Modelling hydrological connectivity of tropical floodplain wetlands via a combined natural and artificial stream network

The ecological condition and biodiversity values of floodplain wetlands are highly dependent on the hydrological connectivity of wetlands to adjacent rivers. This paper describes a method for quantifying connectivity between floodplain wetlands and the main rivers in a wet tropical catchment of northern Australia. We used a one‐dimensional hydrodynamic model to simulate time‐varying water depths across the stream network (i.e. rivers, streams and man‐made drains). The timing and duration of connectivity of seven wetlands (four natural and three artificial) with the two main rivers in the catchment were then calculated for different hydrological conditions. Location and areal extent of the wetlands and the stream network were identified using high‐resolution laser altimetry, and these data formed key inputs to the hydrodynamic model. The model was calibrated using measured water depths and discharges across the floodplain. An algorithm was developed to identify contiguous water bodies at daily time steps, and this gave the temporal history of connection and disconnection between wetlands and the rivers. Simulation results show that connectivity of individual wetlands to both rivers varies from 26 to 365 days during an average hydrological condition. Location, especially proximity to a main river, and wetland type (natural stream or artificial drain) were identified as key factors influencing these levels of connectivity. Some natural wetlands maintain connection with the river for most or all of the year, whereas the connectivity of some artificial wetlands varies from 26 to 36 days according to their patterns of network connection to adjacent rivers – a result that has important implications for the accessibility of these types of wetland to aquatic biota. Using readily available river gauge data, we also show how connectivity modelling can be used to identify periods when connectivity has fallen below critical thresholds for fish movement. These connectivity patterns within the floodplain network are central to the setting of river flows that will meet environmental requirements for biota that use floodplain wetlands during their life history. Copyright © 2013 John Wiley & Sons, Ltd.     

Validation of a full hydrodynamic model for large‐scale hydrologic modelling in the Amazon

A key aspect of large river basins partially neglected in large‐scale hydrological models is river hydrodynamics. Large‐scale hydrologic models normally simulate river hydrodynamics using simplified models that do not represent aspects such as backwater effects and flood inundation, key factors for some of the largest rivers of the world, such as the Amazon. In a previous paper, we have described a large‐scale hydrodynamic approach resultant from an improvement of the MGB‐IPH hydrological model. It uses full Saint Venant equations, a simple storage model for flood inundation and GIS‐based algorithms to extract model parameters from digital elevation models. In the present paper, we evaluate this model in the Solimões River basin. Discharge results were validated using 18 stream gauges showing that the model is accurate. It represents the large delay and attenuation of flood waves in the Solimões basin, while simplified models, represented here by Muskingum Cunge, provide hydrographs are wrongly noisy and in advance. Validation against 35 stream gauges shows that the model is able to simulate observed water levels with accuracy, representing their amplitude of variation and timing. The model performs better in large rivers, and errors concentrate in small rivers possibly due to uncertainty in river geometry. The validation of flood extent results using remote sensing estimates also shows that the model accuracy is comparable to other flood inundation modelling studies. Results show that (i) river‐floodplain water exchange and storage, and (ii) backwater effects play an important role for the Amazon River basin hydrodynamics. Copyright © 2011 John Wiley & Sons, Ltd.     

Vertical surface water–groundwater exchange processes within a headwater floodplain induced by experimental floods

Restoring hydrologic connectivity between channels and floodplains is common practice in stream and river restoration. Floodplain hydrology and hydrogeology impact stream hydraulics, ecology, biogeochemical processing, and pollutant removal, yet rigorous field evaluations of surface water–groundwater exchange within floodplains during overbank floods are rare. We conducted five sets of experimental floods to mimic floodplain reconnection by pumping stream water onto an existing floodplain swale. Floods were conducted throughout the year to capture seasonal variation and each involved two replicate floods on successive days to test the effect of varying antecedent moisture. Water levels and specific conductance were measured in surface water, soil, and groundwater within the floodplain, along with surface flow into and out of the floodplain. Vegetation density varied seasonally and controlled the volume of surface water storage on the floodplain. By contrast, antecedent moisture conditions controlled storage of water in floodplain soils, with drier antecedent moisture conditions leading to increased subsurface storage and slower flood wave propagation across the floodplain surface. The site experienced spatial heterogeneity in vertical connectivity between surface water and groundwater across the floodplain surface, where propagation of hydrostatic pressure, preferential flow, and bulk Darcy flow were all mechanisms that may have occurred during the five floods. Vertical connectivity also increased with time, suggesting higher frequency of floodplain inundation may increase surface water–groundwater exchange across the floodplain surface. Understanding the variability of floodplain impacts on water quality noted in the literature likely requires better accounting for seasonal variations in floodplain vegetation and antecedent moisture as well as heterogeneous exchange flow mechanisms. Copyright © 2016 John Wiley & Sons, Ltd.     

