Changes In Channel Morphology Research Articles

Characterization of Downstream Channel Morphology of a Monsoon Dominated Dwarkeswar River in West Bengal

Abstract Downstream changes in channel morphology and its flow were investigated over a monsoon dominated Dwarkeswar river from the western part of West Bengal. The basin has developed over the Proterozoic Granite Gneiss Complex to recent Holocene alluvium, forming three distinctive geomorphological regions of the basin e.g., dissected plateau, plate-fringe and alluvial plain area. Sixty cross-sections along the entire main stream were surveyed and bankfull channel parameters were measured. Sediment samples from each location were collected and Manning’s roughness (n) value for respective reaches were estimated and flow velocity, discharge, stream power and shear stress were calculated. The result has shown that channel width and channel capacity in the dissected plateau are increasing downstream, in the plateau-fringe area the trends are relatively constant. But in alluvial plain, width (87.3%), flow area (81.3%), discharge (84.1%), W/D ratio (91.3%) decreases downstream. Downstream decreasing channel width and channel capacity are strongly related to elevation, slope, HSI, TSI and SSI. Extremely low slope, channel switching, cohesive bank materials and vast flood plain width facilitates the river flow to spill over from the channel and spread to the surrounding area resulting diminishing discharge and thereby the channel morphology in downstream. Frequency of flow crossing bankfull limit has increased downstream resulting frequent flood diminishing channel morphology with distance down the valley.

Seven decades of hydrogeomorphological changes in a near‐natural (Sense River) and a hydropower‐regulated (Sarine River) pre‐Alpine river floodplain in Western Switzerland

ABSTRACTHydropower alteration of the natural flow and sediment regime can severely degrade hydromorphology, thereby threatening biodiversity and overall ecosystem processes of rivers and their floodplains. Using sequences of aerial images, we quantified seven decades (1938/1942–2013) of spatiotemporal changes in channel and floodplain morphology, as well as changes in the physical habitats, of three floodplain river reaches of the Swiss pre‐Alps, two hydropower‐regulated and one near‐natural. In the Sarine River floodplain, within the first decades of hydropower impairment, the magnitude and frequency of flood events (Q2, Q10, Q30) decreased substantially. As a result, the area of pioneer floodplain habitats that depend on flood activity and sediment dynamic, such as bare sediments, decreased dramatically by approximately 95%. However, by 2013 vegetated areas had generally increased in comparison to the pre‐regulation period in 1943, indicating general vegetative colonization. Between 1943 and 2013, the active channel underwent essential narrowing (up to 62% width reduction in the residual flow reach) and habitat turnover rates were very low (5% of the total floodplain area changed habitat type five to six times). In contrast, from the 1950s onwards, the near‐natural floodplain of the Sense River experienced recurrent narrowing and widening, and frequent changes between bare and vegetated areas, reflecting the shifting habitat mosaic concept typical for natural floodplains. In the three reaches investigated, we found that the active floodplain width and erosion of vegetated areas were primarily controlled by medium to large floods (Q10, Q30), which combined with reduced time intervals between ordinary floods ≥ Q2 most likely mobilized streambed sediments and limited the ability of vegetation to establish itself on bare gravel bars within the parafluvial zone. These findings can contribute to restoration action plans such as controlled flooding and sediment replenishments in the Sarine and other floodplain rivers of the Alps. © 2020 John Wiley & Sons, Ltd.

Assessment of Changes in Channel Morphology in a Mountain River Regulated Using Grade Control Structures

Assessment of Changes in Channel Morphology in a Mountain River Regulated Using Grade Control Structures

Geomorphological changes of the Cetina River channels since the end of the nineteenth century, natural vs anthropogenic impacts (the Dinarides, Croatia)

