Interacting effects of multiple factors on the morphological evolution of the meandering reaches downstream the Three Gorges Dam

Elucidating the influence of dams on fluvial processes can inform river protection and basin management. However, relatively few studies have focused on how multiple factors interact to affect the morphological evolution of meandering reaches. Using hydrological and topographical data, we analyzed the factors that influence and regulate the meandering reaches downstream the Three Gorges Dam (TGD). Our conclusions are as follows. (1) The meandering reaches can be classified into two types based on their evolution during the pre-dam period: G1 reaches, characterized by convex point bar erosion and concave channel deposition (CECD), and G2 reaches, characterized by convex point bar deposition and concave channel erosion (CDCE). Both reach types exhibited CECD features during the post-dam period. (2) Flow processes and sediment transport are the factors that caused serious erosion of the low beaches located in the convex point bars. However, changes in the river regime, river boundaries and jacking of Dongting Lake do not act as primary controls on the morphological evolution of the meandering reaches. (3) Flood discharges ranging from 20,000 to 25,000 m³/s result in greater erosion of convex point bars. The point bars become scoured if the durations of these flows, which are close to bankfull discharge, exceed 20 days. In addition, the reduction in bedload causes the decreasing of point bar siltation in the water-falling period. (4) During the post-dam period, flood abatement, the increased duration of discharges ranging from 20,000 to 25,000 m³/s, and a significant reduction in sediment transport are the main factors that caused meandering reaches to show CECD features. Our results are relevant to other meandering reaches, where they can inform estimates of riverbed change, river management strategies and river protection.     

多因素对三峡水库下游弯曲河型演变的迭加效应（英文）

Elucidating the influence of dams on fluvial processes can inform river protection and basin management.However,relatively few studies have focused on how multiple factors interact to affect the morphological evolution of meandering reaches.Using hydrological and topographical data,we analyzed the factors that influence and regulate the meandering reaches downstream the Three Gorges Dam(TGD).Our conclusions are as follows.(1)The meandering reaches can be classified into two types based on their evolution during the pre-dam period:G1 reaches,characterized by convex point bar erosion and concave channel deposition(CECD),and G2 reaches,characterized by convex point bar deposition and concave channel erosion(CDCE).Both reach types exhibited CECD features during the post-dam period.(2)Flow processes and sediment transport are the factors that caused serious erosion of the low beaches located in the convex point bars.However,changes in the river regime,river boundaries and jacking of Dongting Lake do not act as primary controls on the morphological evolution of the meandering reaches.(3)Flood discharges ranging from 20,000 to 25,000 m3/s result in greater erosion of convex point bars.The point bars become scoured if the durations of these flows,which are close to bankfull discharge,exceed 20 days.In addition,the reduction in bedload causes the decreasing of point bar siltation in the water-falling period.(4)During the post-dam period,flood abatement,the increased duration of discharges ranging from 20,000 to 25,000 m3/s,and a significant reduction in sediment transport are the main factors that caused meandering reaches to show CECD features.Our results are relevant to other meandering reaches,where they can inform estimates of riverbed change,river management strategies and river protection.     

Acoustic Doppler current profiler surveys along the Yangtze River

An acoustic Doppler current profiler is used to characterize the river velocity against the morphology of the Yangtze River from Chonqing to the sea. High flow velocities occur in the Three Gorges section and lower velocities in the middle and lower reaches of the river. This is largely due to the change in river pattern from a high gradient deeply-cut valley to a flat fluvial plain. Flow velocities fluctuate in the middle Yangtze due to the presence of meander bends of different length. There are numerous smaller velocity fluctuations in the lower Yangtze channel that reflect multichannel pattern with numerous sand bars and a river morphology affected by bedrock outcrops. Water depths of 40–100 m occur in the Three Gorges valley but decrease to 15–40 m in the middle and lower Yangtze. At the Gezhou Reservoir, 30 km downstream of the Three Gorges damsite velocity drops to low (< 1.0 m s^− 1) 20 km reach. A second low velocity (< 0.5 m s^− 1) zone, about 20 km in length, is located in the lower Yangtze near the coast probably due to the tidal influence. The results from this research will serve as a datum for evaluating changes to the river once the Three Gorges dam is completed in 2009.     