The interplay between extrinsic and intrinsic controls in determining floodplain wetland characteristics in the South African drylands

Controls on the characteristics of floodplain wetlands in drylands are diverse and may include extrinsic factors such as tectonic activity, lithology and climate, and intrinsic thresholds of channel change. Correct analysis of the interplay between these controls is important for assessing possible channel–floodplain responses to changing environmental conditions. Using analysis of aerial imagery, geological maps and field data, this paper investigates floodplain wetland characteristics in the Tshwane and Pienaars catchments, northern South Africa, and combines the findings with previous research to develop a new conceptual model highlighting the influence of variations in aridity on flow, sediment transport, and channel–floodplain morphology. The Tshwane–Pienaars floodplain wetlands have formed in response to a complex interplay between climatic, lithological, and intrinsic controls. In this semi‐arid setting, net aggradation (alluvium >7 m thick) in the wetlands is promoted by marked downstream declines in discharge and stream power that are related to transmission losses and declining downstream gradients. Consideration of the Tshwane–Pienaars wetlands in their broader catchment and regional context highlights the key influence of climate, and demonstrates how floodplain wetland characteristics vary along a subhumid to semi‐arid climatic gradient. Increasing aridity tends to be associated with a reduction in the ability of rivers to maintain through‐going channels and an increase in the propensity for channel breakdown and floodout formation. Understanding the interplay between climate, hydrology and geomorphology may help to anticipate and manage pathways of floodplain wetland development under future drier, more variable climates, both in South African and other drylands. Copyright © 2016 John Wiley & Sons, Ltd.     

Recent increase of river–floodplain suspended sediment exchange in a reach of the lower Amazon River

We analyzed variation of channel–floodplain suspended sediment exchange along a 140 km reach of the lower Amazon River for two decades (1995–2014). Daily sediment fluxes were determined by combining measured and estimated surface sediment concentrations with river–floodplain water exchanges computed with a two‐dimensional hydraulic model. The average annual inflow to the floodplain was 4088 ± 2017 Gg yr^?1 and the outflow was 2251 ± 471 Gg yr^?1, respectively. Prediction of average sediment accretion rate was twice the estimate from a previous study of this same reach and more than an order of magnitude lower than an estimate from an earlier regional scale study. The amount of water routed through the floodplain, which is sensitive to levee topography and increases exponentially with river discharge, was the main factor controlling the variation in total annual sediment inflow. Besides floodplain routing, the total annual sediment export depended on the increase in sediment concentration in lakes during floodplain drainage. The recent increasing amplitude of the Amazon River annual flood over two decades has caused a substantial shift in water and sediment river–floodplain exchanges. In the second decade (2005–2014), as the frequency of extreme floods increased, annual sediment inflow increased by 81% and net storage increased by 317% in relation to the previous decade (1995–2004). Copyright © 2017 John Wiley & Sons, Ltd.     

Groundwater–surface water interactions in a large semi‐arid floodplain: implications for salinity management

Flow regulation and water diversion for irrigation have considerably impacted the exchange of surface water between the Murray River and its floodplains. However, the way in which river regulation has impacted groundwater–surface water interactions is not completely understood, especially in regards to the salinization and accompanying vegetation dieback currently occurring in many of the floodplains. Groundwater–surface water interactions were studied over a 2 year period in the riparian area of a large floodplain (Hattah–Kulkyne, Victoria) using a combination of piezometric surface monitoring and environmental tracers (Cl⁻, δ²H, and δ¹⁸O). Despite being located in a local and regional groundwater discharge zone, the Murray River is a losing stream under low flow conditions at Hattah–Kulkyne. The discharge zone for local groundwater, regional groundwater and bank recharge is in the floodplain within ∼1 km of the river and is probably driven by high rates of transpiration by the riparian Eucalyptus camaldulensis woodland. Environmental tracers data suggest that the origin of groundwater is principally bank recharge in the riparian zone and a combination of diffuse rainfall recharge and localized floodwater recharge elsewhere in the floodplain. Although the Murray River was losing under low flows, bank discharge occurred during some flood recession periods. The way in which the water table responded to changes in river level was a function of the type of stream bank present, with point bars providing a better connection to the alluvial aquifer than the more common clay‐lined banks. Understanding the spatial variability in the hydraulic connection with the river channel and in vertical recharge following inundations will be critical to design effective salinity remediation strategies for large semi‐arid floodplains. Copyright © 2005 John Wiley & Sons, Ltd.     

