Channel Planform Changes Research Articles

Controls on channel planform changes in rivers of the Atlantic Coastal Plain: Findings from the Congaree river valley (South Carolina, USA)

Rivers shaped the landscape of the Atlantic Coastal Plain during the last tens of thousands years. However, their spatial and temporal evolution is understudied. We conducted geophysical, geological and remote sensing surveys in the Congaree River valley, characterized by numerous traces of palaeochannels preserved in the floodplain. The age of the former channels was determined by AMS radiocarbon dating. The main goal of our study was to identify factors controlling the river planform changes and compare the findings with other rivers of the Atlantic Coastal Plain. We found that the Congaree floodplain was shaped by single-thread channels characterized by formation of levees and floodbasins at least 34,000 cal. BP. Large-scale meanders were active between 16,000 and 12,000 cal. BP. They were replaced by meandering channel belts shifting by avulsions between 12,000 and 3,000 cal. BP. Compound meanders have been formed during the last 3000 years. Traces of these channel planforms are also present in other rivers of the Atlantic Coastal Plain. The abandonment of the large-scale meanders was caused by increase in climate humidity, changes in vegetation cover, and inundation of the coastal plain by the ocean level rise, resulting in base-level rise. The avulsions of meander belts in the Early and Middle Holocene were formed in conditions of a humid climate, a series of flooding periods, and continued base-level rise. A relatively drier climate in the Late Holocene caused a change in the meanders’ evolution from the channel belts shifting by avulsions to compound meanders evolving by cutoffs. The last 200 years were dominated by a huge delivery and deposition of fines, caused by deforestation and land-use changes in the river valleys.

The evolution of meandering rivers in sedimentary basins: Insights from the lower Drava (Hungary/Croatia)

AbstractRivers flowing through sedimentary basins are subjected to a variety of controls. The main goal of our study was to identify the effect of external (e.g. climate changes, tectonics) and internal controls (e.g. sediment transport, deposition, vegetation cover) on the evolution of meandering rivers flowing through sedimentary basins using the example of the lower Drava River (Europe, Hungary/Croatia). Field research was conducted along a 50‐km‐long section of the valley. Sedimentary data from boreholes and corings, 35 km of ground‐penetrating radar surveys and analyses of digital maps were conducted to reconstruct channel planform changes. Traces of four meander belts were identified, and 39 AMS radiocarbon dates were used to distinguish the chronology of the fluvial events. The evolution of the lower Drava River comprised alternating periods of deposition (formation of aggrading meander belt) and avulsions. The channel belts were formed owing to upstream sediment delivery and floodplain storage. Changes in climate humidity and the occurrence of high flows influenced the planform of the meanders within particular channel belts. The oldest channel was active at least ~40 000 cal. BP before being reworked by subsequent meanders active between the Late Pleniglacial (30 000–14 700 cal. BP) and Late Glacial (~11 000 cal. BP) periods. The channel belts shifted to the south in the Holocene, between ~11 000 and 250 cal. BP due to the presence of a thrust fault situated diagonally to the Drava Valley. Results show channel width, channel belt width and the surface area of point bar deposits increased in the succeeding generations of meanders and that the style and sedimentary architecture of the channel belts were dominantly dependent on autogenic controls, that is, sediment delivery, aggradation and erosional events (e.g. formation of chute cut‐offs).

Long-term geomorphic adjustments following the recoupling of a tributary to its main-stem river

River restoration and rehabilitation projects are widespread, but rarely include the data needed to fully evaluate if they are successful in achieving their goals or how long the process of readjustment takes before a new ‘recovered’ regime state is reached. Here we present a seven-year post-project dataset detailing the morpho-sedimentary responses of a river to the reconnection of a formerly diverted tributary, and relate observed changes to conditions in the river prior to the reconnection. We describe changes in the tributary and main-stem channels, including changes in channel planform, morphology, and the export of coarse and fine sediment from the tributary to the main-stem river. We use the data to develop a conceptual model of the system's response to the reconnection.Marked geomorphic changes occurred within the first two years after the reconnection. Changes during this ‘shock phase’ included dramatic erosion and subsequent deepening and widening of the tributary channel, rapid development of a confluence bar and an increase in fine sediment delivered to the main-stem. After this shock phase, and despite the continued occurrence of high magnitude flow events, the rate of geomorphic change in the tributary began to decrease, and the rate of growth of the confluence bar slowed. Fine sediment volumes in the main-stem also decreased steadily. After an adjustment phase lasting a total of approximately 4.5 yr (including the initial 2-yr shock phase), the tributary to mainstem system appeared to reach a new dynamic equilibrium that we consider the adjusted regime state. This new regime state was characterised by, among other things, an increase in geomorphic heterogeneity in the tributary and main-stem channels.Changes in both fluvial processes and forms indicate that within 4.5 yr the project was successful in achieving its goal of augmenting sediment and increasing geomorphic heterogeneity. Our conceptual model of adjustment mirrors that developed by Petts and Gurnell (2005), with the river passing through a complex and dynamic adjustment phase before reaching a new regime state. However, unlike the responses to impoundment represented by Petts and Gurnell, our model of river response to rehabilitation charts increases in dynamism and heterogeneity.