Channel deformations of the Irkut river in the Tunkinskaya depression

We examine the intrasecular morphodynamics of the Irkut river within the Tunkinskaya depression and the associated partitions between depressions. An analysis is made of the changes in the channel pattern in plan, and a typization is provided for the channel processes for three time intervals: 1914, 1975 and 2015. We identified eleven morphodynamically uniform reaches of the channel: three, one and seven of them are of the adapted, incised and broad-floodplain types, respectively. The types of morphodynamical development of the river are systematized, and reaches with a different degree of deformation are identified within the most mobile broad-band segment of the river in conditions of free meandering. It is ascertained that the reaches of the braided type and the reach with sharp loop-like bends are the most stable, which is, in either case, due to the consolidation of vegetation by the high floodplain which made it possible to stabilize the movement of the channel in plan. The most strongly altered reaches are determined, where river channel displacements were accompanied by a complete change of the morphodynamical types of bends; the macrobends mainly experienced a regular breakthrough or were complicated by lower-order bends. We identified the reaches with lower-order (local) changes where the development of the bends (for the most part, of the loop-like type) changed into a new evolutionary cycle). We determined the role played by the influence of the territory’s geologo-geomorphological structure, hydrological conditions and natural processes of channel evolution on the dynamics of homogeneous reaches, and on morphological changes within them. The period of the most significant channel deformations, from 1914 to 1976, was determined, which is accounted for by an enhancement in anthropogenic pressure and by the associated increase in suspended load, and the period of decrease in the magnitude of manifestations of channel deformations from 1976 to 2015.     

Complexity, self-organisation and variation in behaviour in meandering rivers

River meanders are natural features on the surface of Earth that present some degree of regularity of form. They range from being highly dynamic to being stable under present conditions. Conventional theory is that meanders develop to an equilibrium form which is related to discharge and sediment load. Other research has demonstrated that many highly active meanders exhibit a continuous evolution over time and a non-linearity in rate of development. Ideas of autogenesis and of self-organised criticality as being an explanation of some meander changes have been proposed. In this paper data from rivers around the world are examined for further evidence of autogenic, self-organised or non-linear behaviour through analysis of change in sinuosity over time for reaches and change in individual bend form, particularly bend curvature and bend elongation. Some examples do exhibit trends of increasing sinuosity over time and a few show limits from which large decreases occur. Several case studies show non-linearity of behaviour and increasing complexity of form. Other case studies, however, do not exhibit such trends. Phase space plots are used to help uncover emergent behaviour but show a variety of patterns. The example of a reach in which multiple cut-offs occurred is analysed for mechanisms of self-organisation of the planform and in the pool-riffle pattern. Riffles are more closely spaced and also more transient in the more rapidly changing and higher sinuosity parts of the channel. Hypothetical trajectories of different meander behaviour, including for bedrock meanders, are plotted but the challenge remains to uncover the conditions for occurrence and for divergence of tendencies to stability and instability. Identification of attractors and phase space of behaviour of different meandering systems offer the potential for application to sustainable channel management.     

Effects of Stronach Dam removal on fluvial geomorphology in the Pine River, Michigan, United States

