Meander Cutoff Research Articles

Landscape evolution of the Ganga-Sai interfluve in response to river dynamics and Late Pleistocene-Holocene climate change via a meander cut-off lake of a plains-fed channel

The floodplains in the interfluve between the Ganga and Sai rivers in the Central Ganga Plains exhibit a broad suite of geomorphic features such as a linear array of meander cut-offs and oxbows representing abandoned sinuous channels. Here we integrate regional and local scale geomorphic analysis using satellite imageries with detailed chronostratigraphic and paleoclimatic reconstruction from a large oxbow lake (Baraila Tal) in the paleochannel belt to infer landscape evolution in this region. The stratigraphic succession at Baraila Tal supported by sedimentology and stable isotope composition of organic matter and micro-gastropod shells suggests that the lake was previously a part of an active fluvial system, and the depositional environment transformed from a fluvial to a lacustrine system in response to geomorphic and climatic changes during the post-LGM period. Integrating geomorphic and paleoclimatic data, we identify four stages (I-IV) of landscape evolution in the Ganga-Sai interfluve region during the Late Pleistocene-Holocene interval: (I) active channel phase of paleo-Sai river before 12 kyr, (II) development of paleochannel belt and meander cut-offs during 12–8 kyr, (III) development of ‘misfit’ Sai river and gullied landscape during 8–7 kyr, and (IV) expansion of ephemeral lakes (wetlands) and subsequent filling post 7 kyr. We have demonstrated the use of paleoclimate and paleovegetation data retrieved from a meander cut-off lake for deciphering landscape evolution.

Assessing geomorphic and hydraulic changes related to an unplanned reconnection of the middle Mississippi River and its floodplain near Miller City, Illinois, USA

The 2016 Len Small Levee breach and the decision not to repair this breach provides a unique opportunity to study the geomorphic and hydraulic changes associated with floodplain reconnection along a large (~30 km long) meander of the Mississippi River known as Dogtooth Bend. This study evaluates recent geomorphic, hydraulic, and sediment-transport capacity changes related to the breaching of the Len Small Levee and the development of cutoff channels across the neck of Dogtooth Bend. We also assess actions undertaken to mitigate further development of cutoff channels across Dogtooth Bend. To address these goals, we perform geomorphic change detection (GCD) analysis and hydrodynamic modeling to assess the geomorphic and hydraulic changes within both the Mississippi's river-connected floodplain and channel.GCD analysis using digital elevation models (DEMs) of the floodplain from pre (2011) and post (2020) levee breach showed net deposition of 23 million tons (MT) of sediment across the reconnected portion of Dogtooth Bend floodplain alone and an additional 34 MT within the batture (the land between the channel's low-water elevation and the flood-mitigation levee). Converting these deposition masses to deposition rates indicates ~5.7 MT yr−1 of deposition within the river-connected floodplain, which is equivalent to ~6 % of the average annual suspended load (~89 MT yr−1) showing this floodplain area is currently a substantial sink for river-transported sediments.In 2021, the U.S. Army Corps of Engineers constructed a grade-control structure down gradient of the levee breach to inhibit the development of cutoff channels across the neck of Dogtooth Bend. Our modeling indicates the addition of this grade-control structure could decrease peak discharge by up to ~15 % and annual flow volume by half assuming channel conveyance remains similar to 2020 conditions. However, sediment-transport-capacity modeling revealed the decrease in flow across Dogtooth Bend attributed to the grade-control structure did not translate into an increase in sediment-transport capacity within the main channel. Assessment of critical shear stress of erosion for fine-grained cohesive materials across Dogtooth Bend suggests conditions favorable to scouring along the actively forming cutoff channels even with the grade-control structure in place. These observations taken together suggest the current river management efforts may not be sufficient in preventing a meander cutoff over the long-term.

Integrated remote sensing and field-based approach to assess the temporal evolution and future projection of meanders: A case study on River Manu in North-Eastern India.

