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Dimensionless morphological ratios (DMR) generally are used in systemic proposals for stream classification and river restoration projects. Often, such morphometric parameters, including field data from channel cross sections, develop into a template for a given geomorphic reference site. In this study, high-resolution Digital Terrain Models (HRDTMs), combined with orthophotographs from 2018 and ground-based surveys, were used to analyze the spatial variability of DMR in a semi-arid ephemeral stream subject to changes in stream power under bankfull conditions. In particular, a channel reach of 2.7 km in length along the Upper Mula stream in southeastern Spain was chosen to test the relationships between the two types of variables. Rosgen's DMR (width to depth ratio, entrenchment ratio, bank height ratio) and hydraulic data at bankfull stage (flow velocity, Froude number, shear stress, mean stream power and energy gradient, among others) were calculated by 1D hydrodynamic modeling and HRDTMs prior entry of field information. The resulting maps allowed comparison of stream power with DMR in relation to the channel stretch class and bed stability. The results showed similar spatial patterns for the width to depth ratio and the bank height ratio. The average values estimated in bend stretches were lower than along straight and slightly sinuous stretches and very similar to those of sub-reaches from the inflection point to the meander bend apex. However, the entrenchment ratio followed a different pattern, according to which the straight and slightly sinuous stretches were the most entrenched and the bend stretches presented a more moderate average entrenchment ratio. In addition, the energy balance and power gradient also experienced spatial variations in relation to the bed stability and DMR. Only in highly incised sub-reaches were such relationships not significant. The lack of a significant correlation between excess energy and bank height ratio or width to depth ratio over short lag distances was also verified, regardless of the affected bedforms. An ANOVA showed important differences between the straight and slightly sinuous stretches and bend stretches, which were strongly influenced by the incision and entrenchment ratios and the maximum bankfull depth. Unlabelled Image • Dimensionless ratios are sensitive to morphological changes in ephemeral streams. • Different relationships between dimensionless ratios and bankfull channel metrics. • Changes in stream power affecting patterns of bed stability and channel adjustments. • Spatial variations in excess energy were poorly correlated with the incision ratios. • Entrenchment and width-to-depth ratios differ according the channel stretch class. [ABSTRACT FROM AUTHOR]

Contemporary river forms and processes can be heavily influenced by the legacies of anthropogenic disturbances to river systems. Knowledge of the historical range of river adjustment can be used to develop an understanding of a river's 'expected' character and behaviour. Drawing upon the case study of Richmond River Catchment, New South Wales, Australia, we track the history of geomorphic river adjustment from the time of European colonisation in the mid-late nineteenth century. We use this study to develop an approach, called the ' Behavioural sensitivity logical tree ' that can be applied to assess and quantify reach scale behavioural sensitivity, defined as the ease with which geomorphic units and associated water, sediment and vegetation interactions adjust within the expected behavioural regime. We use the results to categorise rivers as Fragile , Active sensitive , Passive sensitive , Insensitive and Resistant. Fragile rivers have a behavioural sensitivity >75% and have the propensity to undergo wholesale river change such that a new river type and behavioural regime is created. Active sensitive rivers have a behavioural sensitivity ranging from 50%–75% and have the ability to re-configure within their contemporary behavioural regime. The behavioural sensitivity of Passive sensitive rivers lies between 20%–50%. These rivers have the ability to maintain their behavioural regime and withstand adjustment. Insensitive rivers have a behavioural sensitivity ranging from 5%–20%. They do not readily adjust and may contain significant antecedent elements that limit geomorphic adjustment. Resistant rivers have a behavioural sensitivity <5% and because of the imposed geological setting cannot readily adjust. We further discuss the evolutionary nature of behavioural sensitivity itself and how rivers can dynamically evolve and shift to a different sensitivity category over time in response to different forms of direct and indirect disturbances. Unlabelled Image • Legacies of post-colonisation anthropogenic disturbance can influence contemporary river forms and behaviour. • We develop an approach that can be applied to any catchment to assess and quantify reach scale geomorphic river sensitivity. • The approach can be used to categorise rivers as Fragile, Active sensitive, Passive sensitive, Resistant and Insensitive. • Geomorphic river sensitivity can dynamically evolve with time. [ABSTRACT FROM AUTHOR]

