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11.
Anabranching rivers consist of multiple channels separated by vegetated semi-permanent alluvial islands excised from existing floodplain or formed by within-channel or deltaic accretion. These rivers occupy a wide range of environments from low to high energy, however, their existence has never been adequately explained. They occur concurrently with other types of channel pattern, although specific requirements include a flood-dominated flow regime and banks that are resistant to erosion, with some systems characterized by mechanisms to block or constrict channels, thereby triggering avulsion. The fundamental advantage of an anabranching river is that, by constructing a semi-permanent system of multiple channels, it can concentrate stream flow and maximize bed-sediment transport (work per unit area of the bed) under conditions where there is little or no opportunity to increase gradient. On the basis of stream energy, sediment size and morphological characteristics, six types of anabranching river are recognized; types 1–3 are lower energy and types 4–6 are higher energy systems. Type 1 are cohesive sediment rivers (commonly termed anastomosing) with low w/d ratio channels that exhibit little or no lateral migration. They are divisible into three subtypes based on vegetative and sedimentary environment. Type 2 are sand-dominated, island-forming rivers, and type 3 are mixed-load laterally active meandering rivers. Type 4 are sand-dominated, ridge-forming rivers characterized by long, parallel, channel-dividing ridges. Type 5 are gravel-dominated, laterally active systems that interface between meandering and braiding in mountainous regions. Type 6 are gravel-dominated, stable systems that occur as non-migrating channels in small, relatively steep basins. Anabranching rivers represent a relatively uncommon but widespread and distinctive group that, because of particular sedimentary, energy-gradient and other hydraulic conditions, operate most effectively as a system of multiple channels separated by vegetated floodplain islands or alluvial ridges. 相似文献
12.
The focus of this study is to understand a dramatic avulsion event on the Tisza River. During the Late Pleistocene the river course switched by about 80 km from the east to west of the Great Hungarian Plain (GHP) through the Záhony bend to its present meander belt. The aim of this study is to date this Záhony avulsion: based upon radiocarbon and pollen samples from six cores in the Polgár study area, situated in the west of the GHP at the middle course of the Tisza River. In addition, a grain size composition and heavy mineral analysis has been performed. The results of these analyses reveal a sequence of paleochannels and have been plotted on a high-resolution digital elevation model, illustrating the paleochannel form and age relationships.
The study suggests that the age of this major avulsion event is significantly older than it was previously supposed. Instead of 10 to 11 ka it is, according to our new data, 16–18 ka, and definitely predates the Last Glacial Maximum (LGM). The river cut into the previous, contiguous surface and formed at least one climatic terrace, then drifted gradually westward as a response to a tilt or the differential subsidence of tectonic origin. 相似文献
13.
Coastal morphodynamics and Chenier-Plain evolution in southwestern Louisiana, USA: A geomorphic model 总被引：1，自引：0，他引：1
Using 28 topographic profiles, air-photo interpretation, and historical shoreline-change data, coastal processes were evaluated along the Chenier Plain to explain the occurrence, distribution, and geomorphic hierarchy of primary landforms, and existing hypotheses regarding Chenier-Plain evolution were reconsidered. The Chenier Plain of SW Louisiana, classified as a low-profile, microtidal, storm-dominated coast, is located west and downdrift of the Mississippi River deltaic plain. This Late-Holocene, marginal-deltaic environment is 200 km long and up to 30 km wide, and is composed primarily of mud deposits capped by marsh interspersed with thin sand- and shell-rich ridges (“cheniers”) that have elevations of up to 4 m.In this study, the term “ridge” is used as a morphologic term for a narrow, linear or curvilinear topographic high that consists of sand and shelly material accumulated by waves and other physical coastal processes. Thus, most ridges in the Chenier Plain represent relict open-Gulf shorelines. On the basis of past movement trends of individual shorelines, ridges may be further classified as transgressive, regressive, or laterally accreted. Geomorphic zones that contain two or more regressive, transgressive, or laterally accreted ridges are termed complexes. Consequently, we further refine the Chenier-Plain definition by Otvos and Price [Otvos, E.G. and Price, W.A., 1979. Problems of chenier genesis and terminology—an overview. Marine Geology, 31: 251–263] and define Chenier Plain as containing at least two or more chenier complexes. Based on these definitions, a geomorphic hierarchy of landforms was refined relative to dominant process for the Louisiana Chenier Plain. The Chenier Plain is defined as a first-order feature (5000 km2) composed of three second-order features (30 to 300 km2): chenier complex, beach-ridge complex, and spit complex. Individual ridges of each complex type were further separated into third-order features: chenier, beach ridge, and spit.To understand the long-term evolution of a coastal depositional system, primary process–response mechanisms and patterns found along the modern Chenier-Plain coast were first identified, especially tidal-inlet processes associated with the Sabine, Calcasieu, and Mermentau Rivers. Tidal prism (Ω) and quantity of littoral transport (Mtotal) are the most important factors controlling