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1.
陈立  陈帆  张为  袁晶 《湖泊科学》2020,32(3):840-849
分汊河段冲淤调整会引起汊道分流比的复杂变化.建立准确的分流比计算公式是研究分流比在不同水沙、地形边界条件下变化的基础.以长江上荆江分汊河段——沙市段为例,利用三峡水库蓄水后的实测资料对现有分流比计算式的适用性进行比较,通过引入综合反映两汊糙率、比降差异的因子,建立了较高精度的分流比计算式,在此基础上,分析了沙市分汊段分流比的变化特点.结果表明:(1)当无法准确反映两汊糙率、比降的差异时,枯水分流比计算误差较大,最大计算偏差达15%;(2)两汊平均水深之比与糙率、比降综合影响系数比高度相关,引入两汊平均水深之比,建立了最大计算偏差小于5%的分流比计算式;(3)三峡水库蓄水后沙市段汊道冲淤变化对枯水分流比的影响大于洪水分流比,对微弯分汊段分流比的影响大于顺直分汊段.  相似文献   

2.
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   

3.
气候变化和流域人类活动等综合影响改变了河道演变的自然进程,尤其是流域大型水库的影响,深刻改变了下游的水沙通量与河道演变过程及趋势.以三峡工程为对象,研究坝下游水沙通量及河道演变过程对三峡工程运行的响应关系,可加深大型水利枢纽运行对下游河道演变影响的认识.本文以长江中游河段（宜昌-湖口河段）为研究对象,分析1975-2017年河道冲淤变化,结果表明：近40年来,长江中游河床已发生了累积性的冲刷,近坝段砂卵石河段冲刷强度先增强后减弱,沙质河段冲刷强度呈增强态势.三峡工程运行后不同河型演变特征归纳为：分汊河段江心洲以冲刷为主,上荆江河段内实施的航道整治工程稳定了江心洲形态,但面积受清水下泄的影响呈减小态势,枯水期主汊分流比呈减小态势,部分汊道发生了主支汊交替变化;下荆江及下游分汊河段江心洲面积虽然交替变化,整体上航道整治工程控制了江心洲规模;弯曲河段凸岸侧边滩以冲刷为主,急弯段的凹岸侧深槽淤积,微弯段凸岸边滩冲刷,对应的凹岸侧深槽相对稳定.  相似文献   

4.
This study presents the first detailed field‐based analysis of the morphology of bifurcations within anabranching cobble–gravel rivers. Bifurcations divide the flow of water and sediment into downstream anabranches, thereby influencing the characteristics of the anabranches and the longevity of river islands. The history, morphology, bed grain size, and flow vectors at five bifurcations on the Renous River, New Brunswick, Canada, were studied in detail. The angles of bifurcations within five anabranching rivers in the Miramichi basin were investigated. The average bifurcation angle was 47°, within the range of values cited for braided river bifurcations. Bifurcation angle decreased when anabranches were of similar length. Shields stresses in channels upstream of bifurcations were lower than reported values for braided rivers. Stable bifurcations displayed lower Shields stresses than unstable bifurcations, contrary to experimental results from braided river bifurcations. Bifurcations in anabranching rivers are stabilized by vegetation that slows channel migration and helps to maintain a uniform upstream flow field. The morphology of stable bifurcations enhances their stability. A large bar, shaped like a shallow ramp that increases in elevation to floodplain level, forms at stable bifurcations. Floodplains at stable bifurcations accrete upstream at rates between 0·9 and 2·5 m a?1. Bars may also form within the entrance of an anabranch downstream of the bifurcation node. These bars are associated with bifurcation instability, forming after a period of stability or an avulsion. Channel abandonment occurs when a bar completely blocks the entrance to one anabranch. The stability of channels upstream of bifurcations and the location of bars at bifurcations influence bifurcation stability and the maintenance of river anabranching in the long term. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