Exploring willow decline in an over-greening riparian corridor (River Jarama, Spain)

The analysis of willow forest decline in the Jarama River has been assessed through the study of morphological river conditions in three periods (1956, 1998 and 2018) together with active channel mobility ratios and changes in channel planform and vegetation coverage. Species composition of current riparian corridors at river segment scale and monitored natural recruitment of pioneer species at local scale have been studied. Jarama River has changed from wide active wandering towards narrower meandering channels with dense and continuous riparian over-greening corridors, in which willow formations are threatened, and drought-tolerant species have colonised it. Landform evolution shows an intensive ‘maturation’ associated with ‘anthropisation’ processes during the first period, while stability dominates nowadays. Results show a generalised aging and progressive mortality of shrub willows and the expansion of forests dominated by Fraxinus angustifolia, likely associated to the progressive reduction of channel mobility and the lack of bare gravel bars for pioneer recruitment. The dominant willow species is Salix salviifolia, a drought tolerant willow that has been able to withstand the processes of 'maturation’ and channel narrowing being widely distributed throughout the riparian section. On the other hand, other strict pioneer willows such as Salix eleagnos and Salix purpurea are very rare and are found in the few gravel banks that remain on the shore. Results predict a gradual disappearance of willows in the Jarama River as there is no significant recruitment, and they would last as long if those old specimens may survive.

The evolution of meandering and anabranching rivers in postglacial and loess landscapes of Europe

This study aims to explain the influence of sediments and landforms of postglacial and loess landscapes of Europe on processes forming meandering and anabranching rivers. Based on a literature review, sedimentary architecture, types and grain-size of deposits, and channel planform changes since the Late Pleniglacial were collected for extensive set of European rivers. These data were used to create a conceptual model explaining the evolution of meandering and anabranching rivers in both types of landscape. The present study shows that the inheritance of glacial and fluvioglacial landforms and sediments, and loess formation, influence types of processes forming anabranching and meandering rivers. Point bar accretion formed meandering rivers in postglacial zone whereas oblique accretion dominated in loess areas. Anabranching rivers of postglacial zone evolved through the formation of crevasse channels and meandering anabranches. Anabranching rivers of loess zone formed sustained bifurcations. Postglacial and loess rivers reacted differently to increased deposition and humidity in the Late-Holocene. Meanders of postglacial zone formed channel bars owing to increase in stream power and sediment load. In cases of flow energy decrease, and sustained upstream sediment delivery, they turned into anastomosing rivers. Anabranching rivers in ice-marginal valleys maintained its courses until hydro technical works in the 19th century. Low-energy meandering rivers of loess areas became wetlands owing to deposition of silts. High-energy meandering and anabranching rivers maintained the planforms throughout the Holocene. These findings need to be refined by detailed studies on the evolution of rivers in subarctic zone, and large rivers of Europe.

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.

River lengths in Croatia determined from a topographic map at a scale of 1:25,000

This study presents methodologically consistent length measurements of rivers with a total length of 40 kilometres or more in the Republic of Croatia. These measurements were performed by vectorising river channels from a topographic map at a scale of 1:25,000. Contemporary orthophoto maps were used as additional cartographic sources for verification. The approaches used to determine the starting and ending points of rivers, i.e. sources and mouths or confluences, have been explained and their spatial coordinates were listed. The results showed that in terms of total river length, the 21 rivers in this study differed from the available official data by less than 5%. Considerable differences were found for rivers where significant river engineering works had been carried out to channelize and divert flow, as well as for rivers in the Adriatic Basin where there have been methodological differences in determining the river mouth, and for large lowland rivers where changes in channel planform are common due to natural dynamics and engineering interventions. Moreover, this research revealed that the longest Croatian river, with its source, confluence, and both river banks situated entirely within the territory of Croatia, is the Karašica River, with a total length of 148 km.