Although dam removal has been increasingly used as an option in dam management, and as a river restoration tool, few studies provide detailed quantitative assessment of the geomorphological response of rivers to dam removal. In this study, we document the response of the Pine River, Michigan, to the gradual removal of Stronach Dam. In 1996, prior to the initiation of removal, 31 permanent cross-sectional transects were established in the 10-km study area. These transects were surveyed annually during the course of the removal (1996–2003) and for the three years following removal (2004–2006). Dam removal resulted in progressive headcutting of sediments in the former impoundment, extending upstream 3.89 km of the dam. Over the course of the 10 years since dam removal was initiated, a net total of 92 000 m³ of sediment erosion occurred. The majority of sediments stored in the former reservoir remained in place, with only 12% of the estimated reservoir sediment fill being eroded. Approximately 14% of the net erosion was deposited within the stream channel 1 km downstream of the dam location, with the remainder being transported further downstream or deposited in the floodplain. Sediment fill incision resulted in a narrower and deeper channel upstream, with higher mean water velocity and somewhat coarser substrates. Downstream deposition resulted in a wider and shallower channel, with little change in substrate size composition. Counter-intuitively, water velocity also increased downstream because of the increased slope that developed. Prior to removal, bedforms in the former impoundment were dominated by runs but are showing signs of restoration toward reference conditions. Continuing changes in river geomorphology are evident even three years following removal and are likely to occur for years to come.     

Local- and watershed-scale controls on the spatial variability of natural levee deposits in a large fine-grained floodplain: Lower Pánuco Basin, Mexico

This study examines spatial variations in natural levee deposits within the lower reaches of a large coastal plain drainage system. The Pánuco basin (98,227 km²) drains east-central Mexico, and is an excellent setting to examine the influence of watershed and local controls on the morphology and sedimentology of natural levees. Although many fluvial systems in the U.S. Gulf Coastal Plain have been investigated, the rivers in the Mexican Gulf Coastal Plain have received comparatively little attention. Lateral and downstream characteristics of natural levee morphology and sediment texture are considered within the context of meandering river floodplain deposits. Data sources include total-stations surveying, sediment samples of surficial levee deposits, topographic maps (1:50,000), and aerial photographs (1:40,000). The slope of natural levees average 0.0049 m/m, whereas the texture (D₈₄) of levee deposits averages 0.12 mm. Natural levee characteristics vary due to local- and watershed-scale controls. The lateral reduction in levee height displays a curvilinear pattern that coincides with an abrupt change in sediment texture. The downstream pattern of natural levee texture exhibits the influence of local-scale perturbations superimposed upon a larger watershed-scale trend. Disruption to the fining trend, either by tributary inputs of sediment or reworking of Tertiary valley deposits, is retained for a limited distance. The influence of the channel planform geometry on levee morphology is examined by consideration of the radius of curvature (R_c) of meander bends, and is inversely related to natural levee width. This suggests that the planform geometry of river channels exerts a control on the dispersal of flood sediments, and is responsible for considerable local variability in the floodplain topography. The average width of natural levees increases with drainage area, from an average of 747 m in the Moctezuma to an average of 894 m in the Pánuco. However, in the lower reaches of the Pánuco valley the width of natural levees rapidly decreases, which is associated with fining of the suspended sediment load. Thus, the reduction in natural levee width signifies an abrupt change in the directionality of cause–effect relationships at the watershed-scale. Findings from this study elucidate linkages between meandering river channels and floodplains for a large lowland alluvial valley.     

Urban transformation of river landscapes in a global context

Over the past 50 years considerable progress has been made in understanding the impacts of urban development on river processes and forms. Such advances have occurred as urban population growth has accelerated around the world. Using a compilation of research results from more than 100 studies conducted in a range of areas (58 addressing morphological change), this paper describes how urbanization has transformed river landscapes across Earth’s surface, emphasizing the distribution of impacts in a global comparative context. Urban development induces an initial phase of sediment mobilization, characterized by increased sediment production (on the order of 2–10 times) and deposition within channels, followed by eventual decline that couples with erosion from increased runoff to enlarge channels. Data from humid and temperate environments around the world indicate that channels generally enlarge to 2–3 times and as much as 15 times the original size. Although research has emphasized temperate environments, recent studies of tropical areas indicate a tendency for channel reduction resulting from strong sediment erosion and deposition responses because of intense precipitation and highly weathered soils. Embryonic research in arid environments further suggests variable river responses to urbanization that are characterized by rapid morphological change over short distances. Regardless of location, the persistence of the sediment production phase varies from months to several years, whereas several decades are likely needed for enlarging channels to stabilize and potentially reach a new equilibrium. Urbanizing streams pose particular challenges for management given an inherent changing nature. Successful management requires a clear understanding of the temporal and spatial variations in adjustment processes.     