5.
When studying the evolution of landscape, it is difficult to discriminate the influence of anthropogenic from natural causes, or recognise changes caused by different sources of human action. This is especially challenging when the influence of certain sources is overprinted. For instance, although dam closure is the most common method of altering river courses, dam construction is often preceded by hydro‐technical works such as channel straightening, embankment construction or sediment mining. Both dam construction and the hydro‐technical works that precede dam closure can result in changes in the balance between sediment supply and transport capacity, and often, changes in river planform. The main objective of this study was to verify whether the works preceding dam closure are an important driver of river planform changes on the lower Drava River (Hungary). The case study is based on geological and geophysical surveys, as well as the analysis of historical maps covering an anabranching, 23 km long valley section. We show that channel straightening conducted prior to dam closure resulted in a transition from a meandering to sinuous planform with channel bars. Dam construction itself then caused enhanced incision, exposure of bar surfaces, vegetation encroachment and the formation of an anabranching planform. Based on this study, we developed models of alluvial island and channel planform evolution downstream of dams. Dam construction enhances channel incision, narrowing, and the reduction of flow caused by earlier hydro‐technical works. Many rivers downstream of dams experience episodes of anabranching or wandering, with a multi‐thread pattern replacing sinuous, braided and meandering courses. When incision continues, river patterns evolve from anabranching to sinuous via the attachment of alluvial islands to floodplains. However, the timing and sequence of these changes depend on hydrological and sediment supply regimes, geomorphic settings and anthropogenic actions accompanying dam construction. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

6.
Anabranching rivers evolve in various geomorphic settings and various river planforms are present within these multi‐channel systems. In some cases, anabranches develop meandering patterns. Such river courses existed in Europe prior to intensive hydro‐technical works carried out during the last 250 years. Proglacial stream valleys, inherited from the last glaciation, provided a suitable environment for the development of anabranching rivers (wide valleys floors with abundant sand deposits). The main objective of the present study is to reconstruct the formation of an anabranching river planform characterized by meandering anabranches. Based on geophysical and geological data obtained from field research and a reconstruction of palaeodischarges, a model of the evolution of an anabranching river formed in a sandy floodplain is proposed. It is demonstrated that such a river system evolves from a meandering to an anabranching planform in periods of high flows that contribute to the formation of crevasse splays. The splay channels evolve then into new meandering flow paths that form ‘second‐order’ crevasses, avulsions and cutoffs. The efficiency of the flow is maintained by the formation of cutoffs and avulsions preventing the development of high sinuosity channels, and redirecting the flow to newly formed channels during maximum flow events. A comparison with other anabranching systems revealed that increased discharges and sediment loads are capable of forming anabranching planforms both in dryland and temperate climate zones. The sediment type available for transport, often inherited from older sedimentary environments, is an important variable determining whether the channel planform is anabranching, with actively migrating channels, or anastomosing, with stable, straight or sinuous branches. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

7.
Anthropogenic climate change is expected to change the discharge and sediment transport regime of river systems. Because rivers adjust their channels to accommodate their typical inputs of water and sediment, changes in these variables can potentially alter river morphology. In this study, a hierarchical modeling approach was developed and applied to examine potential changes in reach‐averaged bedload transport and spatial patterns of erosion and deposition for three snowmelt‐dominated gravel‐bed rivers in the interior Pacific Northwest. The modeling hierarchy was based on discharge and suspended‐sediment load from a basin‐scale hydrologic model driven by a range of downscaled climate‐change scenarios. In the field, channel morphology and sediment grain‐size data for all three rivers were collected. Changes in reach‐averaged bedload transport were estimated using the Bedload Assessment of Gravel‐bedded Streams (BAGS) software, and the Cellular Automaton Evolutionary Slope and River (CAESAR) model was used to simulate the spatial pattern of erosion and deposition within each reach to infer potential changes in channel geometry and planform. The duration of critical discharge was found to control bedload transport. Changes in channel geometry were simulated for the two higher‐energy river reaches, but no significant morphological changes were found for a lower‐energy reach with steep, cohesive banks. Changes in sediment transport and river morphology resulting from climate change could affect the management of river systems for human and ecological uses. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