Effects of flow regulation and drought on geomorphology and floodplain habitat along the Colorado River in Canyonlands National Park, Utah

AbstractStreamflow regulation compounded by regional drought has resulted in up to 22% reduction in channel width, changes in channel planform, expansion of riparian vegetation, and alterations to floodplain habitat on the Colorado River in Meander Canyon, Utah. Although some changes in channel width occurred between the 1940s and 1980s, coinciding with major phases of upstream water development, larger decreases in channel width occurred between 1993 and 2006 during periods of exceptionally low annual floods. These findings illustrate that low runoff associated with regional drought and climate change may cause changes in river channel form that accelerate and compound the effects of upstream water development. Declining peak flows have also resulted in disconnection between the wetted channel and floodplains, where inundated back‐levee depressions provide habitat used by two species of threatened and endangered native fish. Despite this disconnection, some back‐levee depressions on the floodplain continue to be inundated by ~1.5‐year recurrence floods via connections created by tributary mouths, floodplain outflow channels, and levee breaches excavated by resident beaver. These changes are shown by analysis of aerial images, high‐resolution bathymetric and topographic measurements, and 2‐dimensional streamflow modeling.

Channel planform changes on the lower Peiros River (Patras, Greece): 1945-2018 comparison

Changes between the active channel planform features of the lower Peiros River in 1945 and 2018 were examined through a multitemporal analysis performed in a GIS environment. The active channel experienced a consistent and rather generalized narrowing process (overall 71.6%, up to 84.3% at the reach-scale) along with localized increases of sinuosity and channel pattern variations associated with the disappearance of secondary channels and bars. Negligible changes in length occurred but the river mouth retreated by approximately 160 m. Large riverine areas disconnected from the active channel were involved in a peri-urbanization process and, thus, occupied by human activities and infrastructures; this resulted in anthropogenic landform modelling and increase in vulnerable elements exposed to hydrogeomorphic hazards. The observed active-channel planform changes could be, at least partly, human-induced. Further research is needed to define the Peiros River geomorphological evolution. The outcomes from this study represent an essential preliminary knowledge base under a river management perspective.

Channel planform dynamics, avulsion and bankline migration: a study in the monsoon-dominated Dwarkeswar river, Eastern India

The progressive and sequential shifting of the banklines and centerlines (thalweg) due to bank erosion and channel bar formation, as well as channel planform changes and avulsion of the Dwarkeswar river of West Bengal during the last 100 years, have been studied. For this purpose, police station maps, Survey of India topographical maps, and satellite images were used along with extensive field surveys. The channel was divided into reaches and segments for measuring the amount of migration and quantifying planform indices. Four types of changes have been observed: (1) channel planform change due to meander development and migration, (2) avulsion and consequent alteration in the flow direction, (3) thalweg shifting as a consequence of mid-channel bar formation, and (4) widening of the channel due to bank erosion. The study reveals that the alluvial plain upper (APU) is the most dynamic reach due to the meandering nature of the river. The thalweg shifted up to 1.71 km in this reach. An avulsion occurred in the lower course of the river during a large flood in 1978. The Jhumjhumi Nala became the main course of the river, and the earlier channel had become completely abandoned due to this avulsion. The sinuosity index of the entire study reach has been increasing since 1920, whereas the braiding index has decreased at most of the segments. Therefore, the channel planform is transforming into the meandering pattern from the braided pattern.

Planform Dynamics of Kankai River using GIS and Remote Sensing

The Kankai river is a rainfed perennial river of eastern Nepal and carries a large quantum of silt, and enters in Kishanganj of West Bengal, India. Recently, a bridge collapsed in a village in the Kishanganj district due to a rise in the water levels of the Kankai river. This paper presents a study of the dynamics of changes in channel planform and measurement of the erosion and accretion of the Kankai river in Bihar, India from 2000 to 2018. Spatial data were processed in GIS to determine the changes in sinuosity, centreline migration rates, and extent of erosion, and deposition of sediment. The maximum river shift was obtained as 800 to 1600 m and the minimum river shift varied from 6 to 70 m. With a sinuosity ratio for the whole reach of the river of 1.45, the Kankai river is considered a sinuous river and needs river training works at the erosion sites.