Large‐scale modelling of channel flow and floodplain inundation dynamics and its application to the Pantanal (Brazil)

For large‐scale sites, difficulties for applying coupled one‐dimensional (1D)/2D models for simulating floodplain inundation may be encountered related to data scarcity, complexity for establishing channel–floodplain connections, computational cost, long duration of floods and the need to represent precipitation and evapotranspiration processes. This paper presents a hydrologic simulation system, named SIRIPLAN, developed to accomplish this aim. This system is composed by a 1D hydrodynamic model coupled to a 2D raster‐based model, and by two modules to compute the vertical water balance over floodplain and the water exchanges between channel and floodplain. Results are presented for the Upper Paraguay River Basin (UPRB), including the Pantanal, one of the world's largest wetlands. A total of 3965 km of river channels and 140 000 km² of floodplains are simulated for a period of 11 years. Comparison of observed and calculated hydrographs at 15 gauging stations showed that the model was capable to simulate distinct, complex flow regimes along main channels, including channel‐floodplain interactions. The proposed system was also able to reproduce the Pantanal seasonal flood pulse, with estimated inundated areas ranging from 35 000 km² (dry period) to more than 120 000 km² (wet period). Floodplain inundation maps obtained with SIRIPLAN were consistent with previous knowledge of Pantanal dynamics, but comparison with inundation extent provided by a previous satellite‐based study indicates that permanently flooded areas may have been underestimated. The results obtained are promising, and further work will focus on improving vertical processes representation over floodplains and analysing model sensitivity to floodplain parameters, time step and precipitation estimates uncertainty. Copyright © 2010 John Wiley & Sons, Ltd.     

A multilinear discrete Nash-cascade model for stage-hydrograph routing in compound river channels

ABSTRACT

Stream gauge-based information is the foundation for many hydrological applications in a river basin including the aquatic-habitat conservation. A simple two-parameter model for routing streamflow depth (alternatively, stream–stage) hydrographs and estimating corresponding discharge hydrographs in river channels is proposed using the multilinear approach, based on Nash-type discrete-cascade model. The applicability of this model is investigated by extending its framework to the realm of compound cross-section trapezoidal channels for both in-bank and overbank flows by using 20 flood events of the Tiber River in the Umbria region of Central Italy, and subsequently comparing the simulated results with the corresponding simulations of the HEC-RAS (Hydrologic Engineering Center – River Analysis System) hydrodynamic model and observed flow depth hydrographs. The field application, comparative study, and uncertainty and sensitivity analysis of the results demonstrate that the proposed multilinear discrete Nash-cascade stage-hydrograph (MDNS) routing model has the potential for routing floods in real-world rivers and canal irrigation systems, especially in operational mode.     

Tributary valley impoundment by trunk river floodplain development: a case study from the KwaZulu‐Natal Drakensberg foothills,eastern South Africa

This paper investigates the origin and geomorphic evolution of Stillerust Vlei, a 189 ha wetland located approximately 150 km northwest of Durban in the temperate submontane foothills of the KwaZulu‐Natal Drakensberg Mountains. The investigation confirms the findings of previous research on the arid to semi‐arid South African interior, which established that many floodplain wetlands in eastern South Africa are located upstream of resistant rock barriers (dolerite intrusions) that cross river courses and form stable local base levels. Upstream of these barriers, rivers laterally plane less resistant Karoo sedimentary rocks (sandstones, mudstones), creating broad, low gradient valleys conducive to the formation of floodplain wetlands. In addition, the study examines how local levee and alluvial ridge accretion on the floodplain of Stillerust Vlei has impounded a small tributary valley, and drawing on observations of similar wetlands in the region, the paper explains the origin and geomorphic evolution of wetlands in floodplain‐abutting valleys, and associated streams that commonly become discontinuous toward their confluence with the trunk (floodplain) river. Controls on the origin and geomorphic evolution of Stillerust Vlei are placed within the context of slope‐channel decoupling and (dis)connectivity in sediment delivery, illustrating that wetlands are environments of deposition. As a result of dynamic trunk‐tributary relations, Stillerust Vlei holds a diversity of geomorphic features, and thus provides potential habitat for a diversity of biota. Copyright © 2008 John Wiley & Sons, Ltd.     