8.
Depositional environments along the tidal river downstream of Óbidos have been proposed as important sinks for up to one third of the sediment discharge from the Amazon River. However, the morphology of the intertidal floodplain and the dynamics of sediment exchange along this reach have yet to be described. River-bank surveys in five regions along the Amazon tidal river reveal a distinct transition in bank morphology between the upper, central and lower reaches of the tidal river. The upper tidal-river floodplain is defined by prominent natural levees that control the transfer of water and sediment between the mainstem Amazon River and its floodplain. Greater tidal influence in the central tidal river suppresses levee development, and tidal currents increase sediment transport into the distal parts of the floodplain. In the lower tidal river, the floodplain morphology closely resembles marine intertidal environments (e.g. mud flats, salt marshes), with dendritic tidal channels incising elevated vegetated flats. Theory, morphology and geochronology suggest that the dynamics of sediment delivery to the intertidal floodplain of the Amazon tidal river vary along its length due to the relative influence and coupling of fluvial and tidal dynamics. © 2018 John Wiley & Sons, Ltd.  相似文献   

9.
Bo Wang  Yi-Jun Xu 《水文研究》2020,34(13):2864-2877
Bed material transport at river bifurcations is crucial for channel stability and downstream geomorphic dynamics. However, measurements of bed material transport at bifurcations of large alluvial rivers are difficult to make, and standard estimates based on the assumption of proportional partitioning of flow and bedload transport at bifurcations may be erroneous. In this study, we employed a combined approach based on observed topographic change (erosion/deposition) and bed material transport predicted from a one-dimensional model to investigate bed material fluxes near the engineering-controlled Mississippi-Atchafalaya River diversion, which is of great importance to sediment distribution and delivery to Louisiana's coast. Yang's (1973) sediment transport equation was utilized to estimate daily bed material loads upstream, downstream, and through the diversion during 2004–2013. Bathymetric changes in these channels were assessed with single beam data collected in 2004 and 2013. Results show that over the study period, 24% of the Mississippi River flow was diverted into the Atchafalaya River, while the rest remained in the mainstem Mississippi. Upstream of the diversion, the bed material yield was predicted to be 201 million metric tons (MT), of which approximately 35 MT (i.e., 17%) passed through the bifurcation channel to the Atchafalaya River. The findings from this study reveal that in the mainstem Mississippi, the percentage of bed material diversion (83%) is larger than the percentage of flow diversion (76%); Conversely, the diversion channel receives a disproportionate amount of flow (24%) relative to bed material supply (17%). Consequently, severe bed scouring occurred in the controlled Outflow Channel to the Atchafalaya River, while riverbed aggradation progressed in the mainstem Mississippi downstream of the diversion structures, implying reduced flow capacity and potential risk of a high backwater during megafloods. The study demonstrates that Yang's sediment transport equation provides plausible results of bed material fluxes for a highly complicated large river diversion, and that integration of the sediment transport equation with observed morphological changes in riverbed is a valuable approach to investigate sediment dynamics at controlled river bifurcations.  相似文献   