This study investigates changes in channel morphology of the Cetina River over the last 130 years affected by climate changes and various human impacts. A detailed digitalisation and analysis of the channel features was performed by using historical maps from the 1880s, archival aerial images from the 1950/60s, and contemporary orthophoto maps from the 2010s. According to these sources, we defined three study periods, and distinguished changes in the pre-dam phase (1880s–1950/60s) and the post-dam phase (1950/60s–2010s). In the pre-dam phase, a partial channel narrowing and drying of marsh zones in the source area were recorded, possibly due to reduced humidity and discharge following the end of the Little Ice Age. A drop in discharge and an increase in sediment supply caused by agricultural intensification most probably affected the expansion of fluvial bars. The increase in sediment supply is particularly evident in considerable river delta progradation of 500 m during this period. In the last 50 years, five hydroelectric power plants were built in the catchment. Consequently, severe reductions in discharge led to channel narrowing by about 50% which occurred in the third of the river length. Apart from the dam construction, a reduction in the area of bars by 85% and an increase in the number of islands by 103% can be related to decreased sediment supply following land abandonment and natural reforestation. This study adds to the knowledge on river morphology and channel adjustments following natural and strong human disturbances in the Mediterranean environment.

The roles of flood magnitude and duration in controlling channel width and complexity on the Green River in Canyonlands, Utah, USA

Predictions of river channel adjustment to changes in streamflow regime based on relations between mean channel characteristics and mean flood magnitude can be useful to evaluate average channel response. However, because these relations assume equilibrium sediment transport, their applicability to cases where streamflow and sediment transport are decoupled may be limited. These general relations also lack the specificity that is required to connect specific characteristics of the streamflow and sediment regime with the dynamics of channel morphological change that create channel complexity, which is often of ecological interest. We integrate historical records of channel change, observations of scour and fill during a snowmelt flood, measurements of sediment transport, and predictions from a two-dimensional streamflow model to describe how annual peak flow magnitude and peak-flow duration interact with the upstream sediment supply to control channel form for a 15-km study reach on the regulated Green River in Canyonlands National Park, Utah. Two major decadal-scale episodes of channel narrowing have occurred within the study area. For each of these episodes, the reduction in average channel width was consistent with the change predicted by hydraulic geometry relations as a function of average flood magnitude. However, channel narrowing occurred during periods of exceptionally low annual floods. The most recent episode of channel narrowing occurred between 1988 and 2009, during low-flow cycles when the 5-yr mean peak flow was less than 60% of the long-term (1959–2016) mean peak flow. These findings, together with findings from previous studies, demonstrate that decreases in peak-flow magnitude caused by streamflow regulation, climate change, or a combination of those factors have driven episodes of channel narrowing on the Green River. Observations of streamflow, sediment-transport, and morphologic change coupled with predictions from a two-dimensional streamflow model indicate that peak flow magnitudes of at least 75% of the long-term mean peak flow are required to transport bed-material sand in suspension in all regions of the multi-thread channel and that the ~2-month duration of the snowmelt flood played an important role in creating conditions necessary to maintain channel conveyance. These results indicate that detailed characterizations of channel response such as these are needed to predict how river channels will respond to changes in streamflow regime that affect annual peak flow magnitude and duration.

Sustainability of a navigation channel in the Nile River: A case study in Egypt

AbstractInland waterways are an environmentally friendly and efficient mode of transport. At locations along the Nile River, Egypt, navigation is hindered requiring expensive, resource‐intensive corrective. An increasing number of navigational bottlenecks have occurred between 1982 and 2015 because of changing river channel morphologies. This study provides a suitable method for evaluating navigation conditions within a reach of the Nile River. A 2D morphological model combined with a geographic information system application was created to assess the navigability of a reach of the Nile River, and different maintenance strategies were evaluated. Different scenarios of varying water levels, dredging operations, flow discharge fluctuations, and river channel morphological changes were considered. Results reveal that dredging operations, alone, cannot be adopted as a permanent solution for navigation bottlenecks, with the riverbed returning to its original state within 10 years. An important output of the method is the generation of reach scale hydro‐morphodynamic maps that show the main morphological features, navigation bottlenecks, and navigation conditions. These maps aid in the development of strategic navigability plans and in evaluating long‐term improvement measures. The method presented is applicability to large river systems worldwide.