RIVER MORPHOLOGY AND CHANNEL STABILIZATION OF THE BRAHMAPUTRA RIVER IN BANGLADESH

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The influence of local hydrogeologic forcings on near‐stream event water recharge and retention (Upper San Pedro River,Arizona)

The rise in stream stage during high flow events (floods) can induce losing stream conditions, even along stream reaches that are gaining during baseflow conditions. The aquifer response to flood events can affect the geochemical composition of both near‐stream groundwater and post‐event streamflow, but the amount and persistence of recharged floodwater may differ as a function of local hydrogeologic forcings. As a result, this study focuses on how vertical flood recharge varies under different hydrogeologic forcings and the significance that recharge processes can have on groundwater and streamflow composition after floods. River and shallow groundwater samples were collected along three reaches of the Upper San Pedro River (Arizona, USA) before, during and after the 2009 and 2010 summer monsoon seasons. Tracer data from these samples indicate that subsurface floodwater propagation and residence times are strongly controlled by the direction and magnitude of the dominant stream–aquifer gradient. A reach that is typically strongly gaining shows minimal floodwater retention shortly after large events, whereas the moderately gaining and losing reaches can retain recharged floodwater from smaller events for longer periods. The moderately gaining reach likely returned flood recharge to the river as flow declined. These results indicate that reach‐scale differences in hydrogeologic forcing can control (i) the amount of local flood recharge during events and (ii) the duration of its subsurface retention and possible return to the stream during low‐flow periods. Our observations also suggest that the presence of floodwater in year‐round baseflow is not due to long‐term storage beneath the streambed along predominantly gaining reaches, so three alternative mechanisms are suggested: (i) repeated flooding that drives lateral redistribution of previously recharged floodwater, (ii) vertical recharge on the floodplain during overbank flow events and (iii) temporal variability in the stream–aquifer gradient due to seasonally varying water demands of riparian vegetation. Copyright © 2011 John Wiley & Sons, Ltd.     

The effects of floods on the temperature of riparian groundwater

The transition area between rivers and their adjacent riparian aquifers, which may comprise the hyporheic zone, hosts important biochemical reactions, which control water quality. The rates of these reactions and metabolic processes are temperature dependent. Yet the thermal dynamics of riparian aquifers, especially during flooding and dynamic groundwater flow conditions, has seldom been studied. Thus, we investigated heat transport in riparian aquifers during 3 flood events of different magnitudes at 2 sites along the same river. River and riparian aquifer temperature and water‐level data along the Lower Colorado River in Central Texas, USA, were monitored across 2‐dimensional vertical sections perpendicular to the bank. At the downstream site, preflood temperature penetration distance into the bank suggested that advective heat transport from lateral hyporheic exchange of river water into the riparian aquifer was occurring during relatively steady low‐flow river conditions. Although a small (20‐cm stage increase) dam‐controlled flood pulse had no observable influence on groundwater temperature, larger floods (40‐cm and >3‐m stage increases) caused lateral movement of distinct heat plumes away from the river during flood stage, which then retreated back towards the river after flood recession. These plumes result from advective heat transport caused by flood waters being forced into the riparian aquifer. These flood‐induced temperature responses were controlled by the size of the flood, river water temperature during the flood, and local factors at the study sites, such as topography and local ambient water table configuration. For the intermediate and large floods, the thermal disturbance in the riparian aquifer lasted days after flood waters receded. Large floods therefore have impacts on the temperature regime of riparian aquifers lasting long beyond the flood's timescale. These persistent thermal disturbances may have a significant impact on biochemical reaction rates, nutrient cycling, and ecological niches in the river corridor.     

