produces bed shear stress. 3 3 Scouring accounts for the most stream erosion. This process is closely related to turbulent flow and the motion of particles in contact with the streambed. 3 3 Most material eroded by streams comes from deposits in their valleys. These include channel deposits, flood deposits, and hillslope deposits. 3 3 Channel geometry changes substantially with variations in discharge and sediment supply. Scouring and deepening increase when discharge rises and reverse when discharge falls. 3 3 Most natural channels are single-thread and most form meandering patterns. Flow in meanders results in undercutting on one bank and deposition on the other, processes essential to the formation of alluvial plains. 3 3 Floodplains abound with distinctive geomorphic features. These include levees, oxbows, back-swamps, and terraces, all related to channel flow. 3 3 The movement of sediment from hillslopes to the sea via streams is not a smooth process. Most sediment is moved during runoff events when discharge rises. 3 3 Most of the sediment exported from the continents is carried by the world’s great rivers. Of these rivers, those in Asia do the most work. 3 3 Watersheds are geomorphic systems whose landforms provide the basic infrastructure for the landscape.
eBook - PDF- Vijay K. Sharma(Author)
- 2010(Publication Date)
- CRC Press(Publisher)
139 6 Fluvial Processes and Depositional Landforms Weathering, surface erosion, and mass movement activities within drainage basins generate sufficient sediments of diverse origin and size composition for transport and deposition by streams as alluvium . This alluvium is deposited as flood plains on land, alluvial fans commonly on land, and river deltas partly on land and partly in adjacent water bodies of sizable extent. Flood plains are the hydrologic response of sediment transport processes within and beyond channel margins. Alluvial fans are a feature of local deposition on surfaces over which streams suddenly lose competence for the transport of sediment load from upland basin sources. River deltas develop in the transitional environment as subaerial and subaqueous depos-its of alluvium at or near the mouth of streams draining into the sea or large fresh-water lakes. FLOOD PLAINS Streams deposit the alluvium within channels and beyond channel banks during nor-mal and overbank flow conditions, developing a low-relief linear topographic surface to the level of mean annual flood that statistically reoccurs once every 1 to 2 years. This surface of alluvial fill is called flood plain, the composition and thickness of which varies with discharge and sediment load characteristics of streams, frequency and magnitude of overbank flows, lateral distance from channel banks, channel pat-tern, and pattern of channel migration. F LOOD P LAIN S EDIMENTS The alluvial fill of most flood plains comprises the bed material of active and inactive channels, sediments laid down at channel margins, sediments deposited overbank, and sediments of mass movement activity adjacent to the valley-side slopes (Vanoni, 1971). These sediments are diagnostic of lateral and vertical accretion processes and processes of slope failure (Figure 6.1), providing for four major sedimentary environments and eight subenvironments of fluvial deposition (Table 6.1).
eBook - PDFPhysical Geography
Made Simple
- Richard H. Bryant(Author)
- 2013(Publication Date)
- Made Simple(Publisher)
However, rather than indicating a stage of erosion, valley shape is more safely regarded as a result of the factors that control slope and stream processes, namely climate, rock type, available relief and geological structure. Terraces are a landform contributing to valley shape and are usually the result of both erosion and deposition. Terraces may be benches cut in solid rock, but more frequently alluvial terraces are formed when a river erodes flood-plain sediments, previously deposited by itself. The river cuts into these deposits because of some environmental change, which in many cases is a climatic one affecting the stream's discharge. In other cases, near river mouths, terraces may have been built and cut in response to sea-level changes. Terrace sediments and morphology are often used as guides in interpreting the geo-morphological history of a region. Deposition A river deposits alluvium when, because of a decrease in energy, it is no longer competent to transport its load. This usually occurs because of a reduction in the gradient of the stream channel, but may also result from an increase in the calibre of the load, perhaps brought in by a tributary into the main stream, or by conditions of accelerated erosion upstream. The first debris to be deposited will be the largest calibre, succeeded downstream by finer material, while the very finest material may continue to be transported even although the river energy has been reduced. This sequence of sediment-ation is found in many of the depositional forms created by rivers. A flood-plain is the most common depositional feature created by all sizes of river, be they very large or just small brooks. The alluvium in a flood-plain is composed of several kinds of deposit.