Anabranching patterns are characteristic of the largest rivers in the world. We have chosen to study the Paraná River because of its worldwide importance among the great rivers and the large database available from over four decades of research. In this paper, we present results on the classification, origin, evolutionary processes, and sedimentary records of islands that form anabranching patterns in the Upper Paraná River in Brazil. These islands are formed by in-channel processes that drive their evolution from central bar islands and by off-channel processes that form floodplain-excised island. Both types of islands can increase in area by the annexation of lateral bars, forming composite islands. The islands formed by floodplain excision are normally larger than the other types, with ages of up to 14.6 ka. The sedimentary records of these islands are relatively simple. Central bar islands present a lower lithosome comprising sandy channel deposits (Sp, St, Sm, and Sm(o) facies) that correspond to central and lateral bar formation, followed by a mud sequence (Fm, Fl, and Fm(o) facies) deposited by vertical accretion events during flood stages. Floodplain-excised islands have the same sedimentary sequence as the floodplain from which they were detached. The in-channel island-forming processes described in this paper may have been operating since the beginning of the Holocene. The understanding of island origin and evolution processes presented in this paper is fundamental to ecological and geomorphological studies, as in the sustainable management of large anabranching rivers. [ABSTRACT FROM AUTHOR]

River-incision rate is widely used to track changes in tectonics or climate over time and space. However, the feasibility of utilizing the spatial variations of river incision to reconstruct past tectonic and climatic processes remains unclear. Here, we focus on the spatial patterns of river incision along the northern Chinese Tian Shan foreland. Three alluvial fans F P , F eH , and F lH are determined as the alluvial fan context of river incision and terrace classification providing the geomorphological framework across the foreland region. Four rivers (i.e. the Kuitun, Jingou, Manas, and Urumqi Rivers from the western, central, and eastern part of the foreland) are used to reconstruct the paleogeomorphology from the reference, which is the best-preserved terrace of each river system with ages clustering in different parts of the Latest Pleistocene-Early Holocene. The depth of incision constrained by the reference terrace of each river is obtained by comparing the present-day topography and the reconstructed one. The resulting profile of channel incision (depth and rate) of each analyzed river displays an overall decreasing-downstream trend from the maximum where the river exits from the mountain range, to zero. Such a trend has been attributed to the progressive lowering of the river gradient that was caused by the adjusted ratio of sediment input versus water discharge induced by climate change. Superimposing on the decreasing-downstream trend, an obvious step can also easily be observed onto the profile of channel incision of each river, downstream of which channel incision is significantly less. The step occurs near the thrust fault controlling growth of the outermost anticline through which each river cuts, thereby implying the key role of local rock uplift in forming the river-incision step. The former observation implies that, at the same timescale, more river incision at the exit from the mountain range does not necessarily mean stronger climatic forcing of incision there. We thus propose that, in a foreland setting, it should be the temporal pattern of river incision rather than its spatial variation that is helpful for unraveling the change in the forcing factor of downcutting. Unlabelled Image • The paleogeomorphology in the foreland is reconstructed from the reference terrace. • The profile of channel incision displays an overall decreasing-downstream trend. • An obvious step superimposes onto the downstream-decreasing trend of incision. • A possible 4-fold easterly decrease in the mean incision rate is observed. • The temporal pattern of river incision is more helpful for unraveling its mechanism. [ABSTRACT FROM AUTHOR]

Human activities are known to impact the physical template of river channels. These impacts can result from deliberate, direct modifications as well as via indirect processes linked to broadscale landscape change. This study examined changes in the physical template of the Barwon-Darling River, a dryland river in southeastern Australia. Historical longitudinal profiles from the late 1800s were compared with contemporary profiles derived from high-definition, side scanning sonar. Comparisons focused on characterising waterhole features as they are a critical biophysical component of dryland rivers. The use of historical data presented several challenges related to small sample size and suspected sampling bias in the historical survey. However, this study demonstrates that these issues are not insurmountable providing the limitations and uncertainties with the data are acknowledged and data analyses are limited to parameters that can distinguish genuine landscape change. The findings revealed a dramatic change in the physical template of the Barwon-Darling River over a 120-yr period. Waterhole depths and distances between waterholes have been altered significantly. The magnitude and trajectory of change was found to be scale-dependent, with the greatest observable change aligned with the presence or absence of low-level weirs. Waterholes influenced by low-level weirs have increased in depth because of localised impoundment, whilst the distance between deep waterholes (>4 m in depth) has declined substantially. In contrast, the maximum depths of waterholes located outside the influence of weir pools has declined by 1.6 m and the distance between deep waterholes has more than doubled in several reaches. These declines are likely to be caused by sediment accumulation in waterholes associated with anthropogenic increases in sediment flux and a decline in the river's capacity to entrain and transport sediment throughout the system. • Waterhole depths and distances between waterholes have been altered over 120 years. • The trajectory and rate of change is aligned with low level-weir occurrence. • Maximum waterhole depths declined substantially in non-weir pool locations. • Distances between deep waterholes have doubled in several non-weir pool locations. • Historical data can be valuable if data limitations are acknowledged. [ABSTRACT FROM AUTHOR]