Количественная оценка плановых переформирований РЕКИ Камчатки

A quantitative assessment of channel planform changes on the 600-km channel of the Kamchatka River was carried out. Possibilities of semi-automatic GIS-interpretation of satellite images were used for retrospective analysis of such an extended section of a large river. It was estimated that 37% of the left bank length and 44% of the right bank length are eroded. The amount of material coming into the channel due to riverbanks erosion significantly exceeds the sediment runoff, which determines channel type and regime of channel changes. Processes of bank erosion vary on the scale of the longitudinal profile and different channel types. Downstream according to decrease of channel slope, a decrease of bank erosion intensity is observed. The most important factor of bank erosion on the river local section is the volume of material coming from the upstream sections. A quantitative analysis and forecast of planform changes of a river section requires a differential assessment for different morphodynamic channel types, taking into account characteristics of channel processes in the upstream river sections. The bank erosion prognostic equations obtained for the meandering channel and the floodplain-channel multiple branches of the Kamchatka River are discussed.

Channel geomorphology and riparian vegetation interactions along four anabranching reaches of the Upper Yellow River

The fluvial biogeomorphic succession (FBS) phase model is used to appraise biogeomorphic interactions along four anabranching reaches of the Upper Yellow River. Geomorphic features and vegetation distribution patterns are combined to assess the dominant FBS phase. A notable increase in shrub and tree cover and a decrease in sedges occur with decreasing elevation from 4200 to 2200 m asl. The Maduo reach (4185 m) is characterized by the biogeomorphic phase, while the Dari (3960 m) reach is at the pioneer phase, with short shrubs playing a prominent role. Both the Maqu (3450 m) and the Guide (2117 m) reaches are ecological phase dominated (the Guide reach has been impacted by flow regulation in recent decades). Reaches other than the Maduo have experienced an increase in post-pioneer vegetation (woody plants) in recent decades, with a decrease in pioneer plant communities (grass and sedges). The Dari reach experienced the most dynamic channel planform changes, with a marked increase in short shrub patches from 2007 to 2017. An increase in woody vegetation underpinned increasingly stable geomorphic dynamics in the Maqu reach. Human activities have restricted geomorphic impacts of expanded areas of woody riparian vegetation in the Guide reach. Variability in dominant FBS phases exerts a primary control upon channel behavior and evolutionary traits in the four study reaches.

Measuring channel planform change from image time series: A generalizable, spatially distributed, probabilistic method for quantifying uncertainty

AbstractChannels change in response to natural or anthropogenic fluctuations in streamflow and/or sediment supply and measurements of channel change are critical to many river management applications. Whereas repeated field surveys are costly and time‐consuming, remote sensing can be used to detect channel change at multiple temporal and spatial scales. Repeat images have been widely used to measure long‐term channel change, but these measurements are only significant if the magnitude of change exceeds the uncertainty. Existing methods for characterizing uncertainty have two important limitations. First, while the use of a spatially variable image co‐registration error avoids the assumption that errors are spatially uniform, this type of error, as originally formulated, can only be applied to linear channel adjustments, which provide less information on channel change than polygons of erosion and deposition. Second, previous methods use a level‐of‐detection (LoD) threshold to remove non‐significant measurements, which is problematic because real changes that occurred but were smaller than the LoD threshold would be removed. In this study, we present a new method of quantifying uncertainty associated with channel change based on probabilistic, spatially varying estimates of co‐registration error and digitization uncertainty that obviates a LoD threshold. The spatially distributed probabilistic (SDP) method can be applied to both linear channel adjustments and polygons of erosion and deposition, making this the first uncertainty method generalizable to all metrics of channel change. Using a case study from the Yampa River, Colorado, we show that the SDP method reduced the magnitude of uncertainty and enabled us to detect smaller channel changes as significant. Additionally, the distributional information provided by the SDP method allowed us to report the magnitude of channel change with an appropriate level of confidence in cases where a simple LoD approach yielded an indeterminate result. © 2020 John Wiley & Sons, Ltd.