eBook - PDF- Timothy Foresman, Alan H. Strahler(Authors)
- 2012(Publication Date)
- Wiley(Publisher)
Val- leys form as rock is weathered and then eroded away by fluvial agents. a. Peaks and ravines Erosion by water, coupled with mass wasting, has carved out this ravine in a biosphere reserve on the Kamchatka Peninsula in eastern Russia. b. Fans Deposition of sediment by a stream has formed these alluvial fans in Wrangell-Saint Elias National Park in Alaska. Erosion,Transportation,andDeposition365 The landforms shaped by the progressive removal of bedrock are erosional landforms. Fragments of soil, rego- lith, and bedrock that are removed from the parent rock mass are transported and deposited elsewhere, making an entirely different set of surface features—the depositional landforms (Figure 12.1). particles are then transported by water, either in solution as ions or as sediment of many sizes. The sediments are finally deposited downstream, where they build up, forming plains, levees, fans, and deltas. Waves, glacial ice, and wind also carve out landforms, but these processes are restricted to certain areas on the globe, as we will see in later chapters. THE PLANNER ✓ ✓ Peak Canyon Fan Floodplain Depositional landforms Erosional landforms R a v i n e c. Canyons Plunging down the slope of South Africa’s great Eastern Escarp- ment, the Blyde River eroded this steep, colorful canyon in flat-lying sedimentary rocks. d. Floodplain The Kustatan River, Alaska, carrying sediment-laden water from nearby glaciers, deposited this floodplain on its way to Cook Inlet, near Anchorage. 366CHAPTER12 LandformsMadebyRunningWater Slope Erosion Fluvial erosion starts on the uplands as soil erosion (see Where Geographers Click). When falling raindrops hit bare soil, their force lifts soil particles, which fall back into new positions, creating splash erosion (Figure 12.2). A tor- rential rainstorm can disturb as much as 225 metric tons of soil per hectare (about 100 U.S. tons per acre). On a sloping ground surface, splash erosion shifts the soil slowly downhill.
eBook - PDFLandscape Planning
Environmental Applications
- William M. Marsh(Author)
- 2015(Publication Date)
- Wiley(Publisher)
Erosion is concentrated on the outsides of bends and slightly downstream from them where it cuts back the bank, forming an undercut bank. Deposition occurs on the insides of bends, forming features called point bars (Fig. 14.8a). Each year or so, a new increment is added to the point bar while the river erodes away a comparable amount on the opposite bank, the undercut bank. In this way the river shifts laterally, gradually changing its location in the valley and, at selected sites, cutting back the land on the edge of the valley. But the river can also undergo sudden changes in location when it erodes new segments of channel and abandons old ones. This is especially commonplace where Why meanders? Processes and landforms 14.6 FLOODPLAIN FORMATION AND FEATURES 305 a meander forms a large loop and the river erodes toward itself from opposite sides of the loop, eventually breaching the meander (Fig. 14.8b). The old channel is aban- doned because the new route is steeper and thus more efficient. The old channel forms a small lake, called an oxbow, but in time it fills with sediments and organic debris, becoming a wetland. Such features are often vividly defined by the patterns of vegeta- tion on the valley floor, and the aerial photographs in Fig. 3.3 show typical examples. 14.6 FLOODPLAIN FORMATION AND FEATURES If we step out of the channel in most stream valleys, we enter an area of fairly flat ground made up of alluvial deposits. This is river plain, or alluvial plain, and in most quarters it is referred to as floodplain because it is the ground that first receives floodwaters when the stream overtops its banks. Floodplain Formation. Floodplains form mainly by the channel processes just described, that is, the lateral shifting of streams on their valley floors. The process works as follows. When the river flows against the high ground at the edge of its valley, called the valley wall, it undercuts the wall, which fails and thereby retreats a short distance.
eBook - PDF- Harry Y. McSween, Jr, Jeffrey E. Moersch, Devon M. Burr, William M. Dunne, Joshua P. Emery, Linda C. Kah, Molly C. McCanta(Authors)
- 2019(Publication Date)
- Cambridge University Press(Publisher)
Fluvial, alluvial, and lacustrine landforms thereby provide insights into climate, surface, and sedimentologic processes on planetary bodies. 14.1 Volatile Landscapes If the confluence of requirements is unusual for aeolian landscapes (Section 13.1), that description is even more apt for landscapes shaped by flowing volatiles. Earth is, once again, a poor example in this regard. Earth is 70 per- cent covered by vast oceans that have played a controlling role in the evolution of our planet. In addition to their overwhelming physiochemical effects on the composition of terrestrial rocks, they are both source and sink for the hydrologic cycle, which leaves unmistakable geomorphic signatures on land surfaces. No other body in our Solar System has an active hydrologic cycle in which tempera- ture and pressure conditions permit a vast volatile reser- voir to change among the three different phases of matter. So of what use for planetary geology is studying our planet’s hydrologic landforms? Exactly because water is so constrained to specific temperature–pressure conditions, hydrologic landforms provide distinct clues to changes in planetary conditions over time. Thus, the existence of ancient river (fluvial), lake (lacustrine), and even ocean (marine) deposits pro- vides unmistakable evidence that Mars has not always been the cold desert that we see today. And because water is requisite for life as we know it, correctly interpreting hydrologic landforms is integral to astrobiological explor- ation (Chapter 16). Other volatiles besides water can participate in cycling, as on Saturn’s largest satellite, Titan. And any cycling of volatiles also involves move- ment of sediments, which produces fundamental geologic changes in the distributions and compositions of planet- ary materials. Thus, for the forensic science of geology, we study volatile landforms for critical clues in the dis- cernment both of planetary evolution and the potential for life.