10.
《国际泥沙研究》2020,35(4):365-376
The Yom River is one of the four major sediment sources to the Chao Phraya River in Thailand. Human activities and changes in climate over the past six decades may have affected the discharge and sediment load to some extent. In the current study, the river discharge and sediment characteristics in the mainstream of the Yom River were investigated using the field observation data from 2011 to 2013 and the historical river flow and sediment data from 1954 to 2014 at six hydrological stations operated by the Royal Irrigation Department of Thailand (RID). The non-parametric Mann-Kendall test and double mass curve were used to analyze the sediment dynamics and temporal changes in the discharge of the Yom River. The results revealed that the sediment was mainly transported in suspension, and the bed-to-suspended sediment loads ratio varied between 0 and 0.05. The daily suspended sediment load (SSL) in the upper and middle basins had a strong correlation with the daily discharge and could be represented by power equations with coefficients of determination higher than 0.8. The daily suspended sediment load in the lower basin did not directly depend on the corresponding discharge because of the reduction in river slope and water diversion by irrigation projects. It also appeared that the river discharges and sediment loads were mainly influenced by climate variation (floods and droughts). Moreover, the average sediment transport of the upper, middle, and lower reaches were 0.57, 0.71, and 0.35 million t/y, respectively. The sediment load in the lower basin decreased more than 50% as a result of changes in the river gradient (from mountainous to floodplain areas). The results from sediment analysis also indicated that the construction of the Mae Yom Barrage, the longest diversion dam in Thailand, and land-use changes did not significantly affect the sediment load along the Yom River.  相似文献   

11.
Least action principle (LAP) in rivers is demonstrated by maximum flow efficiency (MFE) and is the foundation of variational mechanics based on energy and work rather than Newtonian force and momentum. Empirical evidence shows it to be the primary control for the adjustment of alluvial channels. Because most rivers flow with imposed water and sediment loads down valley gradients they have largely inherited, they self‐regulate energy expenditure to match the work they are required to do to remain stable. Overpowered systems develop a variety of channel patterns to expend excess energy and remain stable. Australia offers an opportunity to study low‐energy rivers closely adjusted to very low continental gradients. The anabranching Marshall and single‐thread Plenty Rivers flow down nearly straight channels with average H numbers [ratio between excess bed shear and width/depth (W/D) ratio] close to the optimum of 0.3 for stationary equilibrium. Ridge‐form divisions of the original channel width create anabranches that radically alter W/D ratios relative to bed shear, the same being true for short‐wide islands on the large low‐gradient Yangtze River in China. In contrast, Mount Chambers Creek in Australia's tectonically more active Flinders Ranges is accreting an alluvial fan with unstable distributary channels exhibiting H numbers well below the optimum. LAP also explains profound biases in Earth's stratigraphic record. Because meandering is an energy‐shedding mechanism, sinuous rivers sequester relatively little sediment resulting in all sequences being just a few tens of metres thick. In contrast, low‐energy braided disequilibrium systems can sequester sediment piles over a kilometre in thickness and tens of kilometres wide. LAP provides a new paradigm for river research by identifying the attractor state controlling river channel evolution. It links advances in theoretical physics to fluvial geomorphology, stratigraphy and hydraulic engineering and opens opportunities for diverse investigations in Earth system science. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

12.
长江下游仪征河段处于枯季潮流界的上边界,揭示其汊道分流属性及滩槽联动演变机制,对河势控制工程及深水航道工程实践具有重要意义.本研究收集了1955—2021年水文泥沙及地形等资料,在汊道分流关系及调整成因上:世业洲右汊的分流属性为枯水倾向型汊道,即低流量时期分流比大于高流量时期;1959—2021年期间,世业洲右汊分流比经历了“稳定-下降-上升”的调整过程,上游河段滩槽格局调整及流域来沙减少引起的汊道间不均衡冲刷是分流关系调整的主因;流域流量过程调整、河道崩岸等综合影响引起1959—2017年期间世业洲右汊分流比为减小态势,航道工程实施起到了调控汊道关系的功能,世业洲右汊分流比为增加态势.在滩槽联动演变关系上:仪征河段进口段以展宽为主,世业洲左汊展宽程度大于右汊,左汊河床形态变化与进口段滩槽形态的一致性关系优于右汊,即上游进口段滩槽演变、流域来沙量减少等综合作用会加速了左汊发展;2015年南京以下12.5 m深水航道二期工程建设以来,工程区域淤积且洲体完整性增强,且深槽冲刷及河槽容积增大,表明航道工程已实现汊道分流关系及滩槽调控的功能.  相似文献   