Insights on the kinematics of deep-seated gravitational slope deformations along the 1915 Avezzano earthquake fault (Central Italy), from time-series DInSAR

Small meandering channels of about 1 m wide on an intertidal mudflat in the Westerschelde estuary the Netherlands) were studied with the aim to improve understanding of the effect of highly cohesive bed and bank sediment on channel inception and meander geometry and dynamics. The study is supported by experiments and modelling. The estuarine meandering channels are less dynamical than alluvial meandering rivers, and the dynamics are more localised. Moreover, the high thresholds for bed sediment erosion and for bank failure lead to two processes, uncommon in larger rivers, that cause most of the morphological change. First, the beds of the channels are eroded by backward migrating steps under hydraulic jumps, while the remainder of the bed surface along the channel is hardly eroded. Second, channel banks erode i) where eroding steps locally cause undercutting of otherwise stable channel banks and ii) in very sharp bends where the flow separates from the inner-bend channel boundary and impinges directly on the bank on the opposite side of the channel. Further morphological change is probably induced by rainfall splash erosion and by storm waves that weaken the mud, and by large mud fluxes from the estuary. The steps were successfully reproduced in laboratory flume experiments. An existing model for step migration predicted celerities consistent with field and laboratory observations and demonstrated a strong dependence on the threshold for erosion. Bank stability models confirm that banks and steps only fail when undercut and weakened by waves, rain or excess pore pressure in agreement with observations. The effects of a high threshold for bank erosion was implemented in an existing meander simulation model that reproduced the observed locations of bank erosion somewhat better than without the threshold, but flow separation and its effect on meander bends remains poorly understood.     

三峡水库下游弯曲河型演变规律调整及其驱动机制

大型水库的兴建深刻改变了下游水沙输移特点,进而导致河床演变规律显著调整,水库下游弯曲河型对水沙过程改变响应敏感,是水库下游河床演变、航道整治、河势控制等方面研究的关键区域。本文基于1996-2016年的实测水文、地形资料,对长江三峡水库下游弯曲河型的演变规律及其驱动机制开展研究,结果表明：① 三峡水库蓄水前,下荆江存在“凸淤凹冲”、“凸冲凹淤”两类弯曲河型,而三峡水库蓄水后均表现为“凸冲凹淤”的一致性规律;② 在水库拦沙作用的影响下,下荆江河段平滩河槽存在累积性冲刷现象,冲刷部位集中于枯水河槽与基本河槽之间的低滩,冲淤部位调整主要由变化的流量过程所驱动,上游河势、河道边界以及支流入汇等因素均有一定驱动作用;③ 在三峡水库蓄水后缺乏大洪水的情况下,凸岸水流挟沙力随流量增加逐渐增强,水流对凸岸冲蚀力度在平滩流量级附近（20000~25000 m3/s）达到最强,平滩流量附近流量级的持续时间超过20天时,弯曲河道发生凸冲凹淤现象。而悬沙中造床粗沙的减少,增强了水流冲刷强度,加剧了凸岸的冲蚀程度。     