inlet stability. Greater discharge and/or tidal prism increase the ability of river and estuarine systems to interrupt longshore sediment transport, maintain and naturally stabilize tidal entrances, and promote updrift deposition. Thus, prior to human modification and stabilization efforts, the Mermentau River entrance would be classified as wave-dominated, Sabine Pass as tide-dominated, and Calcasieu Pass as tide-dominated to occasionally mixed.Hoyt [Hoyt, J.H., 1969. Chenier versus barrier, genetic and stratigraphic distinction. Am. Assoc. Petrol. Geol. Bull., 53: 299–306] presented the first detailed depositional model for chenier genesis and mudflat progradation, which he attributed to changes in Mississippi River flow direction (i.e., delta switching) caused by upstream channel avulsion. However, Hoyt's model oversimplifies Chenier-Plain evolution because it omits ridges created by other means. Thus, the geologic evolution of the Chenier Plain is more complicated than channel avulsions of the Mississippi River, and it involved not only chenier ridges (i.e., transgressive), but also ridges that are genetically tied to regression (beach ridges) and lateral accretion (recurved spits).A six-stage geomorphic process-response model was developed to describe Chenier-Plain evolution primarily as a function of: (i) the balance between sediment supply and energy dissipation associated with Mississippi River channel avulsions, (ii) local sediment reworking and lateral transport, (iii) tidal-entrance dynamics, and (iv) possibly higher-than-present stands of Holocene sea level. Consequently, the geneses of three different ridge types (transgressive, regressive, and laterally accreted) typically occur contemporaneously along the same shoreline at different locations. 相似文献
14.
European settlement in southeastern Australia led to rapid changes in the morphology of many upland streams. However, our knowledge of the nature of these changes is limited as historical records and preserved palaeochannels are rare. In this study we compare a well‐preserved section of the late Holocene palaeochannel of Gilmore Creek to its present channel. We used a combination of map and aerial photograph interpretation, field survey, OSL dating and discharge analysis to describe and compare the modern and palaeochannels and establish a firm date for the timing of channel change. In common with many other streams in southeastern Australia Gilmore Creek's late Holocene channel meandered across a stable well‐vegetated and frequently inundated floodplain. After about 1830 European settlers quickly modified the catchment by clearing riparian and hillslope vegetation, introducing grazing animals and other exotic species and mining for alluvial gold in the headwaters. The OSL dates show that between about 1850 and 1880 the small meandering channel aggraded with coarse sands and then up to about 1 m of silty sand was deposited over the floodplain. Declining sediment input from upstream channel avulsion before 1890 resulted in the establishment of a straighter, larger capacity channel that incised to the level of basal cobbles and, in places, to bedrock. The dramatic change in channel pattern resembles that described on the Cann River in eastern Victoria following the removal of riparian vegetation and within‐channel coarse woody debris. At Gilmore Creek increased channel capacity has greatly reduced the average frequency of floodplain inundation. High values of specific stream power suggest that channel morphology is now well adjusted to the present flow regime. Copyright © 2007 John Wiley & Sons, Ltd. 相似文献
15.
Anastomosing rivers have multiple interconnected channels that enclose flood basins. Various theories potentially explain this pattern, including an increased discharge conveyance and sediment transport capacity of multiple channels, deltaic branching, avulsion forced by base‐level rise, or a tendency to avulse due to upstream sediment overloading. The former two imply a stable anabranching channel pattern, whereas the latter two imply disequilibrium and evolution towards a single‐channel pattern in the absence of avulsion. Our objective is to test these hypotheses on morphodynamic scenario modelling and data of a well‐documented case study: the upper Columbia River. Proportions of channel and floodplain sediments along the river valley were derived from surface mapping. Initial and boundary conditions for the modelling were derived from field data. A 1D network model was built based on gradually varied flow equations, sediment transport prediction, mass conservation, transverse slope and spiral meander flow effects at the bifurcations. The number of channels and crevasse splays decreases in a downstream direction. Also, measured sediment transport is higher at the upstream boundary than downstream. These observations concur with bed sediment overloading from upstream, which can have caused channel aggradation above the surrounding floodplain and subsequent avulsion. The modelling also indicates that avulsion was likely caused by upstream overloading. In the model, multi‐channel systems inevitably evolve towards single‐channel systems within centuries. The reasons are that symmetric channel bifurcations are inherently unstable, while confluenced channels have relatively less friction than two parallel channels, so that more discharge is conveyed through the path with more confluences and less friction. Furthermore, the present longitudinal profile curvature of the valley could only be reproduced in the model by temporary overfeeding. We conclude that this anastomosing pattern is the result of time‐varying sediment overloading and is not an equilibrium pattern feature, and suggest this is valid for many anastomosing rivers. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
16.