Distinct export dynamics for dissolved and particulate phosphorus reveal independent transport mechanisms in an arable headwater catchment

This paper investigates particulate phosphorus (PP) and soluble reactive phosphorus (SRP) concentrations at the outlet of a small (5 km²) intensively farmed catchment to identify seasonal variability of sources and transport pathways for these two phosphorus forms. The shape and direction of discharge‐concentration hystereses during floods were related to the hydrological conditions in the catchment during four hydrological periods. Both during flood events and on an annual basis, contrasting export dynamics highlighted a strong decoupling between SRP and PP export. During most flood events, discharge‐concentration hystereses for PP were clockwise, indicating mobilization of a source located within or near the stream channel. Seasonal variability of PP export was linked to the availability of stream sediments and the export capacity of the stream. In contrast, hysteresis shapes for SRP were anticlockwise, which suggests that SRP was transferred to the stream via subsurface flow. Groundwater rise in wetland soils was likely the cause of this transfer, through the hydrological connectivity it created between the stream and P‐rich soil horizons. SRP concentrations were the highest when the relative contribution of deep groundwater from the upland domain was low compared with wetland groundwater. Hence, soils from non‐fertilized riparian wetlands seemed to be the main source of SRP in the catchment. This conceptual model of P transfer with distinct hydrological controls for PP and SRP was valid throughout the year, except during spring storm events, during which PP and SRP exports were synchronized as a consequence of overland flow and erosion on hillslopes. Copyright © 2015 John Wiley & Sons, Ltd.     

Insight into variability of spring and flash flood events in Lithuania

In this research, variability of spring (from 1 March to 30 May) and flash (from 1 June to 30 November) floods in rivers of different regions was analysed. The territory of Lithuania is divided into three regions according to hydrological regime of the rivers: Western, Central, and Southeastern. The maximum river discharge data of spring and flash floods [a total of 31 water gauging stations (WGS)] were analysed. Comparison of the data of four periods (1922–2013, 1941–2013, 1961–2013, and 1991–2013) with the data of the reference period (1961–1990) was performed. Analysis included the longest discharge data set of the Nemunas River at Smalininkai WGS (1812–2013) as well. Mixed patterns of flood changes in Lithuanian rivers were detected. The analysis of flood discharges of the Nemunas River indicated that both spring and flash floods in Lithuania were getting smaller.     

城市化对水系结构及其连通性的影响——以秦淮河中、下游为例

针对当前城市化所引起水系衰减、河流连通受阻以及由此所引起洪涝与水环境的问题,以秦淮河中、下游为例,选取1979和2006年两期流域遥感影像,分析了城市化影响下的下垫面变化特征;选取1980s和2009年的地形图对河流水系进行提取,借鉴景观生态学中河流廊道空间结构分析方法,通过不同时期水系分级,探讨了城市化对水系结构及其连通性的影响.结果表明:(1)城市化使得2006年城镇用地面积相比1979年增加84.54 km²,增加了9倍多,大量林草地、耕地以及水域转变成城镇用地;(2)河流长度在过去的30年里减少了41%,河道主干化趋势明显;河流发育呈现由多元到单一、由复杂到简单的趋势;(3)连通性参数连接率、实际结合度分别由原来的1.28、0.43下降到0.79、0.26,河流的连通性呈下降趋势.该研究将为城市化地区河流水系保护提供支持与参考.     

Historical variability and feedbacks among land cover,stream power,and channel geometry along the lower Canadian River floodplain in Oklahoma

In 1820, the lower Canadian River meandered through a densely forested floodplain. By 1898, most of the floodplain had been cleared for agriculture and changes in channel geometry and specific stream power followed, particularly channel widening and straightening with a lower potential specific stream power. In 1964, a large upstream hydropower dam was constructed, which changed the flow regime in the lower Canadian River and consequently the channel geometry. Without destructive overbank floods, the channel narrowed rapidly and considerably due to encroachment by floodplain vegetation. The lower Canadian River, which was once a highly dynamic floodplain‐river system, has now been transformed into a relatively static river channel. These changes over the past 200 years have not been linear or independent. In this article, we use a variety of data sources to assess these historical changes along the lower Canadian River floodplain and identify feedbacks among floodplain cultivation, dam construction, specific stream power, and channel width, slope, and sinuosity. Finally, we combine the results of our study with others in the region to present a biogeomorphic response model for large Great Plains rivers that characterizes channel width changes in response to climate variability and anthropogenic disturbances. Copyright © 2011 John Wiley & Sons, Ltd.     