Terrace aggradation during the 1978 flood on Powder River, Montana, USA

Flood processes no longer actively increase the planform area of terraces. Instead, lateral erosion decreases the area. However, infrequent extreme floods continue episodic aggradation of terraces surfaces. We quantify this type of evolution of terraces by an extreme flood in May 1978 on Powder River in southeastern Montana. Within an 89-km study reach of the river, we (1) determine a sediment budget for each geomorphic feature, (2) interpret the stratigraphy of the newly deposited sediment, and (3) discuss the essential role of vegetation in the depositional processes.Peak flood discharge was about 930 m³ s^− 1, which lasted about eight days. During this time, the flood transported 8.2 million tons of sediment into and 4.5 million tons out of the study reach. The masses of sediment transferred between features or eroded from one feature and redeposited on the same feature exceeded the mass transported out of the reach. The flood inundated the floodplain and some of the remnants of two terraces along the river. Lateral erosion decreased the planform area of the lower of the two terraces (~ 2.7 m above the riverbed) by 3.2% and that of the higher terrace (~ 3.5 m above the riverbed) by 4.1%. However, overbank aggradation, on average, raised the lower terrace by 0.16 m and the higher terrace by 0.063 m.Vegetation controlled the type, thickness, and stratigraphy of the aggradation on terrace surfaces. Two characteristic overbank deposits were common: coarsening-upward sequences and lee dunes. Grass caused the deposition of the coarsening-upward sequences, which had 0.02 to 0.07 m of mud at the base, and in some cases, the deposits coarsened upwards to coarse sand on the top. Lee dunes, composed of fine and very fine sand, were deposited in the wake zone downstream from the trees. The characteristic morphology of the dunes can be used to estimate some flood variables such as suspended-sediment particle size, minimum depth, and critical shear velocity. Information about depositional processes during extreme floods is rare, and therefore, the results from this study aid in interpreting the record of terrace stratigraphy along other rivers.     

Seasonal variations of sediment discharge from the Yangtze River before and after impoundment of the Three Gorges Dam

Over the past decades, > 50,000 dams and reforestation on the Yangtze River (Changjiang) have had little impact on water discharge but have drastically altered annual and particularly seasonal sediment discharge. Before impoundment of the Three Gorges Dam (TGD) in June 2003, annual sediment discharge had decreased by 60%, and the hysteresis of seasonal rating curves in the upper reaches at Yichang station had shifted from clockwise to counterclockwise. In addition, the river channel in middle-lower reaches had changed from depositional to erosional in 2002.During the four years (2003–2006) after TGD impoundment, ~ 60% of sediment entering the Three Gorges Reservoir was trapped, primarily during the high-discharge months (June–September). Although periodic sediment deposition continues downstream of the TGD, during most months substantial erosion has occurred, supplying ~ 70 million tons per year (Mt/y) of channel-derived sediment to the lower reaches of the river. If sand extraction (~ 40 Mt/y) is taken into consideration, the river channel loses a total of 110 Mt/y. During the extreme drought year 2006, sediment discharge in the upper reaches drastically decreased to 9 Mt (only 2% of its 1950–1960s level) because of decreased water discharge and TGD trapping. In addition, Dongting Lake in the middle reaches, for the first time, changed from trapping net sediment from the mainstem to supplying 14 Mt net sediment to the mainstem. Severe channel erosion and drastic sediment decline have put considerable pressure on the Yangtze coastal areas and East China Sea.     

Valley confinement as a factor of braided river pattern for the Platte River

The Platte River in Nebraska has evolved in the twentieth century from a predominantly braided river pattern to a mélange of meandering, wandering, anastomosed, island braided, and fully braided reaches. Identifying the factors that determine the occurrence of a fully braided main channel was the objective of this study. Aerial photography, gage flow data, ground-surveyed cross sections, bed material samples, and the results of sediment transport modeling were used to examine factors that control spatial change in main river pattern of the central Platte River. Valley confinement is identified as the determining factor of braided river in nine of eleven divisions of the central Platte River. Flow reduction and the interruption of sediment supply are identified as determining factors preventing fully braided river in the remaining two of eleven reaches.Valley confinement, the topography which limits the width of the floodplain, was initially measured as width between historical banks (predevelopment river banks). This metric was later refined to width between confining features (historical banks, remnant bars, bridge abutments, protected banks and levees). Under existing conditions, the main channel of the central Platte River is fully braided when valley confinement (width between confining features) is 600 m or less and begins to divide into the multiple channels of an anastomosed pattern when valley confinement (width between confining features) exceeds 600 m When Platte River flow is divided between two to four major anabranches, a fully braided pattern in the main channel of the main anabranch requires a more confined valley of 400 m or less.Valley confinement is demonstrated to be the dominant factor in determining river pattern in the central Platte River, although this factor is not normally considered in the continuum of channel pattern model. Conclusions from this study can be used to increase the occurrence of fully braided main channel in the central Platte River, to aid habitat recovery for endangered or threatened bird species that favor this river pattern. Consideration of valley confinement with river continuum factors can aid river managers by improving predictions of river pattern in response to management actions.     

