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In sedimentary geology and fluvial geomorphology, avulsion is the rapid abandonment of a river channel and the formation of a new river channel. Avulsions occur as a result of channel slopes that are much less steep than the slope that the river could travel if it took a new course. ==Deltaic and net-depositional settings== Avulsions are common in deltaic settings, where sediment deposits as the river enters the ocean and channel gradients are typically very small.〔Marshak, Stephen (2001), Earth: Portrait of a Planet, New York: W.W. Norton & Company, ISBN 0-393-97423-5 pp. 528–9〕 This process of avulsion in deltaic settings is also known as delta switching. Deposition from the river results in the formation of an individual deltaic lobe that pushes out into the sea. An example of a deltaic lobe is the bird's-foot delta of the Mississippi River, pictured at right with its sediment plumes. As the deltaic lobe advances, the slope of the river channel becomes lower because the river channel is longer but has the same change in elevation (see slope or gradient). As the slope of the river channel decreases, it becomes unstable for two reasons. First, water under the force of gravity will tend to flow in the most direct course downslope. If the river could breach its natural levees (i.e., during a flood), it would spill out onto a new course with a shorter route to the ocean, thereby obtaining a more stable steeper slope.〔 Second, as its slope is reduced, the amount of shear stress on the bed will decrease, resulting in deposition of more sediment within the channel and thus raising of the channel bed relative to the floodplain. This will make it easier for the river to breach its levees and cut a new channel that enters the ocean at a steeper slope. When this avulsion occurs, the new channel carries sediment out to the ocean, building a new deltaic lobe.,〔Stanley, Steven M. (1999) Earth System History. New York: W.H. Freeman and Company, ISBN 0-7167-2882-6 p. 136〕〔Marshak, pp. 528–9〕 The abandoned delta eventually subsides.〔Stanley, p. 136〕 This process is also related to the distributary network of river channels that can be observed within a river delta. When the channel does this, some of its flow can remain in the abandoned channel. When these channel switching events happen repeatedly over time, a mature delta will gain a distributary network.〔Easterbrook, Don J.''Surface Processes and Landforms Second Edition''Prentice Hall, New Jersey: 1999.〕 Subsidence of the delta and/or sea-level rise can further cause backwater and deposition in the delta. This deposition fills the channels and leaves a geologic record of channel avulsion in sedimentary basins. On average, an avulsion will occur every time the bed of a river channel aggrades enough that the river channel is superelevated above the floodplain by one channel-depth. In this situation, enough hydraulic head is available that any breach of the natural levees will result in an avulsion.〔Bryant, M., P. Falk, and C. Paola (1995), Experimental study of avulsion frequency and rate of deposition, Geology (Boulder), 23, 365–368.〕〔Mohrig, D., P. L. Heller, C. Paola, and W. J. Lyons (2000), Interpreting avulsion process from ancient alluvial sequences; Guadalope-Matarranya system (northern Spain) and Wasatch Formation (western Colorado), Geological Society of America Bulletin, 112, 1787–1803.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Avulsion (river)」の詳細全文を読む スポンサード リンク
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