Assessing channel morphology and prediction of centerline channel migration of the Barak River using geospatial techniques

Barak River is highly meandering rivers flowing through the alluvial plains of Assam in India. However, due to dynamic system, it is found that channel being subjected to regular shifting which creates uncertainty to the habitants residing nearby the river. Therefore, it is anticipated to carry out a study regarding changes in channel morphology and prediction of centerline channel migration during 1984–2030, using multiperiod Landsat remote sensing images along with autoregressive integrated moving average model (ARIMA). From morphometric analysis, it was found that the mean value of meander length (ML), meander width (MB), and meander ratio (MR) indicates an increasing trend, while sinuosity (C), wavelength (λ), and radius of curvature (RC) show a decreasing trend. The outcome of ARIMA model specifies that channel shifting of mid-line is going to change suddenly either to rightward or leftward directions. Throughout the whole alluvial part of the Barak River, rightward side is recognized as major concern. Observed and predicted values have shown a good R2 value (R2 = 0.89 and R2 = 0.88) at CS-30 and CS-18 respectively. Also, lowest RMSE is observed at CS-12 and highest RMSE is observed at CS-21. Finally predicted values were generated for the estimation of centerline channel shifting between two time intervals (2017–2023 and 2023–2030), which shows that the channel shifting of the river basin will occur at many regions particularly at critical sections. Overall, the findings of this study could be used further in river training works and in understanding the future dynamics of channel.

Downstream Decreasing Channel Capacity of a Monsoon-dominated Bengal Basin River: A Case Study of Dwarkeswar River, Eastern India

Downstream changes in channel morphology and flow over the ephemeral Dwarkeswar River in the western part of the Bengal Basin, eastren India were investigated. The river stretches from the Proterozoic Granite Gneiss Complex to the recent Holocene alluvium, forming three distinctive geomorphological regions across the river basin: the pediplane and upper and lower alluvial areas. Sixty cross-sections from throughout the main trunk stream were surveyed and the bankfull width, depth, cross-sectional area, and maximum depth were measured. Sediment samples from each location were studied and the flow velocity, stream power, Manning’s roughness coefficient, and shear stress were estimated. The results show that the bankfull channel cross-section area, width, width-to-depth ratio, and channel capacity increased between the beginning and middle of the river. Thereafter, the size of the river started to decrease in the lower alluvial area. This was characterized by gentle gradients, cohesive bank materials with grass cover, and channel switching. Within the lower part of the river, the channel capacity was observed to diminish as the drainage area increased. This increased the bankfull flow frequency and accelerated large floodwater losses in the floodplain via overbank flows and floodways.

Assessment of the geomorphic effectiveness of controlled floods in a braided river using a reduced-complexity numerical model

Abstract. Most Alpine rivers have undergone significant alterations in flow and sediment regimes. These alterations have notable effects on river morphology and ecology. One option to mitigate such effects is flow regime management, specifically through the reintroduction of channel-forming discharges. The aim of this work is to assess the morphological changes induced in the Piave River (Italy) by two distinct controlled-flood strategies, the first characterized by a single artificial flood per year and the second by higher-magnitude but less frequent floods. This work involved applying a two-dimensional reduced-complexity morphodynamic model (CAESAR-LISFLOOD) to a 7 km long reach, characterized by a braided pattern and highly regulated discharges. Numerical modelling allowed the assessment of morphological changes for four long-term scenarios (2009–2034). The scenarios were defined considering the current flow regime and the natural regime, which was estimated by a stochastic physically based hydrologic model. Changes in channel morphology were assessed by measuring active-channel width and braiding intensity. A comparison of controlled-flood scenarios to a baseline scenario (i.e. no controlled floods) showed that artificial floods had little effect on channel morphology. More channel widening (13.5 %) resulted from the high-magnitude flood strategy than from the application of the other strategy (8.6 %). Negligible change was observed in terms of braiding intensity. The results indicate that controlled floods do not represent an effective solution for morphological recovery in braided rivers with strongly impacted flow and sediment regimes.