Morphological changes and human impact in the Entella River floodplain (Northern Italy) from the 17th century

In this article the morphological changes undergone by the Entella River (Northern Italy) over the last four centuries has been investigated. The historical analysis has allowed reconstruction of fluvial evolution and shoreline dynamics and demonstrated their relationship to human disturbance over a very long period compared to most previous studies in Italy and Europe. A set of 12 historical and current maps and aerial photos, ranging from the 17th century to the present, were entered into a Geographic Information System (GIS) in order to calculate four morphological parameters (i.e., channel length, width, sinuosity and centreline shifting) and the distance of the shoreline.Modification of the Entella River over the 360-years period 1656–2016 include: i) a reduction in channel length by 128 m (3%); ii) a mean narrowing of 56 m (46%); iii) a decrease in total sinuosity from 1.10 to 1.05, with a variation in river pattern, from sinuous to straight; iv) a total shifting of the centreline of 30 m. This evolutionary trend is consistent with most of the previous studies on Italian and European rivers. Conversely, the total channel shortening and the recent phase of substantial morphological stability, with a slight increase in channel width and length, seems to be in contrast with the results of other studies.We correlated the channel planform changes and the regression of the shoreline, at least until the latter 20th century, to the reduction in sediment supply produced by the morphological modifications due to human intervention. At the beginning of the 19th century and, later, from the 1950s to the end of the 20th century, channelization, channel diversion, land-use changes and coastal defences accounted for this supply change to the Entella River and its floodplain.Furthermore, our findings reveal that human disturbance has contributed to increased flood risk in the plain, through the progressive reduction in width of the riverbed and the increasing urbanization along its riverbanks. At the same time, channelization seems to have a negative effect not only on the morphological evolution of the river, but also in terms of prevention and flood risk reduction within the floodplain.

Effect of instream sand mining on hydraulic variables of bedload transport and channel planform: an alluvial stream in South Bengal basin, India

Sand mining adversely behaves with changing river system, especially in the bedload transport process. This study aims to find out the relationships between river water flows, sediment transport regime in bedload transport and also tries to determine how instream mining affect the sediment inflow and channel planform change. The field study has been conducted in three selected sites on Kangsabati River situated in the lower, middle and upper course. Several established equations were applied to derive the results from water flow and sediment regime. GSTAT was used to prepare coarse medium (CM) diagrams and tractive current deposits to analyze the trend of sediment throughout the river course. Sediment and bedload transport were measured using Ackers and White (J Hydraul Eng Div ASCE 99(hy11):2041–2060, 1973) and Meyer-Peter–Muller methods (IAHSR 2nd meeting, Stockholm, appendix 2. IAHR, 1948). Shear stress and critical shear stress were computed using Shields (Application of similarity principles and turbulence research to bedload movement, 1936) and DuBoys equations whereas Friend and Sinha’s method (Braiding and meandering parameters, vol 75, No 1, Geological Society, London, 1993) was used to detect irregular planform response caused by sand mining. Regression analysis denotes that mining intensity can be change the shear stress (R2 = 0.972); velocity (R2 = 0.683); grain diameter (R2 = 0.555) and sediment concentration (R2 = 0.997) on bedload transport. Mining activities can be changes of hydraulic responses as well as interruption of sediment and bedload transport throughout the channel. In context of sand mining, planform response denotes that thalweg shifting leads to massive sandchar deposition in upper course, huge channel incision causes river bank erosion in middle course and pool–riffle alteration expanded in channel and pit area in lower course.

Channel planform changes along the Scrivia River floodplain reach in northwest Italy from 1878 to 2016

AbstractA detailed, quantitative, multitemporal analysis of historical maps, aerial photos, and satellite images was performed to investigate the channel planform changes that occurred along the Scrivia River floodplain from 1878 to 2016. Various channel planform features, including channel length, area, width, braiding, sinuosity, lateral migration, activity, and stability, were computed through an innovative geographic information system–based procedure, starting from manually digitized active-channel polygons. Three active-channel morphological evolution stages are evident from: (1) 1878 to the 1950s; (2) the 1950s to the end of 1990s; and (3) the end of 1990s onward. In the first period, the river was generally able to migrate in its floodplain, shaping the riverscape. Active-channel narrowing and increasing channel stability characterize the second period. The most recent phase shows an inversion of the morphological evolutionary trend. This last phase is characterized by a slight generalized widening related to the reactivation of stabilized surfaces and to bank-erosion processes. Particularly from the 1950s to the 1990s, in-channel sediment mining and channelization with consequent occupation of riverine areas strongly affected the Scrivia River. These factors, together with floods, are thought to be the most likely causes of such consistent and fast morphological changes.