Co‐evolution of riparian vegetation and channel dynamics in an aggrading braided river system,Mount Pinatubo,Philippines 
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下载免费PDF全文 Increased bank stability by riparian vegetation can have profound impacts on channel morphology and dynamics in low‐energy systems, but the effects are less clear in high‐energy environments. Here we investigate the role of vegetation in active, aggrading braided systems at Mount Pinatubo, Philippines, and compare results with numerical modeling results. Gradual reductions in post‐eruption sediment loads have reduced bed reworking rates, allowing vegetation to finally persist year‐round on the Pasig‐Potrero and Sacobia Rivers. From 2009–2011 we collected data detailing vegetation extent, type, density, and root strength. Incorporating these data into the RipRoot model and BSTEM (Bank Stability and Toe Erosion Model) shows cohesion due to roots increases from zero in unvegetated conditions to > 10·2 kPa in densely‐growing grasses. Field‐based parameters were incorporated into a cellular model comparing vegetation strength and sediment mobility effects on braided channel dynamics. The model shows both low sediment mobility and high vegetation strength lead to less active systems, reflecting trends observed in the field. The competing influence of vegetation strength versus channel dynamics is a concept encapsulated in a dimensionless ratio between timescales for vegetation growth and channel reworking known as T*. An estimated T* between 1·5 and 2·3 for the Pasig‐Potrero River suggests channels are still very mobile and likely to remain braided until aggradation rates decline further. Vegetation does have an important effect on channel dynamics, however, by focusing flow and thus aggradation into the unvegetated fraction of braidplain, leading to an aggradational imbalance and transition to a more avulsive state. The future trajectory of channel–vegetation interactions as sedimentation rates decline is complicated by strong seasonal variability in precipitation and sediment loads, driving incision and armoring in the dry season. By 2011, incision during the dry season was substantial enough to lower the water‐table, weaken existing vegetation, and allow for vegetation removal in future avulsions. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
17.
Air photo interpretation and field survey were used to examine rates and patterns of planform change over the last 40 years on an 80 km reach of the Luangwa River, Zambia. The river, a tributary of the Zambezi, is a 100–200 m wide, medium sinuosity sand‐bed river (sinuosity index 1·84). High rates of channel migration (<33 m a−1) and cutoffs on meandering sections are frequent. Some meandering reaches, however, have remained relatively stable. A form of anastomosing with anabranches up to 14 km in length is also a characteristic. Patterns of meander development vary between bends but all can be described in relation to traditional geomorphic models; change occurs by translation, rotation, double‐heading, concave bank bench formation and cutoff causing river realignment. At the local scale spatial variability in bank resistance, induced by floodplain sedimentology, controls rate of bank erosion, and valley‐side channel ‘deflection’ is also apparent. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
18.