A common phenomenon associated with alluvial rivers is their meander evolution, eventually forming cutoffs. Point bar deposits and ox-bow lakes are the products of lateral bend migration and meander cutoff. The present study focuses on identifying the meanders of River Manu and their cutoffs. Moreover, this study compares the temporal evolution and predicts the progress of selected meanders of River Manu. In the present research, the Survey of India topographical map, satellite imagery, and geographic information system (GIS) technique were used to examine the evolution of the Manu River meander. Subsequently, a field visit was done to the selected cutoffs and meanders of River Manu to ascertain the present status and collect data. It has been observed that many cutoffs have undergone temporal changes, and their sizes have decreased. Some have become dried or converted to agricultural fields. The width of River Manu has decreased in all the selected bends from 1932 to 2017. The sinuosity index has changed from 2.04 (1932) to 1.90 (2017), and the length of the river has decreased by 7 km in 85 years (1932–2017). The decrease in length is evident from lowering the number of meanders. Uniformity coefficient and coefficient of curvature of the bank soil samples were calculated, indicating that the soil is poorly graded and falls under the cohesionless category. Based on cross-section analysis, sediment discharge, grain-size analysis of the bank material, channel planform change, and radius of curvature, it can be stated that almost all the selected bends have the probability of future cutoff. The highest probabilities were observed in bend 3 (Jalai) and bend 4 (Chhontail). This work is aimed to provide planners with decisions regarding the construction of roads and bridges in areas that show the huge dynamicity of river meandering.

On the Prediction of the Characteristic Times of River Meander Cutoff Sequence: Theoretical Model and Comparison With Laboratory and Field Observations

AbstractRiver meander dynamics inevitably interferes with a number of human activities and productive processes. Therefore, investigating the involved physical processes and modeling their space‐time evolution represent a crucial requirement in terms of sustainable river management and restoration planning. In the present study, a deterministic integral‐differential equation that governs river‐bend growth and death in the absence of natural or anthropic forcing is for the first time derived and solved by resorting to cardinal fluid‐mechanical equations such as Navier‐Stokes’ in the Lamb‐Oseen version. The related 1‐D model, which accounts for morphology and sedimentology via the meander migration rate and its radius of curvature, proves to be able to grasp the periodic nature of the phenomenon. Additionally, it exhibits an overall very good agreement with field pre‐cutoff and post‐cutoff observations, as well as with the outcome of an ad hoc‐designed laboratory experiment that simulated near‐cutoff conditions. Hence, it may represent a fast and easy tool to monitor river bend hydro‐geomorphological evolution, particularly when the signs of the incipient instability suggest quantifying the time left to its routine exploitation and to timely plan, where needed, suitable management and restoration interventions.

Bedrock gorge incision via anthropogenic meander cutoff

Abstract Bedrock river-gorge incision represents a fundamental landscape-shaping process, but a dearth of observational data at >10 yr timescales impedes understanding of gorge formation. I quantify 102 yr rates and processes of gorge incision using historical records, field observations, and topographic and image analysis of a human-caused bedrock meander cutoff along the North Fork Fortymile River in Alaska (USA). Miners cut off the meander in 1900 CE, abruptly lowering local base level by 6 m and forcing narrowing and steepening of the channel across a knickpoint that rapidly incised upstream. Tectonic quiescence, consistent rock erosivity, and low millennial erosion rates provide ideal boundary conditions for this 102 yr gorge-formation experiment. Initial fast knickpoint propagation (23 m/yr; 1900–1903 CE) slowed (4 m/yr; 1903–1981 CE) to diffusion (1981–2019 CE) as knickpoint slope decreased, yielding an ~350-m-long, 6-m-deep gorge within the pre–1900 CE channel. Today, diffusion dominates incision of a 500-m-long knickzone upstream of the gorge, where sediment transport likely limits ongoing adjustments to the anthropogenic cutoff. Results elucidate channel width, slope, discharge, and sediment dynamics consistent with a gradual transition from detachment- to transport-limited incision in fluvial adjustment to local base-level lowering.