The Zagros Folded belt, as one of the most tectonically active areas in the world, contains NW–SE trending whaleback anticlines that are growing vertically and laterally. Morphometric indices are widely used in tectonic geomorphological research because of their low cost and relative ease of application. Drainage basins and their networks adapt to tectonically induced changes in surface slope, and hence their qualitative and quantitative analysis can reveal aspects of active anticlinal growth. This study evaluates the morphometric properties of drainage basins and their networks developed on the fold limbs, using this quantitative data to detect lateral propagation of the Gorm anticline in the Zagros Simply Folded Belt (ZSFB). The Gorm anticline was considered to be suitable for this approach because of the variability of drainage basins and their networks along the axis of the anticline and also transversely across its forelimb and backlimb. The morphometric properties of stream networks including drainage density (Dd), drainage frequency (Df), ratio of 1st-order streams to the total number of streams of all orders (N1/N), length of 1st-order streams (L1), spacing ratio (R) of main streams on flanks, and drainage patterns were analyzed in three zones of the anticline (southeastern (SE), central, and northwestern (NW)). Also, the morphometric indices of drainage basins such as basin area (Ba), shape (Bs), asymmetry factor (AF), hypsometric integral (HI), a new proposed crescentness index (CI), and sinuosity of anticline divide (S AD) were evaluated. Results show that an increase in N1/N ratio and L1 from the center towards both the southeastern and northwestern fold noses suggests lateral growth of the anticline towards the southeast and northwest. The SE and NW zones are characterized by the smaller, elongated and crescent-shaped basins, lower elevations, lower dip slopes, higher hypsometric integral, and lower sinuosity of anticline ridge. These results collectively suggest that the southeastern and northwestern fold noses are younger, less eroded and laterally growing. The formation of crescent-shaped basins with asymmetric forked drainages developed in the SE and NW zones are considered to be useful indicators of lateral growth of the Gorm anticline. Although higher values of Ba, S AD , Df and S indexes in the central part of the fold can be attributed to the older relative age of this area, the activity of the Nezamabad Fault and the exposure of some weak rocks also play some roles in explaining the increase the values of parameters. Overall, the N1/N, L1, CI, S AD indexes, as well as the fan-shaped and asymmetric forked drainage pattern are the most useful tectonic geomorphological criteria to detect lateral propagation of anticlines. • Drainage morphometry was analyzed to detect lateral growth of the Gorm anticline. • The crescentness index (CI) of basins as a new morphometric index is presented. • The CI index and sinuosity of the anticline divide (S AD) imply lateral fold growth. • The length of 1st-order streams is useful criteria to detect lateral fold growth. • Data show that the fold northwestern plunge is younger than the southeastern one. [ABSTRACT FROM AUTHOR]

The supply, transfer and deposition of sediment from channel headwaters to lowland sinks, is a fundamental process governing upland catchment geomorphology, and can begin to be understood by quantifying 2D river planform adjustments over time. This paper presents a catchment scale methodology to quantify historic patterns of 2D channel planform adjustment and considers geomorphic controls on 2D river stability. The methodology is applied to 18 rivers (total length = 24 km) in the upland headwaters of the previously glaciated Wasdale catchment (45 km2), Lake District, northwest England. Planform adjustments were mapped from historic maps and air photographs over six contiguous time windows covering the last 150 yr. A total of 1048 adjustment and stable reaches were mapped. Over the full period of analysis (1860–2010) 32% (8 km) of the channels studied were adjusting. Contrasts were identified between the geomorphic characteristics (slope, catchment area, unit specific stream power, channel width and valley bottom width) of adjusting and stable reaches. The majority of adjustments mapped were observed in third and fourth order channels in the floodplain valley transfer zone, where the channels were laterally unconfined (mean valley bottom widths of 230 ± 180 m), with low sediment continuity. In contrast, lower order channels were typically confined (mean valley bottom widths of 31 ± 43 m) and showed relative 2D lateral stability. Hence, valley bottom width was found to be important in determining the available space for rivers to adjust. Over the full period of analysis 38% of planform adjustments involved combined processes, for example, as bar and bend adjustments. The study demonstrates the importance of stream network hierarchy in determining spatial patterns of historic planform adjustments at the catchment scale. The methodology developed provides a quantitative assessment of planform adjustment patterns and geomorphic controls, which is needed to support the prioritisation of future river management and restoration. Unlabelled Image • Catchment-wide river planform adjustment patterns and geomorphic controls are quantified • From 1860 to 2010, 32% of channels surveyed were adjusting in the Wasdale catchment, UK • Adjusting and stable reaches show contrasts in valley and channel width, slope and stream power • From 1860 to 2010, 38% of adjustments occurred as combined processes such as bar and bend adjustments • Stream order hierarchy determines the spatial pattern of adjustments in headwater catchments [ABSTRACT FROM AUTHOR]