Floodplain stratigraphy of the ice jam dominated middle Yukon River: a new approach to long‐term flood frequency

Floodplain stratigraphy is used as a new method for reconstructing ice jam flood histories of northern rivers. The method, based on reconstruction of the sedimentary record of vertically‐accreting floodplains, relies on stratigraphic logging and interpretation of floodplain sediments, which result from successive ice jam floods, and radiocarbon dating of inter‐flood organic material for chronology. In a case study along a reach of the Yukon River that straddles the Yukon–Alaska border, the method is used to develop a record of ice jam flooding for the last 2000 years. Detailed chronostratigraphic logs from three sites along the Yukon River indicates that the long‐term recurrence interval varies depending on location, but ranges from approximately once in 25 years to once in 38 years (or a probability of ca 3–4% in any given year). This is broadly similar to the 4·5% probability of recurrence calculated from archival and gauged data at Dawson City, Yukon Territory, for the period 1898–2006. Two of the three study locations, with sufficient chronology, suggest a decrease in flood frequency in the last several hundred years relative to the preceding period at each site, broadly corresponding to the Little Ice Age, suggesting climate exerts some control over long‐term ice jam flood frequency. This study demonstrates that the floodplain sedimentary record offers the potential to extend records of ice jam flooding in remote, ungauged northern rivers and provides a broader temporal context for assessing the frequency and variability of ice jam flooding. Copyright © 2008 John Wiley & Sons, Ltd.     

Identifying and mitigating dam-induced declines in river health:Three case studies from the western United States

River health can be defined as the degree to which riverine energy source,water quality,flow regime, habitat and biota match the natural conditions.In a healthy river,physical process and form remain actively connected and able to mutually adjust,and biological communities have natural levels of diversity and are resilient to environmental stress.Both physical diversity and biodiversity influence river health.Physical diversity is governed by hydrology,hydraulics,and substrate,as reflected in the geometry of the river channel and adjacent floodplain,which create habitat for aquatic and riparian organisms.Biodiversity is governed by biological processes such as competition and predation,but biodiversity also reflects the diversity,abundance and stability of habitat,as well as connectivity. Connectivity within a river corridor includes longitudinal,lateral,and vertical dimensions.River health declines as any of these interacting components is compromised by human activities.The cumulative effect of dams and other human alterations of rivers has been primarily to directly reduce physical diversity and connectivity,which indirectly reduces biodiversity.Restoration and maintenance of physical diversity and biodiversity on rivers affected by dams requires quantifying relations between the driver variables of flow and sediment supply,and the response variables of habitat,connectivity,and biological communities.These relations can take the form of thresholds(e.g., entrainment of streambed sediment) or response curves(e.g.,fish biomass versus extent and duration of floodplain inundation).I use examples from Wyoming,Colorado,and Arizona in the western United States to illustrate how to quantify relations between driver and response variables on rivers affected by dams.     

Hydraulic and geomorphic processes in an overbank flood along a meandering,gravel‐bed river: implications for chute formation

Hydraulic interactions between rivers and floodplains produce off‐channel chutes, the presence of which influences the routing of water and sediment and thus the planform evolution of meandering rivers. Detailed studies of the hydrologic exchanges between channels and floodplains are usually conducted in laboratory facilities, and studies documenting chute development are generally limited to qualitative observations. In this study, we use a reconstructed, gravel‐bedded, meandering river as a field laboratory for studying these mechanisms at a realistic scale. Using an integrated field and modeling approach, we quantified the flow exchanges between the river channel and its floodplain during an overbank flood, and identified locations where flow had the capacity to erode floodplain chutes. Hydraulic measurements and modeling indicated high rates of flow exchange between the channel and floodplain, with flow rapidly decelerating as water was decanted from the channel onto the floodplain due to the frictional drag provided by substrate and vegetation. Peak shear stresses were greatest downstream of the maxima in bend curvature, along the concave bank, where terrestrial LiDAR scans indicate initial floodplain chute formation. A second chute has developed across the convex bank of a meander bend, in a location where sediment accretion, point bar development and plant colonization have created divergent flow paths between the main channel and floodplain. In both cases, the off‐channel chutes are evolving slowly during infrequent floods due to the coarse nature of the floodplain, though rapid chute formation would be more likely in finer‐grained floodplains. The controls on chute formation at these locations include the flood magnitude, river curvature, floodplain gradient, erodibility of the floodplain sediment, and the flow resistance provided by riparian vegetation. Copyright © 2015 John Wiley & Sons, Ltd.