Quantifying Coastal Fluvial Morphodynamics Over the Last 100 Years on the Lower Rio Grande, USA and Mexico

AbstractDownstream changes in fluvial channel morphology are commonly observed in association with the backwater zone, where rivers transition from quasi‐uniform flow with normal‐flow depth to gradually varying flow. This transition is linked to changes in channel morphology and mobility and resulting fluvial stratigraphy. However, the majority of systems studied to date are perennial rivers with relatively consistent flow conditions. Here we investigate the evolution of a large river with significant flood‐to‐baseflow variability as it transverses and builds a large delta. We provide the first comprehensive study of the morphology and morphodynamics of the lower Rio Grande, a major continental drainage system that enters the western Gulf of Mexico. We quantify the morphology of the current Rio Grande channel and document spatial trends in channel geometry and kinematics using lidar, historical surveys, and hydrographic analysis. The modern Rio Grande channel morphology does not significantly vary toward the coast. Rather, the channel width, levee, and bed slopes remain nearly constant for ~200 river km. We find historical migration rates between 10 and 100 m/yr with no significant reduction toward the coast in contrast to previously studied systems. We propose that this invariant channel geometry and sustained high migration rates are signatures of the channel not requiring adjustment within the lower coastal reach to accommodate baseflow conditions, and the channel remains continuously adjusted solely to peak flow conditions.

Release of treated effluent into streams: A global review of ecological impacts with a consideration of its potential use for environmental flows

Abstract Worldwide, the addition of treated wastewater (i.e. effluent) to streams is becoming more common as urban populations grow and developing countries increase their use of wastewater treatment plants. Release of treated effluent can impair water quality and ecological communities, but also could help restore flow and maintain aquatic habitat in water‐stressed regions. To assess this range of potential outcomes, we conducted a global review of studies from effluent‐fed streams to examine the impacts of effluent on water quality and aquatic and riparian biota. We identified 147 quantitative studies of effluent‐fed streams, most of which were from the U.S.A. and Europe. Over 85% of the studies identified water quality as a primary study focus, including basic physical and chemical parameters, as well as trace organic contaminants. Nearly 60% of the studies had at least some focus on aquatic or riparian biota, primarily fish, aquatic invertebrates, and basal resources (e.g. algae). Effluent inputs generally impaired water quality near discharge points, mainly through increased water temperature, nutrients, and concentrations of trace organic contaminants, but also via decreased dissolved oxygen levels. The majority of ecological studies found that basal resources, aquatic invertebrates, and fish were negatively affected in a variety of ways (e.g. biodiversity losses, replacement of sensitive with tolerant species). However, several studies showed the importance of effluent in providing environmental flows to streams that had been dewatered by anthropogenic water withdrawals, especially in semi‐arid and arid regions. Knowledge gaps identified include the abiotic impacts of effluent, such as changes in channel morphology and hydrology (e.g. how nutrient‐rich and warmer effluent affects infiltration rates or interactions with groundwater), the effects of effluent on plants and vertebrates (e.g. amphibians, birds), and the impact of effluent‐induced perennialisation on naturally intermittent or ephemeral streams. Although effluent‐fed streams often exhibit signs of ecological impairment, there is great potential for these systems to serve as refuges of aquatic biodiversity and corridors of ecological connectivity when wastewater treatment standards are high, especially in semi‐arid and arid regions where natural streams have been dewatered.