Dating the landscape evolution around the Chauvet-Pont d\u2019Arc cave

The Chauvet cave (UNESCO World Heritage site, France) is located in the Ardèche Gorge, a unique physical and cultural landscape. Its setting within the gorge—overlooking a meander cutoff containing a natural arch called the Pont d’Arc—is also remarkable. Investigating possible associations between sites’ physical and cultural settings, chronologies of human occupation, and access conditions has become a major theme in archeological research. The present study aims to reconstruct the landscape of the Pont d'Arc meander cutoff during the Upper Paleolithic, when humans were present in the Chauvet Cave. We used uranium-series and electron spin resonance analyses to date the formation of the Pont d’Arc natural arch in the Combe d’Arc meander cutoff, near the Chauvet Cave. Results show that the meander became totally cutoff between 108 and 138 ka (95%). Hence, the natural arch formed before the Upper Paleolithic and the first known human presence in the Chauvet Cave, dated to 37 ka cal BP. These results allowed us to reconstruct a key part of the landscape surrounding the Chauvet Cave when it was being used by Upper-Paleolithic societies.

A century of stream burial due to urbanization in the Tokyo Metropolitan Area

The alteration of hydrologic systems due to urbanization causes many problems including urban flood inundation, water quality degradation, and stream ecosystem damage related to river channel modifications such as stream burial. The purpose of this study was, therefore, to clarify the factors affecting the process of stream burial by performing a time series investigation of the changes in six river systems flowing through the Tokyo Metropolitan Area from 1909 to 2020 using historical topographic maps. The observed changes revealed that the stream burial process consists of: (1) river improvements such as meander cutoff and integration of flow channels by branch abolishment, (2) stream burial of relatively small tributaries, and (3) stream burial of large tributaries and part of the mainstream. Stream burial was remarkable in the center of the Tokyo Metropolitan Area, where the river channels and condition of stream burial were nearly the same in 2020 as they were in 1945. However, meandering channels partly remained in the tributaries of relatively large stream systems. Straight channels constituted about 10% of all channels in 1909, but constituted greater than 40% in 2020, suggesting that the remaining open channels were considerably influenced by river improvements such as meander cutoff. Furthermore, the degree of stream burial was observed to be strongly related to the proportion of urbanization and the scale of the target river system. Stream burial was found to be advanced not only by changes in land use, but also by social factors including earthquake and war damage reconstruction projects as well as measures to counter flooding and improve water quality. The findings of this study advance research into evaluating measures and goals for urban river restoration projects.

Integration of channel meander abandonment age uncertainty into a stochastic channelized system reconstruction method

The logic of sedimentary channelized systems is only partly preserved in the depositional record. We propose a new method to reconstruct 3D channelized systems from incomplete observations made on surface or subsurface images that integrates the uncertainty on meander abandonment chronology. Starting with the youngest channel path observed and the abandoned loops in the meander belt, the method generates local relative chronologies using cross-cutting analysis. It thus proposes a possible global chronology of channel meander abandonments by going back in time while updating the system configuration. This global chronology is obtained through a stochastic procedure that orders the youngest abandoned meanders with regard to their position and orientation relative to the current channel path. Ordered abandoned meanders are then integrated in the main channel path during the reverse migration process so that the global stochastic ordering is made at each time step of the migration algorithm consistently with the updated channel belt. The proposed approach is parameterized by the spatio-temporal density of meander cutoff along the channel belt, which can be inferred from analog databases. The application of our tool on a seismic stratal slice of the McMurray Formation in Canada provides various possible global chronologies of the abandoned meanders. We analyze the variability of these chronologies and highlight its impact on the various possible subsurface 3D global architectures of the paleo-system and on the associated subsurface heterogeneity.