Drainage basins are useful recorders of tectonics and climate. The Datong River, which drains the Qilian Mountains, China, is one of the most elongated river basins of the world. How this unusual basin configuration was established remains unknown, despite this area being ideal for assessing the controlling effects of tectonics and climate on the drainage evolution. In this study, we reconstruct the history of the drainage evolution of the Datong River, using integrated provenance methods (U Pb dating, heavy mineral and gravel compositions) and river terrace dating. The drainage system of the modern Datong River began to form at ~1100 ka, and the drainage basin was limited to the area of the lower modern Datong River. Since then, it has gradually extended through headward erosion. The river channelized into the Menyuan Basin between ~620 ka and ~420 ka, and it captured the upper Datong River between ~130 ka and ~60 ka. We suggest that the rapid uplift of the Qilian Mountains disrupted and longitudinally deflected the transverse rivers over the past 1100 ka. Additionally, intensified precipitation and glacial meltwater enhanced the river incision and captured the middle and upper Datong River during 620–420 ka and 130–60 ka, respectively. This model shows that the direction of drainage pattern evolution is controlled by tectonics and that the timing of drainage reorganization is controlled by climate change in the active fold-thrust belt. • Reconstructed the Quaternary evolutionary history of the Datong River using provenance methods, and river terraces dating. • The Datong River channelized into the Menyuan Basin between ~620 ka and ~420 ka, and finally formed after 60 ka. • Coupled the regional of tectonics and climate on the evolution of drainage of the Datong River. [ABSTRACT FROM AUTHOR]

This paper documents ranges of streambed and bank erosion observed across suburban northern Kentucky via time-series surveys at 61 stream monitoring sites from the last ~10 yr. Average erosion rates in streams draining undeveloped watersheds (<5% total impervious area, TIA) were nominal: 0.5 cm/yr of incision (range of −5.8 to 11 cm/yr) and 1.0 cm/yr of widening (range of −58 to 20 cm/yr). By contrast, streams draining developed watersheds (>5% TIA) averaged 1.5 cm/yr of incision (range of −9.2 to 36 cm/yr) and 9.4 cm/yr of widening (range of −11 to 61 cm/yr). The suburban streams also followed predictable patterns of evolution consistent with the "classic" Channel Evolution Model (CEM) of Schumm et al. (1984) , with the initial incision period (Stage 2) coinciding with bed coarsening followed by widening (Stage 3). Out of 45 sites draining >5% TIA, only four were in a state of dynamic equilibrium (Stage 1), two of which were attributable to stabilization via stream restoration and another was attributable to an upstream stormwater retrofit that substantially restricted erosive discharges. The other stable suburban site drained a watershed that was only 6.5% TIA and was just upstream of a reach undergoing incision and headcutting, implying that it would likely be experiencing incision (Stage 2) soon. Although the suburban sites spanned a gradient of development age and extent, >90% fell into unstable CEM categories (Stages 2 through 4), with only one site exhibiting features emblematic of potential geomorphic recovery (Stage 4 trending to Stage 5), attributable to an upstream stormwater retrofit. This case study underscores the predictable nature of long-term channel instability in gravel/cobble streams draining conventionally-developed suburban watersheds in support of more mechanistically based stormwater management strategies tailored to prevent the geomorphic degradation commonly associated with the urban stream syndrome. [ABSTRACT FROM AUTHOR]