Responses and resilience of tallgrass prairie streams to patch‐burn grazing

Abstract Patch‐burn grazing (PBG) can promote terrestrial heterogeneity and biodiversity, but can temporarily increase stream nutrients and ecosystem metabolism, and alter macroinvertebrate assemblages. The impacts of grazing on stream channel morphology and post‐PBG recovery patterns are unclear. We assessed the influence of grazing in PBG managed grassland streams in Missouri, USA, and subsequent recovery when grazing ceased for 2 years. We hypothesized that grazing would degrade water quality, stream biotic integrity and channel morphology, but that riparian fencing would mitigate these effects. We predicted that biological and chemical variables in unfenced streams would return to pre‐PBG levels within 2 years after grazing ceased, but channel morphology would not. Six small headwater streams (two in ungrazed control watersheds, two in PBG watersheds with 10 m fenced riparian zones and two in PBG unfenced) were sampled over 7 years; 2 years before PBG, 3 years during PBG and 2 years post‐PBG. We sampled macroinvertebrates and water chemistry monthly when water was present and surveyed channel morphology at least once each study period. During grazing, unfenced watersheds showed the greatest changes in channel width, depth and area. During the post‐PBG period, one of the two unfenced watersheds showed partial recovery of channel morphology. Although grazing increased concentrations of nutrients and chlorophyll‐a, concentrations returned to pre‐PBG conditions after grazing ceased, indicating recovery. Very fine organic sediments increased in the unfenced watersheds compared to the control during grazing but recovered afterwards. Contributions of Chironomidae to total invertebrate abundance increased in the unfenced watersheds during grazing, and then decreased during the post‐PBG period. Riparian fencing mostly mitigated effects of grazing on the streams. Unfenced streams were resilient to effects of grazing in a PBG managed grassland, with most metrics recovering within 2 years after grazing ceased, except for channel morphology. Synthesis and applications. Grazing in a patch‐burn grazed managed grassland coupled with riparian fencing could be an effective conservation tool for land managers of prairies, with relatively modest influences on stream water quality and biotic integrity. Persistent changes in stream channel morphology and effects of longer periods of grazing deserve further research.

Does variable channel morphology lead to dynamic salmon habitat?

AbstractStream channel morphology forms the template upon which hydraulic aspects of aquatic habitat are created, yet spatial and temporal variability in habitat imposed by changing morphology is not well understood. This paper presents a conceptual model linking sediment supply patterns to spatial and temporal variability in channel form and aquatic habitat. To evaluate this model, change over time in three habitat variables is quantified using a 2D hydrodynamic modeling approach. A 45‐year record of topographic data from Carnation Creek, a catchment in coastal British Columbia, is used for the flow modeling. Using the Nays2DH modeling platform, water depths and velocities are simulated in eight channel segments located at different positions relative to locations of historical colluvial input using seven flow levels ranging from 3% to 400% of mean annual discharge (0.02 to 3.31 m s ). Results indicate that habitat availability changes through time as a result of sediment supply‐driven changes to channel morphology and wood loads, but patterns in habitat vary as a function of dominant channel segment morphology. Spatial and temporal variability in morphology also influences the relationship between habitat availability and river discharge, leading to non‐stationary habitat‐discharge rating curves. When habitat areas are predicted by applying these curves to daily flow series spanning annual dry seasons, over 50% of the variance in cumulative seasonal habitat area can be explained by year‐to‐year changes in channel morphology and wood loading, indicating that changing morphology is an important factor for driving temporal habitat variability. This variance is related to the morphological variability of a channel segment, which in turn is associated with the segment position relative to zones of colluvial input. Collectively, these results suggest that variability in habitat is impacted by channel morphology, and can be evaluated partly on the basis of a channel's sediment supply regime. © 2019 John Wiley & Sons, Ltd.

Генетическая составляющая типизации речных русел

The paper represents the analysis of the existing, most common classifications (typifications) of river channels. It is displayed that almost in all approaches, regardless of the methodological basis, there is a morphogenetic component. Restriction of the whole variety of conditions for river channels development by one or two or three factors of channel formation confines the possibilities of channel analysis, distinguishing the main regularities of channel dynamics and management of channel processes in practical problems solution. In the case study of the Ob’ River the estimation was performed of the changes of channel morphology and dynamics along the river as the consequences of the multifactor nature of channel processes. The main attention is paid to the morphodynamic classification of river channels by the Moscow State University, which is characterized by the most complete account of their morphology and channel changes regime. A block structure of this classification reflects the sequence of structural levels of channel processes manifestations (mountain and lowland rivers as a reflection of different flow turbulence and forms of sediment transport, incised and wide-floodplain channels which develop in different geological and geomorphological conditions, morphodynamic channel types themselves – meandering, braided and relatively straight and the diversity of their varieties, etc.) – so this classification demonstrates complex interrelations between its structural levels. Accounting of channel stability, size of bed-material load, direction of vertical channel development (river entrenchment (incision)/accumulation of sediments) and anthropogenic channel change gives to the classification a universal character, makes it possible for regional generalizations and mapping of territories and river basins, gives an opportunity to use it as a basis of channel processes management in water economy and water transport.