Haiyan Yang 《水文研究》2020,34(17):3702-3717
Gravel-bed braided rivers are highly energetic fluvial systems characterized by frequent in-channel avulsions, which govern the morphodynamics of such rivers and are essential for them to maintain a braided planform. However, the avulsion mechanisms within natural braided rivers remain unclear due to their complicated hydraulic and morphodynamic processes. Influenced by neighbouring channels, avulsions in braided rivers may differ from those of bifurcations in single-thread rivers, suggesting that avulsions should be studied within the context of the entire braid network. In this study, braiding evolution processes in gravel-bed rivers were simulated using a physics-based numerical model that considers graded bed-load transport by dividing sediment particles into multiple size fractions and vertical sediment sorting by dividing the riverbed into several vertical layers. The numerical model successfully produced braiding processes and avulsion activities similar to those observed in a laboratory river. Results show that bend evolution of the main channel was the fundamental process controlling the occurrence of avulsions in the numerical model, with a cyclic process of channel meandering by lateral migration that transitioned to a straight channel pattern by avulsion. The radius of bend curvature for triggering avulsions in the numerical model was measured and it was found that the highest probability for a channel bend to generate an avulsion occurs when its radius of curvature is approximately 2.0–3.3 times the average anabranch width. Other types of avulsion were also observed that did not occur specifically at meander bends, but upstream meander evolution indirectly influenced such avulsions by altering channel pattern and discharge to those locations. This study explored the processes and mechanisms of several types of avulsion, and proposed factors controlling their occurrence, namely increasing channel curvature, high shear stress, tributary discharge, riverbed gradient and upstream channel pattern, with high shear stress being a direct indicator. Furthermore, avulsions in a typical gravel-bed braided river, the Waimakariri River in New Zealand, were analysed using sequential Google Earth maps, which confirmed the conclusions derived from the numerical simulation. 相似文献
19.
Maarten G. Kleinhans Kim M. Cohen Jantine Hoekstra Janneke M. IJmker 《地球表面变化过程与地形》2011,36(15):2011-2027
Rivers may dramatically change course on a fluvial plain. Such an avulsion temporarily leads to two active channels connected at a bifurcation. Here we study the effect of dynamic meandering at the bifurcation and the effect of channel width adjustment to changing discharge in both downstream branches on the evolution of a bifurcation and coexisting channels. As an example, we reconstructed the last major avulsion at the Rhine delta apex. We combined historical and geological data to reconstruct a slowly developing avulsion process spanning 2000 years and involving channel width adjustment and meandering at the bifurcation. Based on earlier idealised models, we developed a one‐dimensional model for long‐term morphodynamic prediction of upstream channel and bifurcates connected at the bifurcation node. The model predicts flow and sediment partitioning at the node, including the effect of migrating meanders at the bifurcation and channel width adjustment. Bifurcate channel width adaptation to changing discharge partitioning dramatically slows the pacing of bifurcation evolution because the sediment balance for width adjustment and bed evolution are coupled. The model further shows that meandering at the bifurcation modulates channel abandonment or enlargement periodically. This explains hitherto unrecognised reactivation signals in the sedimentary record of the studied bifurcation meander belts, newly identified in our geological reconstruction. Historical maps show that bifurcation migration due to meander bend dynamics increases the bifurcation angle, which increases the rate of closure of one bifurcate. The combination of model and reconstruction identifies the relevant timescales for bifurcation evolution and avulsion duration. These are the time required to fill one downstream channel over one backwater length, the time to translate one meander wavelength downstream and, for strong river banks, the adaptation timescale to adjust channel width. The findings have relevance for all avulsions where channel width can adjust to changing discharge and where meandering occurs. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
20.
Little is known about controls on river avulsion at geological time scales longer than 104 years, primarily because it is difficult to link observed changes in alluvial architecture to well‐defined allogenic mechanisms and to disentangle allogenic from autogenic processes. Recognition of Milankovitch‐sale orbital forcing in alluvial stratigraphy would provide unprecedented age control in terrestrial deposits, and also exploit models of allogenic forcing enabling more rigorous testing of allocyclic and autocyclic controls. The Willwood Formation of the Bighorn Basin is a lower Eocene fluvial unit distinctive for its thick sequence of laterally extensive lithological cycles on a scale of 4 to 10 m. Intervals of red palaeosols that formed on overbank mudstones are related to periods of relative channel stability when gradients between channel belts and floodplains were low. The intervening drab, heterolithic intervals with weak palaeosol development are attributed to episodes of channel avulsion that occurred when channels became super‐elevated above the floodplain. In the Deer Creek Amphitheater section in the McCullough Peaks area, these overbank and avulsion deposits alternate with a dominant cycle thickness of ca 7·1 m. Using integrated stratigraphic age constraints, this cyclicity has an estimated period of ca 21·6 kyr, which is in the range of the period of precession climate cycles in the early Eocene. Previous analyses of three older and younger sections in the Bighorn Basin showed a similar 7 to 8 m spacing of red palaeosol clusters with an estimated duration close to the precession period. Intervals of floodplain stability alternating with episodes of large‐scale reorganization of the fluvial system could be entirely autogenic; however, the remarkable regularity and the match in time scales documented here indicate that these alternations were probably paced by allogenic, astronomically forced climate change. 相似文献