Geography
River Erosion Landforms
River erosion landforms are features created by the erosive action of rivers. They include meanders, oxbow lakes, river cliffs, and river terraces. Meanders are sinuous bends in a river's course, while oxbow lakes form when a meander is cut off from the main river. River cliffs are steep slopes formed by erosion, and river terraces are flat, raised surfaces alongside a river.
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12 Key excerpts on "River Erosion 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- 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- David Huddart, Tim A. Stott(Authors)
- 2019(Publication Date)
- Wiley-Blackwell(Publisher)
The morphology of the river channel, including its cross‐ sectional shape, size, longitudinal profile and planform pattern, is the result of sediment erosion, transport, and deposition processes taking place within the controls imposed by the geology and ter- rain of the drainage basin. Rivers are constantly evolving and adjusting as a response to the sequence of normal flow, flood peaks and droughts, which are controlled by the regional climate, local weather and 17.9 Fluvial Channel Geomorphology 363 Dendritic (a) (b) (c) Parallel Rectangular Annular Contorted Multi–basinal Radial Trellis Figure 17.7 (a) Drainage patterns. (b) Dendritic drainage, Dirty Bend, Utah, USA. (c) Radial drainage, Bromo volcano, Java. Source: http://rst.gsfc.nasa.gov. 17 Fluvial Processes and Landform-Sediment Assemblages 364 catchment hydrology. In this respect the channel geomorphology can be best explained if distinctions are made between those factors which drive the flu- vial system in producing the channel, those which characterize the physical boundaries of the channel and those which respond to the driving and bound- ary conditions and define the channel form. 17.9.1 River Erosion Processes There are four principal erosion processes: ● Corrasion takes place when the river picks up sed- iment, which acts like sandpaper and wears away the rock by abrasion. It is effective during flood stages and is the major process by which rivers erode horizontally and vertically. It can produce bedrock hollows and potholes, where gravels can be trapped in pre‐existing holes, and because the current is turbulent, the gravel can be swirled around in these holes which enlarge vertically. ● Corrosion or solution is where the rock is dis- solved in the water. It occurs ubiquitously and continuously and takes place independent of river discharge or velocity, although it can be affected by the chemical composition of the water, particu- larly from the soil system.
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 - 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- Mateo Gutierrez(Author)
- 2012(Publication Date)
- CRC Press(Publisher)
The most important geologic erosional process on the Earth’s surface is water that runs off the land (Morisawa, 1968). Water creates most landscapes and forms flood plains where many of our cities are built. Fluvial action generates steep slopes that can cause landslides as their instability increases. Glacial melt water flows down braided channels and transports glacial materials downstream. Exorheic rivers deliver water to oceans but those that flow though endor- heic areas deposit their loads in interior basins. Finally, under- ground currents move particles under the Earth’s surface (Morisawa, 1985). 8.2 Brief history of fluvial geomorphology Fluvial geomorphology developed along with the science of geomorphology. During the 17th and 18th centuries, hydrau- lic engineers such as Perrault, Surrell, Guettard, Desmarest, and De Saussure studied the role of fluvial action in degrada- tion and landscape formation in spite of the diluvialist ideas that prevailed during this time. These ideas were gradually rejected by the work of Scottish geologists Hutton (18th cen- tury) and Lyell (19th century) who finally put an end to theory of catastrophism, which had prevailed for many centuries. As scientific geology developed, geomorphology followed. At the end of the 19th century, North American geologists Dutton and Powell provided the most interesting advances with their documentation of the erosional activity of rivers in canyon formation. To Powell we owe the concept of the base level of erosion; he considered the existence of local base levels and the sea as the general base level. He also clas- sified rivers as consequent, antecedent, and superimposed. At the end of the 19th and beginning of the 20th century, the North American geologist Gilbert contributed his ideas on processes of fluid mechanics, capacity and competence, the concept of grade, and the interdependence of variables within a fluvial system (Morisawa, 1985).