Is the topography playing a dual role in controlling downstream channel morphology of a monsoon dominated Dwarkeswar River, Eastern India?

This study focuses on the role of topography in downstream changes in channel morphology of a monsoon dominated Dwarkeswar River with sixty cross-sections. Bankfull channel characteristics, flow characteristics, sediment characteristics and channel pattern were analysed with respect to dissected plateau, plateau-fringe and alluvial plain using statistical analysis. There was a significantly positive relation with distance from source and width (r = 0.874), width/depth ratio (r = 0.607) and flow area (r = 0.926) in the dissected plateau, although a significantly negative relation was found in the alluvial plain (width (r = −0.963), width/depth ratio (r = −0.897), and flow area (r = −0.821)). Overall, the channel dimensions diminished in the alluvial area by 6–7 times. In both these cases, TSI values were higher than the HSI. In the PCA-1, topographic factors were more dominant and the role of drainage area, elevation, slope, basin shape and depth of sediment were opposite to each other in comparison with dissected plateau and alluvial plain.

Characterisation of channel morphological pattern changes and flood corridor dynamics of the tropical Tana River fluvial systems, Kenya

Historical morphological changes are useful tools for understanding river evolutionary trends and condition. This study characterised planform changes and floodplain dynamics of the Tana River fluvial systems, Kenya, between 1975 and 2017 using an integrated remote sensing and geographical information system (GIS) methodological framework. The river has witnessed significant alterations in channel morphology over a period of 42 years, indicating a highly active and meandering nature with a generally rising sinuosity in the downstream direction. The annual lateral river movement rate reduced significantly after the river regulation, which occurred between 1981 and 1988. Land cover change detection of the study reaches revealed tremendous spatial and temporal progressive disappearance of the riparian vegetation and dynamic channel morphological characteristics, trends that may continue in the coming years. The approach adopted in this study can be applied to other river reaches within study area with similar protocols or modifications according channel type and reach condition to provide water managers with evidence on the dynamic fluvial channel plan morphology for sustainable planning management and restoration activities.

Morphologic adjustments of actively evolving highly curved neck cutoffs

ABSTRACTNeck cutoffs and their resultant oxbow lakes are important and prominent features of riverine landscapes. Detailed field‐based research focusing on the morphologic evolution of neck cutoffs is currently insufficient to fully characterize cutoff evolution. High‐resolution bathymetric data were collected over 3 years for the purpose of determining channel morphology and morphologic change on three actively evolving neck cutoffs. Results indicate the following general trends in morphologic adjustment: (1) a longitudinal bar in the upstream meander limb that develops near the entrance to the abandoned bend; (2) a deep scour hole in the downstream meander limb immediately downstream of the cutoff channel; (3) erosion of the bank opposite the cutoff in the downstream meander limb; (4) a cutoff bar in the downstream meander limb at the junction corner of the cutoff channel and the downstream meander limb; and (5) perching of the exit of the abandoned bend above the cutoff channel due to channel bed incision. The results presented herein were used to develop a conceptual model that depicts the morphologic evolution of highly curving neck cutoffs. The findings of this research are combined with recent analyses of the three‐dimensional flow structure through neck cutoffs to provide a mechanistic explanation for the morphodynamics of neck cutoffs. © 2019 John Wiley & Sons, Ltd.