A 2000 year record of palaeofloods in a volcanically-reset catchment: Whanganui River, New Zealand

Palaeofloods in the Whanganui River, North Island, New Zealand are investigated using floodplain sedimentary archives at two locations in the lower Whanganui catchment. The ca. AD 232 Taupo volcanic eruption transformed the lower valley of the Whanganui River, emplacing a substantial volume of volcanogenic mass flow material and providing a new starting point for subsequent alluvial sedimentation. At Atene a high–resolution archive of flood sediments is preserved in a valley meander cutoff in the lower reaches of the Whanganui Gorge, where a ~9 m core was extracted. At Crowley House further down valley, two ~5 m cores were also extracted from a terrace-confined floodplain. Organic material from these cores allows the timing of floods at these sites to be constrained using 11 radiocarbon dates (ten from Atene, one from Crowley House). Flood magnitudes are reconstructed using XRF core-scanned geochemistry as a proxy for flood unit grain size. An age-depth model at Atene identifies three distinct phases of sedimentation with above average flood activity recorded at 1450–1125, 950, 650–500, and 400–325 cal. yr BP, which can be linked to the El Nino Southern Oscillation (ENSO) and strengthening of the Southern Hemisphere Westerly Wind circulation. Large floods also cluster in the late 1800s, reflecting a combination of enhanced storminess and land cover change, which also resulted in deeper erosion of regolith in the catchment, revealed by cosmogenic analysis at Crowley House. Climatic and non-climatic drivers are responsible for floods in the Whanganui catchment over the past ~2000 years, with the largest floods occurring during La Nina and positive Southern Annular Mode conditions. The timing of the largest single flood in the Whanganui in this period is consistent with the volcanic-resetting event itself of AD 232. This study demonstrates the close relationship between regional climate variability in the south-western Pacific Ocean and the occurrence of extreme floods in New Zealand, and the importance of using multi-centennial length hydrological series for effective flood risk assessment.

Tracing of palaeochannels of Bakulahi river system in Uttar Pradesh, India

Palaeochannels can be good possible reservoirs of groundwater and are good rechargeable aquifers. Bakulahi is a plain-fed tributary of the Sai River flowing from north-west to south-east direction with about 177-km length and about 841-km2 catchment area. It emanates from interconnected series of tals (large- to medium-sized shallow depressions) situated in the Raebareli district, Uttar Pradesh, India. Course of the Bakulahi River is shifted laterally and left many fluvial landforms—palaeochannels, oxbow lakes, meander cutoff, etc. The present study is based on the on-screen digitization of the Bakulahi River basin boundary, drainage network, palaeocourses, and other surface water bodies from high-resolution remote-sensing data (1 m × 1 m) available at Google Earth. The digitized details from remote sensing data are validated with some field observations. Different thematic maps are prepared by converting Google Earth feature files (.kmz/.kml) into GIS feature files (.shp) and importing layers into the GIS environment. There are about 115 palaeochannels covering about 137-km2 area (16.29%); about 6000 surface water bodies in forms of ponds/tanks having 23.36-km2 area (2.74%) and 40 oxbow lakes are mapped within the basin. This is the first scientific attempt to trace palaeocourses of the Bakulahi River. The present paper also answers the question about the origin of the Bakulahi River. Demarcation of precise basin boundary through manual method by using high-resolution remotesensing data is another contribution of this exercise which is otherwise a difficult task without the availability of high resolution DEM.