黄河中游土壤侵蚀与下游古河道三角洲演化的过程响应

根据黄土高原土壤侵蚀的周期特点，结合华北平原古河道，古三角洲的演化过程，应用泥沙输移的过程响应,分析了晚更新世以来黄河中游黄土高原土壤侵蚀与下游古河道，三角洲演化的关系，在人类历史之前，黄土高原土壤侵蚀基本上遵循自在生态环境演化规律，强裂侵蚀期发生在干冷向湿湿气候转化的过渡期，在强裂侵蚀的初期是古道形成期，强烈侵蚀期发生在干冷向温湿气候转化的过渡期，在强裂侵蚀的期是古河道形成期，强烈侵蚀的外营力迭加了人为作用，黄河下游河游泳以改道，三角洲横向扩展发生在强烈侵蚀的衰退期，人类历史时期，土壤侵蚀的外营力迭加了人为作用，破坏了地质历史时期的规律性，土壤侵蚀强度越来越强，基本上按照旱涝变化频率而演化，干冷期降雨不均匀系数增加，土训侵蚀加重，径流量较少，河床以淤积为主，是古河道形成期，正常年黄河泥少输移比接近于一，是三角洲进积期，温湿期降雨量增加，径流量加大，下游河流改道，三角洲横向发展。     

Fluvial process and morphology of the Brahmaputra River in Assam, India

The Brahmaputra River finds its origin in the Chema Yundung glacier of Tibet and flows through India and Bangladesh. The slope of the river decreases suddenly in front of the Himalayas and results in the deposition of sediment and a braided channel pattern. It flows through Assam, India, along a valley comprising its own Recent alluvium. In Assam the basin receives 300 cm mean annual rainfall, 66–85% of which occurs in the monsoon period from June through September. Mean annual discharge at Pandu for 1955–1990 is 16,682.24 m³ s^{− 1}. Average monthly discharge is highest in July (19%) and lowest in February (2%). Most hydrographs exhibit multiple flood peaks occurring at different times from June to September. The mean annual suspended sediment load is 402 million tons and average monthly sediment discharge is highest in June (19.05%) and lowest in January (1.02%). The bed load at Pandu was found to be 5–15% of the total load of the river. Three kinds of major geomorphic units are found in the basin. The river bed of the Brahmaputra shows four topographic levels, with increasing height and vegetation. The single first order primary channels of this braided river split into two or more smaller second order channels separated by bars and islands. The second order channels are of three kinds. The maximum length and width of the bars in the area under study are 18.43 km and 6.17 km, respectively. The Brahmaputra channel is characterised by mid-channel bars, side bars, tributary mouth bars and unit bars. The geometry of meandering tributary rivers shows that the relationship between meander wavelength and bend radius is most linear. The Brahmaputra had been undergoing overall aggradation by about 16 cm during 1971 to 1979. The channel of the Brahmaputra River has been migrating because of channel widening and avulsion. The meandering tributaries change because of neck cut-off and progressive shifting at the meander bends. The braiding index of the Brahmaputra has been increasing from 6.11 in 1912–1928 to 8.33 in 1996. During the twentieth century, the total amount of bank area lost from erosion was 868 km². Maximum rate of shift of the north bank to south resulting in erosion was 227.5 m/year and maximum rate of shift of the south bank to north resulting in accretion was 331.56 m/year. Shear failure of upper bank and liquefaction of clayey-silt materials are two main causes of bank erosion.     