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 - ePubNatural Hazards
Earthquakes, Volcanoes, and Landslides
- Ramesh Singh, Darius Bartlett, Ramesh Singh, Darius Bartlett(Authors)
- 2018(Publication Date)
- CRC Press(Publisher)
The Earth experiences alterations upon its relief by exogenous dynamic processes. The most significant of these processes is erosion, which sculpts the superficial layer of the crust. The main processes of erosion are due to moving water, ice, wind and gravity. Erosion depends on various factors, such as climate, rock, soil, morphology and land uses. The climatic conditions determine the wind regime, which is responsible for wind erosion and erosion due to wave action and currents. The amount of rainfall is also dependent on the climate. The more rain, the more surface runoff and erosion that are produced, which induces a higher rill and interrill erosion, as well as riverbed erosion. The rock type determines the erosion type developed on it. For example, a coastal limestone cliff will undergo wave erosion as well as bioerosion, because this lithology is prone to both mechanical and chemical erosion. Lastly, the morphology and land uses pose their own interaction on erosion. Human-made interventions on the plant cover of a slope can enhance rill erosion by reducing the plant volume. Finally, each erosion type produces a variety of distinct geomorphic features.Erosion is the natural phenomenon of the combined effect of weathering and transportation of the weathered products. It is a two-dimensional process that involves both a static and a dynamic aspect. The Earth surface and subsurface are made of rocks that experience tectonic forces, and also the Earth surface is exposed to winds, heat, rainfall and ocean waves, and as a result, the rocks are weathered and deformed. These forces try to eliminate Earth’s relief, leading it towards a peneplain. The static aspect of erosion relies on the weathering process of rocks, which takes place in situ, whereas the dynamic process includes the transportation of the weathered products (rock fragments). This double quality distinguishes erosion from plain weathering.The first stage of erosion is the detachment of rock fragments, in shapes and sizes that depend on the type of weathering, which might be either mechanical or chemical. The detached particles overlie the parent rock. According to the prevailing climatic and topographic conditions, the transportation of the fragments takes place, comprising the second stage of erosion. This is accomplished via moving water, wind, ice (glaciers) and gravity. These four major factors alter the surface topography and renew it in the course of geologic time. The last stage is the deposition of the transferred material.Moving water is a significant eroding agent. Large amounts of precipitation produce high quantities of superficial runoff. Increased runoff during rainfall moves notable sediment masses in a drainage basin from its high-relief part towards its low-relief part. This is accomplished through riverbed erosion, embankment erosion and mass movements on the valley sides as streams engrave downward.
eBook - PDF- James Petersen, Dorothy Sack, Robert Gabler, , James Petersen, Dorothy Sack, Robert Gabler(Authors)
- 2014(Publication Date)
- Cengage Learning EMEA(Publisher)
Fluvial Processes Stream Erosion Fluvial erosion is the removal of rock material by flowing water. Fluvial erosion consists of the chemical removal of ions from rocks and the physical removal of rock fragments (clasts). Physical removal of rock fragments includes breaking off new pieces of bedrock from the channel bed or sides and moving them as well as picking up and removing preexisting clasts that were temporarily resting on the channel bottom. The removal of rock material by erosion does not necessar-ily mean that the landscape is undergoing long-term lowering. If sediments eroded from the bed of a stream channel are replaced by the deposition of other fragments transported in from upstream, there will be no net drop in the position of the chan-nel bottom. Such lowering, known as channel incision, occurs only when there is net erosion compared to deposition. Net ero-sion results in the lowering of the affected part of the landscape and is termed degradation . Net deposition of sediments results in a building up, or aggradation , of the landscape. One way that streams erode occurs when stream water chemically dissolves rock material and then transports the ions away in the flow. This fluvial erosion process, called corro-sion , has only a limited effect on many rocks but can be sig-nificant in certain rock types, such as limestone. Suspension Saltation Traction © Cengage Learning ■ FIGURE 14.13 Transportation of solid load in a stream. Clay and silt particles are carried in suspension. Sand typically travels by suspension and saltation. The largest (heaviest) particles move by traction. What is the difference between traction and saltation? 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).
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)
490 C H A P T E R 1 7 • 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 rocks lying upstream from the resistant rock forming the tempo- rary base level (see Fig. 17.7). Many streams in this type of setting spill from lake to lake in their upper courses, either over open land (like the Niagara River at Niagara Falls, between Lake Erie and Lake Ontario) or through narrow gorges. In either case, a lake will eventually be eliminated—become a regular segment of the stream channel—if the outflowing stream lowers the lake basin outlet enough by erosion or if fluvial deposition at the inflow points fills in the lake basin with sediment. 17-3b Middle Stream Course In the middle section of the ideal longitudinal profile, the stream flows over a moderate gradient and on a moderately smooth channel bed. Here the stream valley includes a floodplain, but remaining ridges beyond the floodplain still form definite valley walls. The stream lies closer to its base level, flows over a gentler gradient, and thus directs less energy toward vertical erosion than in its upper course. The flow still has considerable energy, however, because of the downstream increase in the related vari- ables of water volume, discharge, and velocity. The river now uses much of its available energy for transporting the considerable load that it has accumulated and for lateral erosion of the channel sides. Because of lateral erosion, the middle course of the stream develops a definite meandering channel pattern with its sinuous bends that wander over time across the valley floor. The stream erodes a cut bank on the outside of meander loops, where the channel is deep and centrifugal force accelerates stream velocity. The cut bank is a steep slope, and slumping may occur there, particularly when there is a rapid fall in water level. Slumping on the outside of meander bends contributes to the effect of lateral erosion by the stream and adds load to the stream.
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