Geography
River Deposition Landforms
River deposition landforms are created by the deposition of sediment carried by rivers. Examples include river deltas, formed at the mouth of a river where it meets a body of water, and alluvial fans, which develop where a river flows out of a mountainous area onto a flat plain. These landforms are important for understanding the processes of erosion and sediment transport.
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11 Key excerpts on "River Deposition Landforms"
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 - PDF- Mateo Gutierrez(Author)
- 2012(Publication Date)
- CRC Press(Publisher)
9 FLUVIAL GEOMORPHOLOGY II 1. Fluvial sedimentation 2. Floodplains 3. Alluvial fans 4. Fluvial terraces 5. Large rivers 6. Flooding 7. Flood risk 8. Flood prevention and mitigation 9.1 Fluvial sedimentation The material that a river picks up and transports is depos- ited downstream or along valleys as in piedmonts and coastal plains. An analysis of deposits can provide information on velocities and conditions and dynamics of the processes oper- ating at the time of sedimentation (Morisawa, 1985). Fluvial sediments are deposited primarily in topographical disconti- nuities and as valley fill and marginal deposits. Topographical discontinuities are created by faults, concave crustal struc- tures, glacial scours, marine abrasion, and other factors; these conditions lead to alluvial fans and other landforms. Valley fill involves a set of complex processes that produce flood plains and terraces. Marginal deposits result from sedimentation in still water where transport velocity has decreased such as deltas and playa deposits (Chorley et al., 1984). The latter will be analyzed in Chapter 11 on Littoral Geomorphology. Although most eroded material finally arrives at the mouth of a river, part of it is temporarily deposited along its course. During transport each particle has a “fall” velocity that rep- resents the sedimentation threshold for the particle. If the current velocity is less than the fall velocity, the particle will be deposited. Alluvial deposits are economically important as groundwater sources and as sources of aggregate for con- struction. Some alluvium contains heavy minerals, diamonds, and gold, which are actively mined in rivers and terraces in many places in the world. Ancient playa and delta deposits are frequently sources of petroleum and natural gas deposits (Chorley et al., 1984). 9.2 Floodplains A floodplain is an alluvial surface that is frequently inun- dated by an adjacent river.
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- 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 - 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)
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 382 leads to considerable lateral shifting of the channel and cre-ation of a large depositional plain. The lower floodplain of a major river is much wider than the width of its meander belt and shows evidence of many changes in course ( ■ Fig. 14.23). The stream migrates laterally through its own previously deposited sediment in a channel composed exclusively of allu-vium. During floods, these extensive floodplains, known as alluvial plains , become inundated with sediment-laden water that contributes deposits to the large natural levees and to the already thick alluvial valley fill of the floodplain in general. 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 envi-ronment provides evidence of the meandering of a river over time. Especially during floods, meander cut-offs occur when a stream seeks a shorter, steeper, straighter path, and it breaches the levees, leaving behind a former meander bend now isolated from the new channel position. If the meander cut-off remains filled with water, which is common, it forms an oxbow lake ( ■ Fig. 14.24). Sometimes people attempt to control streams by build-ing up levees artificially in order to keep the river in its chan-nel. During times of reduced discharge, however, when a river has less energy, deposition occurs in the channel. Thus, in an artificially constrained channel, a river tends to raise the level of its channel bed. In some instances, as in China’s Huang He and the Yuba River in northern California, depo-sition has raised the streambed above the surrounding flood-plain. Flooding presents an especially serious danger in this situation, with much of the floodplain lying below the level of the river.
No longer available |Learn more- James Petersen, Dorothy Sack, Robert Gabler(Authors)
- 2016(Publication Date)
- Cengage Learning EMEA(Publisher)
Natural levees accumulate along the banks of these distributary channels. Continued deposition and delta formation extend the delta plain and create new land beyond the original shoreline. Rich alluvial deposits and the abundance of moisture allow vegetation to become established quickly on these fertile deposits and further secure the delta’s position. Delta plains, such as those of the Mekong, Indus, and Ganges Rivers, form important agricultural areas that feed the dense populations of many parts of Asia. Because of their very low elevation, extremely gentle gradient, and minimal relief, however, delta plains are especially susceptible to extensive flooding, and great loss of life has occurred on densely populated delta plains struck by tropical cyclones and storm surges. The same three factors of low elevation, gradient, and relief make delta plains vulnerable to effects of rising sea level. Different types of deltas form in different settings. Where the Mississippi River flows into the Gulf of Mexico, the river has con- structed the type of delta called a bird’s-foot delta (● Fig. 17.29a). Bird’s-foot deltas are constructed in settings where the influence of the fluvial system far exceeds the ability of waves, currents, and tides of the standing water body to rework the deltaic sediment into coastal landforms or to transport it away. Numerous distributaries slightly above sea level extend far out into the receiving water body. Occasional changes in the distributary channel system occur when a major new distributary is cut that siphons flow away from a previous one, causing the center of deposition to switch to a new location far from its previous position. The appearance in map view of the natural levees extending toward the present and former depositional centers leaves the delta resembling a bird’s foot. Other types of deltas do not project into their receiving water bodies as far or with as much spatial irregularity as bird’s foot del- tas.