黄河流域河型转化现象初探

黄河以其高含沙水流以及下游河道的高沉积速率而著称于世。迄今的研究, 主要针对黄河中下游流域的 侵蚀、水文泥沙和河床演变方面的研究, 而对黄河流域主支流发生河型转化的现象关注不够。在黄河的不同河段, 河型的变化频繁, 类型多样, 现象复杂, 是研究者不可回避的科学问题。本文选取黄河上游第一弯的玛曲河段、黄河 上游末段托克托附近河段及黄河下游高村上下河段来研究河型转化的形式及影响因素。玛曲河段沿流向发生网状 河型→弯曲河型→辫状河型的转化现象, 该系列转化呈现出由极稳定河型向极不稳定河型的转化, 这与世界上通 常可以观察到的沿流向不稳定河型向稳定河型转化的情况完全相反。这主要受到地壳的抬升、上下峡谷卡口、水动 力特征、边界沉积物特征及植被的区域分布等因素的控制。托克托附近沿流向发生了弯曲河型→顺直河型转化的 现象, 这是较稳定河型向极稳定河型的转化, 主要受到边界沉积物、水动力等因素的控制。高村上下河段沿流向发 生的辫状河型→弯曲河型转化的现象, 是由极不稳定河型向较稳定河型转化的现象, 河道边界沉积物及水动力是 其主要控制因素, 人工大堤只是限制了河道摆动的最大幅度, 对河型的性质影响不大, 但其上游河段修筑的水库导 致下泻的水流在辫状河段的侵蚀能力增强而使其边界沉积物粗化, 并将泥质物大量沉积在弯曲河段, 客观上促进 了河型的转化。     

Cutoffs galore!: occurrence and causes of multiple cutoffs on a meandering river

The creation of cutoffs and of oxbow lakes is a well-known phenomenon of meandering rivers, but views on the extent to which they are inherent in meander behaviour have varied. Assumptions of meander behaviour have shifted from those of stability and equilibrium to recognition of gradual evolution and increased complexity of form. Alternative explanations of cutoff occurrence are discussed here in relation to a remarkable set of cutoffs that occurred in one reach of the River Bollin, UK, for which long-term historical evidence of meander evolution existed and which has been monitored for change and processes over the last 20 years. The cutoffs occurred during the high floods of winter 2000–2001. A series of hypotheses is examined, including the occurrence of floods and effects of hydrological changes. Although the flood events actually caused the cutoffs, the long-term pattern accords with ideas of chaotic behaviour and sinuosity of a river reaching a critical state at which clustering of meander cutoffs takes place. It is suggested that the occurrence of the cutoffs can be explained as inherent in meander behaviour.     

Morphodynamics of a pseudomeandering gravel bar reach

A large number of rivers in Tuscany have channel planforms, which are neither straight nor what is usually understood as meandering. In the typical case, they consist of an almost straight, slightly incised main channel fringed with large lateral bars and lunate-shaped embayments eroded into the former flood plain. In the past, these rivers have not been recognised as an individual category and have often been considered to be either braided or meandering. It is suggested here that this type of river planform be termed pseudomeandering.A typical pseudomeandering river (the Cecina River) is described and analysed to investigate the main factors responsible for producing this channel pattern. A study reach (100×300 m) was surveyed in detail and related to data on discharge, channel changes after floods and grain-size distribution of bed sediments. During 18 months of topographic monitoring, the inner lateral bar in the study reach expanded and migrated towards the concave outer bank which, concurrently, retreated by as much as 25 m. A sediment balance was constructed to analyse bar growth and bank retreat in relation to sediment supply and channel morphology. The conditions necessary to maintain the pseudomeandering morphology of these rivers by preventing them from developing a meandering planform, are discussed and interpreted as a combination of a few main factors such as the flashy character of floods, sediment supply (influenced by both natural processes and human impact), the morphological effects of discharges with contrasting return intervals and the short duration of flood events. Finally, the channel response to floods with variable sediment transport capacity (represented by bed shear stress) is analysed using a simple model. It is demonstrated that bend migration is associated with moderate floods while major floods are responsible for the development of chute channels, which act to suppress bend growth and maintain the low sinuosity configuration of the river.     