Hypothesis Testing for Nonlinear Phenomena in the Geosciences Using Synthetic, Surrogate Data

Studying nonlinear and potentially chaotic phenomena in geophysics from measured signals is problematic when system noise interferes with the dynamic processes that one is trying to infer. In such circumstances, a framework for statistical hypothesis testing is necessary but the nonlinear nature of the phenomena studied makes the formulation of standard hypothesis tests, such as analysis of variance, problematic as they are based on underlying linear, Gaussian assumptions. One approach to this problem is the method of surrogate data, which is the technique explained in this paper. In particular, we focus on (i) hypothesis testing for nonlinearity by generating linearized surrogates as a null hypothesis, (ii) a variant of this that is perhaps more appropriate for image data where structural nonlinearities are common and should be retained in the surrogates, and (iii) gradual reconstruction where we systematically constrain the surrogates until there is no significant difference between data and surrogates and use this to understand geophysical processes. In addition to time series of sunspot activity, solutions to the Lorenz equations, and spatial maps of enstrophy in a turbulent channel flow, two examples are considered in detail. The first concerns gradual wavelet reconstruction testing of the significance of a specific vortical flow structure from turbulence time series acquired at a point. In the second, the degree of nonlinearity in the spatial profiles of river curvature is shown to be affected by the occurrence of meander cutoff processes but in a more complex fashion than previously envisaged.

Tidal meander migration and dynamics: A case study from the Venice Lagoon

Meandering patterns are universal features of tidal landscapes and exert a critical influence on the dynamics of tidal channel networks and on the stratigraphy of the intertidal platforms they dissect. Despite their importance in landscape evolution and their ubiquity, tidal meanders have received less attention when compared to their fluvial counterparts. To improve current understanding of tidal meander migration and its possible stratigraphic implications, we analysed a sequence of aerial photographs and satellite images (from 1938 to present) of a meandering tidal channel in the Venice Lagoon, which experienced multiple cutoff events during its evolution. Migration rates of the considered bends are in the range of 0.06–0.17 m/y, whereas migration rates per unit width vary between 0.6 and 2.5% yr−1. Detailed high-resolution geomorphological, sedimentological, and geochronological analyses were carried out for an abandoned meander bend, which experienced one of these cutoffs. Aerial photographs before and after the cutoff event (1938 and 1955, respectively) were used to infer a minimum velocity of migration (about 0.10 m/y). Sediment cores were also collected along a transect crossing through the meander neck in order to evaluate changes in grain size, sedimentation rates during the cutoff event, and gain further insight into the velocity of migration of meander bends. The spatial distribution of sedimentary facies (point-bar sand, oxbow lake, and salt-marsh mud), grain-size analyses, and 210 Pb and 137Cs chronologies highlighted that meander cutoff occurred progressively around 70 years before present with an average migration rate of 0.25 m/year. Such a procedure to estimate meander migration rates from stratigraphic data reveals to be particularly useful in the absence of high-temporal resolution remotely sensed-images. The effectiveness of the methods used and the high spatial and temporal resolution of the data call for further investigations and analyses of the type proposed herein, furthermore highlighting the potential of the study area as modern analogue for ancient tidal deposits.

Meander cutoffs nonlocally accelerate upstream and downstream migration and channel widening

AbstractThe hydrologic and sediment dynamics within and near cutoffs have long been studied, establishing them as effective agents of rapid local geomorphic change. However, the morphodynamic impact of individual cutoffs at the reachwide scale remains unknown, mainly due to insufficient observations of channel adjustments over large areal extents and at high temporal frequency. Here we show via annually resolved, Landsat‐derived channel masks of the dynamic meandering Ucayali River in Peru that cutoffs act as perturbations that nonlocally accelerate river migration and drive channel widening both upstream and downstream of the cutoff locations. By tracking planform changes of individual meander bends near cutoffs, we find that the downstream distance of cutoff influence scales linearly with the length of the removed reach. The discovery of nonlocal cutoff influence supports the hypothesis of “avalanche”‐type behavior in meander cutoff dynamics and presents new challenges in modeling and prediction of rivers' self‐adjusting responses to perturbations.