13.
《国际泥沙研究》2022,37(6):857-870
A macrotidal estuary with mountain streams (MEMS) is characterized by rapidly rising and falling flood peaks and large tidal ranges and is a typical estuary type with strong flow dynamics that is found worldwide. Understanding the morphodynamic evolution of MEMSs has great significance for river management. The roles of the mountain stream river flood process and macrotides on the morphology evolution still needs further quantitative study. Taking the Oujiang Estuary as an example, the evolution mechanisms of bifurcated reaches of a MEMS are studied by analyzing the hydrologic and topographic data and using a two-dimensional numerical model for flow-sediment transport. The results show that (1) under the combined actions of runoff and macrotides, sediment transport is active and morphological evolution is intensive, the main branch and subbranch historically switch frequently, and erosion generally occurs in wet years, while deposition occurs in dry years. (2) The river flood process of mountain streams causes rapid erosion, which rises and falls rapidly with a kurtosis index (K) of approximately 2.9. There is a logarithmic relation between the sediment transport load and K. The volume of sediment transport will be 40% larger than the river flood process with K = 1.2 as for the Yangtze River. (3) Macrotides with large tidal ranges could enhance the discharge asymmetry (ψ). When the ψ is greater than 1, the side branches are dominated by flood tides. Although the current study is site specific, the results are expected to provide a valuable reference for sustainable management in similar estuaries.  相似文献   

14.
Natural bedrock rivers flow in self‐formed channels and form diverse erosional morphologies. The parameters that collectively define channel morphology (e.g. width, slope, bed roughness, bedrock exposure, sediment size distribution) all influence river incision rates and dynamically adjust in poorly understood ways to imposed fluid and sediment fluxes. To explore the mechanics of river incision, we conducted laboratory experiments in which the complexities of natural bedrock channels were reduced to a homogenous brittle substrate (sand and cement), a single sediment size primarily transported as bedload, a single erosion mechanism (abrasion) and sediment‐starved transport conditions. We find that patterns of erosion both create and are sensitive functions of the evolving bed topography because of feedbacks between the turbulent flow field, sediment transport and bottom roughness. Abrasion only occurs where sediment impacts the bed, and so positive feedback occurs between the sediment preferentially drawn to topographic lows by gravity and the further erosion of these lows. However, the spatial focusing of erosion results in tortuous flow paths and erosional forms (inner channels, scoops, potholes), which dissipate flow energy. This energy dissipation is a negative feedback that reduces sediment transport capacity, inhibiting further incision and ultimately leading to channel morphologies adjusted to just transport the imposed sediment load. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

15.
The choice of a river training strategy is extremely important for the Lower Yellow River (LYR). Currently, the wide-river training strategy applies in the training of the LYR. However, remarkable changes in the hydrological processes in the Yellow River basin, as well as immediate pressure from socio-economic development in the Yellow River basin, make it necessary to consider if there is a possibility to change the river training strategy from wide-river training to narrow-river training. This research investigates the impacts of different river training strategies on the LYR through numerical simulations. A one-dimensional (1-D) model was used to simulate the fluvial processes for the future 50 years and a three-dimensional (3-D) model was applied to study typical floods. The study focused on river morphology, the results show that if the present decreasing trend in both water discharge and sediment load persists, the deposition rate in the LYR will further decrease no matter what strategy is applied. Especially, narrow-river training can achieve the aim to increase the sediment transport capacity in the LYR compared with wide-river training. However, if the incoming water and sediment load recovers to the mean level of the last century, main channel shrinkage due to sedimentation inevitably occurs for both wide-river and narrow-river training. Most importantly, this study shows that narrow-river training reduces the deposition amount over the whole LYR, but it provides little help in alleviating the development of the “suspended river”. Instead, narrow-river training can cause aggradation in the transitional reach where the river pattern changes from highly wandering to meandering, further worsening the “hump deposition” there. Because of uncertainty regarding future changes in hydrological processes in the Yellow River basin, and the lack of feasible engineering measures to mitigate “suspended river” and “hump deposition” problems in the LYR, caution should be exercised with respect to changes in the river training strategy for the LYR.  相似文献   