The fluvial cycle at cold–warm–cold transitions in lowland regions: A refinement of theory

A previously established non-linear theory of river cyclicity as a response to climate change states that short phases of fluvial instability occur both at the transition from relatively warm (temperate) to cold (periglacial) and from relatively cold to warm periods. Such instability typically starts with vertical erosion, successively followed by sedimentary fill of the erosive scar. In spite of frequent confirmation of this theory by geomorphological reconstructions, a few problems arise. First, there are fewer incision phases than climatic transitions. Secondly, remnants of erosion at ‘cold–warm’ transitions are scarce, in contrast to obvious erosion relics at ‘warm–cold’ transitions. Furthermore, it appears that the incision style is strikingly different at both kinds of climatic transitions. Similarly, the long stable phases are also expressed in terms of different floodplain development in cold and warm periods. These arguments require a modification of the general non-linear theory.At the transition from relatively temperate to colder conditions, rivers transformed gradually from a regular, low-energy, single-channel course to a periodically high-energy, multi-channel type. The latter (braided) type is characterized by intense lateral movement, rather than by deep vertical erosion. This results in a well-expressed morphology of wide, extensive floodplains and terraces. In contrast, the linear and constrained, meandering channels incise with small width–depth ratio and build floodplains of limited lateral extent. Consequences are twofold: 1) the spatial limitation of the deeply incised, meandering valleys at the beginning of warm periods counts against their recognition; 2) relatively strong, lateral migration of the braided rivers removes most traces of previous (meandering) systems, which contrasts with the limited lateral activity of confined meandering channels. It means that in a ‘warm–cold–warm’ alternation generally only one phase of vertical erosion is preserved, the one that is caused by the high-energy, braided river at the start of the cold period.     

The influence of river training on mountain channel changes (Polish Carpathian Mountains)

The purpose of this paper is to explain the influence of river training on channel changes in mountain rivers. Also considered are the causes of failure of different training schemes. The research was conducted on the regulated Mszanka and Porębianka Rivers, belonging to the Raba River drainage basin in the Polish Flysh Carpathian Mountains. Channel mapping carried out in 2004 drew attention to the contemporary morphology of the channels and the development of their dynamic typology. General changes in channel morphometry and land cover were identified by comparing cartographic sources from various years. Archive material from Cracow's Regional Water Management Authority (RZGW) was used to analyse the detailed channel changes caused by each regulation structure. The material consisted of technical designs of individual training works, as well as plans, longitudinal profiles and cross-sections of trained channel reaches. A series of minimum annual water stages at the Mszana Dolna gauging station was used to determine the tendency of channel bed degradation over 53 years. During the first half of the 20th century, the middle and lower courses of the Mszanka and Porębianka Rivers had braided patterns. The slopes, mostly covered with crops, were an important source of sediment delivery to the river channels. Today, both channels are single-threaded, narrow and sinuous. Downcutting is the leading process transforming the channels. They cut down to bedrock along about 60% of their lengths. The main type of channel is an erosion channel, which occurs also in the middle and lower courses of the rivers. The channel sediment deficit is an important cause for river incision. Sediment supply to the channels was reduced after a replacement of crops on the slopes by meadows or forests. Gravel mining has also caused channel downcutting. The rapid channel changes began after 1959, as systematic training was introduced. Channel regulation seems therefore to be a major factor determining channel adjustment. Debris dams and groynes were built before 1980 and these caused the greatest change of channel pattern, increase of channel gradient and magnitude of river incision. After that date the measures mostly involved drop structures. From then on, the rate of downcutting decreased considerably, but has not ceased. The rivers continued to incise until bedrock was exposed or training structures were destroyed. After that, a tendency to lateral migration and local braiding were observed in the deepened channel. The channels displayed a tendency to return to their morphology and dynamic from before the training. The results demonstrate that river training distorts the equilibrium of channel systems. A channel becomes divided into artificial reaches, which later follow different evolutionary patterns. Most training schemes on mountain channels are ineffective in the long term, as river managers seem to consider a channel at a reach scale only. Individual channel reaches, however, are not independent but rather form a system that must be managed at the entire channel scale.