Development of cutoff-related knickpoints during early evolution of submarine channels

Abstract Submarine channels are often thought of as having relatively simple geometries, with significant along-channel morphologic and stratigraphic continuity. Using high-resolution seismic reflection data from offshore Angola and a kinematic model of channel evolution, we present evidence that channels on the seafloor can develop slope variability as a result of meander cutoff events. When cutoffs develop, the shortened flow paths produce locally steep gradients, thus initiating knickpoints. Waves of knickpoint retreat and the related channel incision explain the occurrence of terraces and associated remnant channel deposits above the youngest channel thalweg. The simple processes of meander cutoff followed by knickpoint retreat are intrinsic to submarine channels and result in significant morphologic variability, erosion, and stratigraphic complexity, without any external forcing. These insights highlight the early evolution of submarine channels, a phase with a record that is commonly fragmented or completely absent as a result of subsequent erosion, and allow a better understanding of the autogenic controls on deep-marine stratigraphy.

Late Holocene pollen record from Fiume Morto (Dead River), a palaeomeander of Tiber River near Ancient Ostia (central Italy)

Pollen and non pollen palynomorph analyses were carried out in a sediment core (MO2) drilled in the southern lobe of a palaeomeander, Fiume Morto (Dead River), in the Tiber delta area, near Ancient Ostia (central Italy). Since the Roman period, the Tiber River flowed close to Ancient Ostia and its saltworks, Salinae Ostiensis. The Tiber meander was cut off during the Tiber River flood of 1557 AD and transformed into an oxbow lake. During the nineteenth century the Fiume Morto pond was reclaimed and at present the area is about 3 km distant from the present shoreline and intensely transformed by human settlements. According to radiocarbon dates, the pollen record, not continuous, spans from the fourth century BC to the nineteenth century AD. It shows first a riverine phase before the meander cut off of sixteenth century AD and probably only the last centuries BC were preserved from erosion. The river deposits record riparian vegetation (mainly tamarisk and alder) with mesophilous (mainly deciduous oaks) and Mediterranean (mainly evergreen oaks, heather and olive tree) elements. The human presence is clear, probably related to the development of Ancient Ostia and evidenced by synanthropic taxa. The second phase corresponds to the oxbow lake formed after the meander cut off. Several peaks of pine pollen are tentatively ascribed to Tiber flood events: the first peak is found just in correspondence with the meander cut off at 1557 AD. The numerous floods we interpret in the following part of the diagram could be linked to the increase in extreme events and precipitation that occurred during the Little Ice Age. The last phase, in which freshwater plants are present and chenopods decrease, shows the saltworks abandonment that occurred in nineteenth century. This study turned out to be of key relevance to reconstruct the palaeoenvironmental evolution of the ancient Holocene Tiber meander during the last two and a half millennia.

Channel migration and meander cutoff in response to high magnitude flood event: a case study from the Meshwa River, North Gujarat, India

Channel migration and meander cutoff in response to high magnitude flood event: a case study from the Meshwa River, North Gujarat, India

Formation and infilling of oxbow lakes in the Ishikari lowland, northern Japan

Oxbow lakes are common landforms in meandering river floodplains. Despite the possible importance of oxbow lakes in floodplain evolution, little is known about sedimentary facies and sedimentation rates of oxbow fills, their formation ages and their persistence in the landscape. We analyzed infilling records by interpretation of borehole cores from four oxbow lakes in the Ishikari lowland, northern Japan. The borehole sediments were interpreted as basal coarse-grained streambed sediments (Unit 1) succeeded by channel fills before complete disconnection (Unit 2) and overbank deposits after complete disconnection (Unit 3). Unit 2 is thick in one core but very thin or absent in the other cores. This difference may result from the diversion angles of the incipient oxbow at the time of cutoff rather than the cutoff mechanism (meander cutoff or local avulsion). Detailed chronology based on the 14C ages, tephra analysis, and 137Cs suggests that the oxbow lakes formed during the last centuries and the sedimentation rates are approximately 45–90 mm/y in Unit 2 and 3.9–22.0 mm/y in Unit 3. Oxbow lakes in the Ishikari lowland may persist in the landscape during 600–1300 years since the initiation of cutoff. The relationship between the production rate of oxbow lakes and their persistence suggests that the production rate of oxbow lakes was high during 1899–1959. The number of oxbow lakes in the landscape may be variable in time scales of several decades or a century depending on the production rate of oxbow lakes.