Arctic Mackenzie Deltachannel planform evolution during 1983–2013 utilisingLandsatdata and hydrological time series

AbstractArctic deltas, such as the Mackenzie Delta, are expected to face major climate change and increased human influence in the near future. Deltas are characterised by highly dynamic fluvial processes, and changing climate will cause considerable evolution of the riverine environment. The changes are difficult to predict with existing knowledge and data. This study quantified channel planform change of the Mackenzie Delta (1983–2013), analysing its temporal and spatial patterns. We addressed the main obstacle of research on large remote areas, the lack of data, by developing a unique work flow that utilised Landsat satellite imagery, hydrological time series, remote sensing‐based change analysis, and automatic vectorisation of channels. Our results indicate that the Mackenzie Delta experienced constant evolution but at a highly varying rate over the 30 years. The study demonstrates that the magnitude and duration of flood peaks and the presence of spring ice breakup floods determine the rate of Arctic delta planform change. Changing winter conditions and spring flood magnitudes may therefore affect the stability of Arctic deltas. However, no clear trends towards decreased recurrence or magnitude of spring floods or increased instability of the delta plain have yet been observed in the Mackenzie Delta. The delta plain was most dynamic at the beginning and at the end of the examined period, corresponding to intense flooding, whereas the rates of change were subtle during the low‐flood period 1994–2007. The largest changes have occurred along the wide Middle Channel and in the outermost delta. Relative to their size, however, smaller meandering channels have been highly dynamic. Hotspots of change in the delta plain are located in anastomosing and braiding channel segments and, at the local scale, in point bars and cut‐banks along meandering channels. Our study describes how Landsat satellite data can be utilised for advancing fluvial geomorphological research in remote areas. However, cloudiness in the delta restricts production of dense time series with simultaneous coverage of the whole area and requires manual preprocessing.

Lateral erosion/accretion area and shrinkage rate of the Linhe reach braided channel of the Yellow River between 1977 and 2014

Quantitative studies on river channel lateral erosion/accretion area changes over time can reveal the characteristics of channel evolution. Taking the 213-km-long Linhe reach braided channel of the Yellow River as an example, area changes in channel bank erosion/accretion in four sub-reaches (S1, S2, S3 and S4) over 19 different periods were evaluated on the basis of remote sensing images captured since 1977. Mean channel shrinkage rate for the whole river reach was also obtained. Results show that the left and right banks of the Linhe reach were dominated by lateral net accretion between 1977 and 2014. The channel area of this section of the Yellow River was characterized by reduction between 1977 and 2001, while periods of alternate erosion and accretion occurred subsequent to 2001. Mean channel shrinkage rate in the Linhe reach braided channel was 6.15 km2/yr between 1977 and 2014, while the most remarkable changes in channel planform occurred in the 1990s. Compared to 1995, channel length and sinuosity increased by 5.8% and 6.6% by 2000, while channel area and mean width decreased by 39.4% and 42.8%, respectively. Significant changes in channel planform and shrinkage of the Linhe reach occurred in the 1990s, mainly as a result of the joint-operation of the Longyangxia and Liujiaxia reservoirs since 1986, which caused substantial reductions in runoff and sediment flux during the annual flooding season. In addition, bank erosion/accretion in the four sub-reaches was affected by the physical properties of local banks, engineering emplaced to protect channel banks, and hydrodynamic differences. However, since the implementation of integrated river management measures from 2000 onwards, these changes have been significantly mitigated and the health of the Linhe reach braided channel of the Yellow River has been restored.

Spatial and temporal variations of channel lateral migration rates in the Inner Mongolian reach of the upper Yellow River

Channel variations of the Inner Mongolian reach in the upper Yellow River are very complicated because of diversity of channel patterns, intense changes in water and sediment, and enhanced human activities in the last decades. It is important to examine the channel migration rates in this river reach. Based on channel planform changes at 236 digital channel cross-sections, the present study estimated the channel migration rates in terms of mid-channel line shift at the digital channel cross-sections during the periods 1975–1990, 1990–2000, 2000–2006, 2006–2010, and 2010–2011. The results show that the average channel migration rate for the whole river reach is 5.0, 7.9, −5.9, 8.6, and −1.0 m−1 in the five different periods, respectively. In both directions to the right and to the left, the highest channel migration rate was always observed in the meandering channels, while the lowest channel migration rate was in the whole Inner Mongolian reach except the Sanhuhekou–Zhaojunfen (S–Z) sub-reach. The temporal variation of channel migration rate was mainly controlled by changing sediment concentration in river flow, while the spatial variation along the straight, braided, and meandering channel reaches was largely affected by material differences of channel banks as reflected by different channel patterns. With the highest average lateral migration rate to the left bank, the meandering channel reach is extremely vulnerable to lateral migration and needs to be protected with engineering practices, because a large population and wide farmland are located on its left floodplain.