eBook - PDF- Andrew S. Goudie(Author)
- 2013(Publication Date)
- Cambridge University Press(Publisher)
Alluvial rivers tend not to preserve records of the very largest floods because these are usually lost through channel erosion and enlargement during high-magnitude events. Conversely, evidence for large flood events is more commonly preserved in bedrock reaches because of their more stable geometries. The interpretation of slackwater deposits may not always be straightforward, as has been demonstrated by debates relating to the accumulation of laminated, silty valley fills such as occur along the Wadi Feiran in Sinai (Figure 5.9a) (Issar and Eckstein, 1969; Smykatz-Kloss et al. 2003), the Kuiseb and other rivers in Namibia (Smith et al. 1993; Eitel et al. 2001; Srivastava et al. 2006) (Figure 5.9b) and the Brachina Gorge in South Australia (Haberlah et al. 2010). Hypotheses for their development include flood deposition as slackwater deposits, accumulation in a lake or swamp as a result of a drainage line being dammed by dune accumulation, river end-point deposits and accumulation as a result of large aeolian silt deposition in the catchment and its translocation from slopes into the channel. Each of these mechanisms has very different environmental/climatic significance (Leopold et al. 2006). 5.19 Some Slope Forms: Hillslopes in Massive Rocks In a review of desert hillslopes, Mabbutt (1977a, p. 39), wrote: On a broad scale, there is a striking lack of slopes of intermediate angle. The hills rise abruptly, island-like, from plains of gentle declivity, a characteristic which suggested to early investigators that the deserts were abandoned seabeds, or which led to erroneous views of the hills as projections through depositional surfaces in landscapes drowned in their own detritus. 240 Rivers and Slopes (a) (b) Figure 5.9 Alluvial silts on desert rivers. (a) The horizontal silt deposits of the Wadi Feiran in Sinai. (b) The Homeb Silts on the Kuiseb River, Namibia. Their origin has been the subject of debate. (ASG)
eBook - PDFRiver Dynamics
Geomorphology to Support Management
- Bruce L. Rhoads(Author)
- 2020(Publication Date)
- Cambridge University Press(Publisher)
Other mechanisms include localized aggradation during floods that produces an elevated erosion-resistant platform and localized incision that isolates part of the valley floor from the active Figure 14.22. (a) Idealized depositional environment for an aggrading gravel-bed braided river (adapted from Selley, 2000). (b) Idealized depositional environment for a sand-bed braided river (redrawn by Sambrook Smith et al., 2006 from Cant and Walker, 1978). 338 The Dynamics of Floodplains braidtrain. Similarly, floodplains can be reactivated by lateral shifts in the position of the braidtrain and reactivation of abandoned floodplain channels by floods. The conversion of bars to floodplain, either in the form of islands within the braidtrain or as extensive surfaces adjacent to the braidtrain, involves stabilization of bar surfaces through growth of vegetation and accumulation of woody debris (Reinfelds and Nanson, 1993; Gurnell et al., 2001; Haschenburger and Cowie, 2009). Studies of braided rivers in New Zealand indicate that floodplains consist of an ensem- ble of discrete depositional units with different stages of devel- opment, frequency of inundation, amounts of accreted fines, vegetation coverage, and surface stability (Reinfelds and Nanson, 1993; Haschenburger and Cowie, 2009). Early stages include both erosional and depositional processes, but dom- inance of the latter leads to expansion of floodplain extent. Over time, as zones of deposition stabilize through increases in surface elevation and vegetation growth, the likelihood of erosion diminishes and the longevity of floodplains increases. The rather substantial extent and thicknesses of accreting fines on the surfaces of established and mature floodplains (Table 14.2) shows that these landforms store large amounts of fine sediment within gravel-bed braided river systems.
eBook - PDFRivers and Floodplains
Forms, Processes, and Sedimentary Record
- John S. Bridge(Author)
- 2009(Publication Date)
- Wiley-Blackwell(Publisher)
8.16 Models of sedimentary features of reoccupied channel belts. (a) channel deposits directly superimposed and two levee deposits separated by floodbasin deposits. (b) As for (a), but levee deposits of different channels cannot be distinguished. (c) Two channel and levee deposits are offset and distinguishable. From Stouthamer (2001). Fluvial deposits that have accumulated over relat-ively short time spans (on the order of millennia or less) have been considered up to this point. Inter-pretation of the origin of these essentially surficial deposits is based on observations from natural en-vironments and laboratory experiments as well as theoretical models. However, the thick and later-ally extensive accumulations of fluvial deposits in sedimentary basins formed over much longer time periods (millions of years). Such accumulations commonly show a range of vertical sequences of different scale (see also Leeder 1993; Miall 1996; Kraus & Aslan 1999), including much thicker ones than those described in previous chapters (Fig. 9.1, Plate 9.1, Table 9.1). It may be possible, given ex-tensive correlated outcrops or seismic data, to define regional unconformities or erosion surfaces that bound sequences of strata that are tens to hundreds of meters thick and tens to hundreds of kilometers in lateral extent (Fig. 9.2). Within these “sequences” there may be characteristic spatial dis-tributions of lithofacies and stratal geometry and orientation, for which a distinct terminology has arisen (review in Miall 1996). The term alluvial architecture (Allen 1978c) refers to the three-dimensional geometry, propor-tion, and spatial distribution of the various types of alluvial deposits (e.g. channel-belt and overbank deposits) in sedimentary basins. The term alluvial architecture has come to be concerned with relat-ively large-scale, long-term aspects of alluvial deposi-tion and erosion.
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