16.
《国际泥沙研究》2020,35(6):600-608
Accumulation of the sediment in the stream of the diversion channels adversely affects its operational systems. Diversion channels are often constructed perpendicular to the main river. In this study, the water flow and sediment transport in the diversion channel with different angles were investigated in an attempt to maximize water discharge and minimize sediment discharge. A physical model with movable bed was used to simulate water and sediment flow with five diversion angles (θ) between (30°–90°). Moreover, three bed width ratios (Br) (the relation between diversion to main channel bed width) between 30% and 50% and five total discharges between (7.25 L/s to 12.25 L/s) were considered for each case of (θ). The results showed, up to 10%, increasing in proportion discharge ratios for 30 and 45 diversion angles compared with 90° diversion angle. The results also showed that the lowest diversion sediment concentration was provided by the (θ) of 30°. Across all scenarios, the average proportion concentration reduction was 64%, compared with 90° diversion angle. Closer observation of the diversion system mechanism confirmed that decreased (θ) result in decreased sediment concentrations in the diversion channels. In conclusion, the diversion channel water and sediment discharge could be effectively managed by changing the (θ) to 30° or 45° instead of 90°.  相似文献   

17.
Earth scientists have traditionally conceptualized rivers and streams as geomorphic machines, whose role is to transfer sediment and to sculpt the landscape. Steady‐state relationships between sediment supply and transport capacity have traditionally been considered normative in fluvial systems. Rivers are hydrological entities, however, whose function is to redistribute excess moisture on land. The geomorphic work of the river – erosion, transport, deposition, etc. – is a byproduct of the hydrological job of the river. There is therefore no reason to expect any particular relationship between sediment supply and transport capacity to develop as a normative condition in fluvial systems. The apparent steady‐state equilibrium slope adjustments of rivers are a byproduct of four basic phenomena: (1) hydraulic selection, which favors channels and branching networks over other flux patterns; (2) water flows along the available path of least resistance; (3) energy dissipation; and (4) finite relaxation times. Recognizing converging trends of stream power or slope and sediment supply as common (but far from inevitable) side effects rather than self‐regulation has important implications for interpreting and predicting fluvial systems, and for river management and restoration. Such trends are variable, transient, contingent, and far from universal. Where they occur, they are an emergent byproduct of fundamental physical mechanisms, not a goal function or attractor state. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

18.
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19.
The upper reach of the Amazon River has a very dynamic morphology, with the highest rates of migration observed in the entire Amazon River. It has an anabranching channel pattern which alternates between a condition of single channel and anabranching structures; in particular, the anabranching structure near Iquitos City shows an interesting channel behavior. Its channels migrate at different rates, where there are processes of narrowing and widening, and also collision and development of new channels. The temporal evolution of the Iquitos anabranching structure is described during the period from 1985 to 2014. The study is carried out by using satellite images to track the migration patterns, which are contrasted to the underlying geological units in the valley. Bathymetry of the structure and several velocity transects were obtained during a field campaign prior to the 2012 historic flood event. This information allowed for numerical modeling in order to compute the hydrodynamic flow field that complements the temporal analysis, aiming to understand the planform migration patterns after the 2012 flood event. It is observed that the geological units play an important role in modulating the migration rates and planform development of the channels. The channels in the structure are in contention to be the main channel, which become the secondary channel after migration. This causes the channels to experience a rise in bed elevation and narrowing of the channel itself; if this trend continues for several more years, these channels will detach from the Iquitos anabranching structure, thus forming paleo‐channels. This geomorphic process is important for horizontal and vertical soil heterogeneity along the floodplain. In general, the analysis shows a complex interaction between the underlying geological units, flow structure, morphology of the bed and planform migration. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

20.
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   

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