No longer available |Learn more- James Petersen, Dorothy Sack, Robert Gabler(Authors)
- 2016(Publication Date)
- Cengage Learning EMEA(Publisher)
Natural levees along the Mississippi River rise up to 5 meters (16 ft) above the rest of the floodplain. A common landform in this deposition-dominated environment provides evidence of the meandering of a river over time. Espe- cially during floods, meander cut-offs occur when a stream seeks a shorter, steeper, and straighter path; breaches through the levees; and leaves a former meander loop isolated from the new channel position (● Fig. 17.27). If the cut-off meander remains filled with water, which is common, it forms an oxbow lake (● Fig. 17.28). ● FIGURE 17.24 Where the upper course of a stream lies in a mountainous region, it might have rapids and waterfalls, but its valley typically has a characteristic V shape, dramatically represented in Yellowstone Canyon, Wyoming. How does the gradient of the Yellowstone River compare with that of the stretch of the Mississippi River shown in Figure 17.23? ● FIGURE 17.25 Characteristics of a meandering river channel. Note that water flowing in a channel has a tendency to flow downstream in a helical, or corkscrew, fashion, which moves water against one side of the channel and then to the opposite side. The up-and-down motion of the water contributes to the processes of erosion, transportation, and deposition. Bank erosion (cut bank) Sediment deposition (point bar) Bank erosion (cut bank) Sediment deposition (point bar) (a) A B Original channel Reduced upward velocity: deposition (b) Lower current Sediment deposition Powerful downward velocity: undercutting, erosion Bank erosion B A Erosion of cut bank Point bar deposition A B (c) Sometimes people attempt to control streams by building up levees artificially to keep the river in its channel. During times of reduced discharge, however, when a river has less energy, deposition occurs in the channel. Thus, in an artificially con- strained channel, a river may raise the level of its channel bed.
eBook - PDF- James Petersen, Dorothy Sack, Robert Gabler, , James Petersen, James Petersen, Dorothy Sack, Robert Gabler(Authors)
- 2021(Publication Date)
- Cengage Learning EMEA(Publisher)
With continued aggradation, a distinctive landform, called a delta because the map view shapes of some resemble the Greek letter delta (D), will be constructed (● Fig. 17.28). Deltas form at the interface between fluvial systems and coastal environments of lakes or the ocean, and therefore originate in part from fluvial and in part from coastal processes. Deltas have a subaqueous (underwater) coastal component, called the prodelta, and a fluvial part, the delta plain, that exists at, to slightly above, the lake level or sea level. Deltas form only at those river mouths where the fluvial sediment supply is high, the underwater topography does not drop too sharply, and waves, currents, and tides cannot transport away all the sediments delivered by the river. These cir- cumstances exist at the mouths of many, but not all, rivers. Delta construction is a slow, ongoing process. A river channel that approaches its base level at a large standing body of water typi- cally has a very low gradient. Lacking the ability to incise its ● FIGURE 17.26 A color infrared aerial photograph showing a simple, recently cut-off meander loop. Vegetation appears on this image in shades of red. Do you see any point bars in this photo? USGS EROS Data Center ● FIGURE 17.27 Features of a large floodplain common in the lower courses of major rivers. Backswamps are low marshy or swampy parts of the floodplain, generally at the water table. Meander Oxbow lake Meander scar Natural levee Natural levee Backswamp BEDROCK ALLUVIUM Point bars Yazoo stream FLOODPLAIN Copyright 2022 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience.
eBook - PDF- James Petersen, Dorothy Sack, Robert Gabler, , James Petersen, Dorothy Sack, Robert Gabler(Authors)
- 2014(Publication Date)
- Cengage Learning EMEA(Publisher)
■ FIGURE 14.25 Satellite views of two types of deltas. (a) The arcuate Horton River delta along the Arctic Coast of Canada. (b) The shape of the Mississippi River delta resembles a bird’s foot. Why are the shapes of some deltas controlled more by fluvial processes whereas the shapes of others are strongly influenced by coastal processes? Copyright 2013 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s). Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. C H A P T E R 1 4 • F L U V I A L P R O C E S S E S A N D L A N D F O R M S 384 Base-Level Changes and Tectonism A change in elevation along a stream’s longitudinal pro-file will cause an increase or decrease in the stream’s gra-dient, which in turn affects the stream’s energy. Elevation changes can occur anywhere along a stream as a result of tectonic uplift or depression; they can also result from rising or falling of base level at the river mouth. Changes of base level for basins of exterior drainage result princi-pally from climate change. Sea level lowers in response to large-scale growth of glaciers and rises with substantial glacier shrinking. Tectonic uplift or a fall in base level gives the stream a steeper gradient and increased energy for erosion and transportation. The landscape and its stream are then said to be rejuvenated because the stream uses its renewed energy to incise its channel to the new base level. Waterfalls and rapids might develop as rejuve-nated channels are deepened by erosion. Tectonic depres-sion of the drainage basin or a rise in sea level reduces the stream’s gradient and energy, enhancing deposition.
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