Meandering-anabranching river channel change in response to flow-sediment regulation: Data analysis and model validation

Anabranching river has been recognized as an independent river pattern. A variety of investigations have attempted to disclose the formation of anabranching rivers, however, most previous studies mainly focused on the controls of anabranching channels formation. The mechanism of transition among sub-channels with varying flow-sediment regime in anabranching rivers has been poorly understood. In this study, we used multi-source data sets to investigate the changes in river channels and island (Wugui Island, about 7 km2) along the 27-km long Jianli Reach (Tashiyi-Tianziyihao) in the middle Yangtze River. Nine Landsat images (1987, 1990, 1994, 1996, 2000, 2003, 2006, 2010 and 2014) with 30 m spatial resolution were used to map the channel planform dynamics. Obvious changes in channel width and morphology of the island were observed during 1987–2014. The width of the left branch had reduced from 1041 m to 340 m, while the width of the right branch had increased from 632 m to 1531 m. During 1994–1996, the main river channel shifted from the left branch to the right. During the period of 1987–2014, the total width of the compound cross-section increased, and the mean length-width ratio of the island increased from 3.59 to 4.32. The mean annual flow discharge increased about 9%, and mean annual suspended sediment concentration and sediment transport coefficient (the ratio of mean annual suspended sediment concentration to mean annul flow discharge) decreased about 75% and 76% respectively from 1951 to 2009, in response to the operation of Gezhouba Dam and Three Gorges Dam. The meandering-anabranching channel planform adjusted by narrowing the island width and enlarging compound cross-section in response to upstream water-sediment regime. The shift of the main river channel from left branch to the right may resulted from the increase in flow discharge and decrease in sediment transport coefficient since 1980s. The adjustment has been intensified by the changes in the sediment transport rate after 1995. After the main channel was switched from left branch to right branch, the flow-sediment diversion capacity was kept stable with a diversion percentage of 90% in right branch. Due to the persistent influence of the Three Gorges Dam, the right branch will function as the main channel for a long time before the meandering-anabranching system achieves equilibrium. The shift and stability of main channel was closely related with the relative magnitude of flow-sediment partition in each branch. The significance of this study is that we successfully manifested the main channel shift process of anabranching river with the support of direct hydromorphological observation and theoretical verification.

The Influence of Channel Morphological Changes on Environmental Flow Requirements in Urban Rivers

Previous research on environmental flows (e-flows) of urban rivers usually assumes that the channel morphology is fixed. However, due to the trapping of sediments by weirs, the channel morphology will undergo significant changes. In this research, the influence of channel morphological changes on e-flow requirements is explored in urban rivers. The hydrological connectivity is considered as a primary factor in e-flows, and three hydrological connectivity scenarios (i.e., high, medium, and low) are explored. The Shiwuli River is adopted as the case study. The results show that e-flows are significantly influenced by changes in river morphology. With an increase in siltation depth, the e-flow requirements will decrease. The sensitivity of e-flows to siltation varies among different river segments, especially in those with low weir heights. In addition, the change ratios of e-flows are different under different hydrological connectivity scenarios. Although siltation is beneficial to the satisfaction degree of e-flow supply, it also leads to a decrease in the flood control ability of rivers. The balance between e-flow and flood reduction is also discussed, and river segments are identified that should be the priority when adopting dredging measures.

Small headwater stream evolution in response to Lateglacial and Early Holocene climatic changes and geomorphological features in the Saint-Gond marshes (Paris Basin, France)

The study focuses on river dynamics and vegetation changes during the last Glacial-Interglacial transition from a headwater stream located in the eastern part of the Paris Basin. We adopt a spatial multiproxy approach combining image analyses, geophysical surveys, sedimentary and pollen record analyses to document the impact of Lateglacial climate changes and geomorphological features on vegetation and fluvial dynamics in the small Boitet catchment (ca. 20 km2). Our results show that the sedimentary record is organized into six successive alluvial sequences reflecting changes in discharge and channel morphology in response to short phases of climate oscillations during the Lateglacial and Early Holocene periods. The Boitet alluvial sequences present some similarities with other NW European rivers that are interpreted as large-scale fluvial system evolution to climate changes. However, some local differences have also been highlighted partly related to the upstream position of the catchment. Among them, two distinctive features of the Boitet catchment are 1) the preservation of the Oldest Dryas deposits, which have been rarely described in this area, and 2) the recognition of multichannel river dynamics during the Oldest Dryas and Younger Dryas. We demonstrate that the fluvial evolution is firstly triggered by climate changes and that land surface features may also influence specifically upstream areas revealing contrasting responses of the river system.