Sediment supply as a driver of river meandering and floodplain evolution in the Amazon Basin

The role of externally imposed sediment supplies on the evolution of meandering rivers and their floodplains is poorly understood, despite analytical advances in our physical understanding of river meandering1, 2. The Amazon river basin hosts tributaries that are largely unaffected by engineering controls and hold a range of sediment loads, allowing us to explore the influence that sediment supply has on river evolution. Here we calculate average annual rates of meander migration within 20 reaches in the Amazon Basin from Landsat imagery spanning 1985–2013. We find that rivers with high sediment loads experience annual migration rates that are higher than those of rivers with lower sediment loads. Meander cutoff also occurs more frequently along rivers with higher sediment loads. Differences in meander migration and cutoff rates between the study reaches are not explained by differences in channel slope or river discharge. Because faster meander migration and higher cutoff rates lead to increased sediment-storage space in the resulting oxbows, we suggest that sediment supply modulates the reshaping of floodplain environments by meandering rivers. We conclude that imposed sediment loads influence planform changes in lowland rivers across the Amazon.

Development of meander cutoffs—a multi-temporal satellite-based observation in parts of Sindh River, Madhya Pradesh, India

Identification of fluvial landforms is of paramount importance in river planform change studies. Satellite remote sensing is a proven technology to identify fluvial landforms and map the associated changes over a period of time because of its synopticity and repetivity. Sindh River, which is the lifeline in central India, changes its course in the alluvial plain, mainly due to meander cutoffs. In this study, multi-temporal satellite data over a period of 23 years were used to find out the behaviour of a meander loop in parts of the Sindh River. It was found that the meander cut off in this loop is a typical neck cutoff unlike chute cutoff in general. The river shows a decreasing trend of sinuosity index during the observation period.

Influence of junction angle on three‐dimensional flow structure and bed morphology at confluent meander bends during different hydrological conditions

AbstractRecent field and modeling investigations have examined the fluvial dynamics of confluent meander bends where a straight tributary channel enters a meandering river at the apex of a bend with a 90° junction angle. Past work on confluences with asymmetrical and symmetrical planforms has shown that the angle of tributary entry has a strong influence on mutual deflection of confluent flows and the spatial extent of confluence hydrodynamic and morphodynamic features. This paper examines three‐dimensional flow structure and bed morphology for incoming flows with high and low momentum‐flux ratios at two large, natural confluent meander bends that have different tributary entry angles. At the high‐angle (90°) confluent meander bend, mutual deflection of converging flows abruptly turns fluid from the lateral tributary into the downstream channel and flow in the main river is deflected away from the outer bank of the bend by a bar that extends downstream of the junction corner along the inner bank of the tributary. Two counter‐rotating helical cells inherited from upstream flow curvature flank the mixing interface, which overlies a central pool. A large influx of sediment to the confluence from a meander cutoff immediately upstream has produced substantial morphologic change during large, tributary‐dominant discharge events, resulting in displacement of the pool inward and substantial erosion of the point bar in the main channel. In contrast, flow deflection is less pronounced at the low‐angle (36°) confluent meander bend, where the converging flows are nearly parallel to one another upon entering the confluence. A large helical cell imparted from upstream flow curvature in the main river occupies most of the downstream channel for prevailing low momentum‐flux ratio conditions and a weak counter‐rotating cell forms during infrequent tributary‐dominant flow events. Bed morphology remains relatively stable and does not exhibit extensive scour that often occurs at confluences with concordant beds. Copyright © 2014 John Wiley & Sons, Ltd.