In large watersheds, sediment storage imposes geologic timescales on the downstream movement of suspended sediment and associated contaminants. Modeling these processes requires temporal and spatial averaging, a task often accomplished using concepts derived from reservoir theory, where residence times and age and storage time distributions of stored sediment are treated as constants, even though all of these quantities should be time-variable under conditions of unsteady sediment transport. Here, concepts from reservoir theory are replaced by deposition and erosion laws to develop a parsimonious quantitative framework for watershed-scale routing of sediment and contaminants that explicitly accounts for time-dependent alluvial storage. Solution of the new equations, combined with appropriate initial and boundary conditions, predicts the age and storage time distributions of stored sediment, the sediment residence time, the mass of stored sediment, and the variations in all of these quantities through time. Model calibration is illustrated by reinterpreting previously published data from the Little Missouri River in North Dakota to create a 3-dimensional chronostratigraphic model of stored sediment, supplemented by observations of contemporary patterns of erosion. Model parameters from the Little Missouri River are then used to demonstrate that approximately 750 yr will be required to remove arsenic-contaminated alluvium supplied by a century of mine tailings discharges to a 112-km reach of the Belle Fourche River in South Dakota. The new approach described here requires very modest computing resources to simulate long timescales across large watersheds, and the fidelity of storage parameters, which ultimately control the timing of sediment delivery, is supported by site-specific stratigraphic observations and sediment dating. Unlabelled Image • New erosion and deposition laws quantify transfers into and out of alluvial storage. • The new framework predicts sediment age and transit time distributions. • Equations are parameterized using 3-d stratigraphic and age data. • Arsenic will persist in floodplains of the Belle Fourche R., S.D., USA, for 750 years. [ABSTRACT FROM AUTHOR]

The lower 800 km of the Missouri River has been incising for decades. However, the 2011 flood scoured more sediment than the previous 12 yr combined. The river rebounded in the following two years, re-depositing over 70% of the sediment scoured during the 2011 flood. Historic evidence suggests that past Missouri River floods may have followed similar scour-rebound patterns. This paper analyzes 16 channel surveys from 1987 to 2014, including 50,723 total cross sections to document the morphological history of the lower 800 km of the Missouri River over the last thirty years, including flood disturbance and long-term trends. The last thirty years on the Missouri River included two basic morphologic regimes: steady incision during low-to-moderate flow periods and rapid scour-rebound responses to floods. Surveys collected during the 2011 flood demonstrate that the river scoured throughout the flood (including the falling limb) and did not begin a rebound phase until after the flood. The analysis also identifies sediment sinks associated with each of these scour regimes (flood scour and incision between floods) that exceed the total volume eroded from the bed during those periods. Results suggest that floodplain deposition may induce the supply limitation that drives flood scour on this large sand-bed river. Additionally, aggregate mining removes substantially more sediment than the total river incision between floods and the reaches with maximum scour during the 1993 and 2011 floods correspond to the zones of aggregate mining. [ABSTRACT FROM AUTHOR]

A large number of landslides have occurred in the upstream reaches of the Jinsha River, Tibetan Plateau due to the intensity of tectonic movement in the area. Remote sensing and field investigation indicate that one of them, the Samaoding paleolandslide, previously blocked the river. Various river-blocking phenomena are well preserved, including the old landslide dam and deposits, fluvial sediments, and hydrostatic sandy sediment. To better understand the evolution of the Samaoding landslide, the authors carried out thermoluminescence (TL) dating and numerical simulations. The TL analysis shows that the landslide occurred at 10.6 ± 0.5 Ka BP. Discrete element method (DEM) simulation of the landslide based on landform restoration provided results that are consistent with field observations. The simulation indicates that the entire landslide process lasted for 80 s, and the sliding mass reached a maximum velocity of 64 m/s. The landslide formed a landslide dam with a length of 1900 m, a width of 600 m, and a depth of 200 m. The simulation results show that the level of the riverbed at that the time of the landslide was at least 25 m higher than it is today. On the combined basis of the simulation results and field observations, the authors propose explanation that the following valley evolution sequence occurred after river blocking. The landslide dam experienced flood overtopping and then was eroded until it had mostly had been transported away by river flow, and the river then rapidly incised the bedrock to form the present-day landform. Based on the field investigations, the authors summarize the failure mechanism of steep-inclined antidip rockslides and found that tectonics play an important role in the formation of landslide dams (or trigger of landslides) and the failure of landslide dams in an active tectonic environment of Tibetan Plateau. • The Samaoding paleolandslide occurred at 10.6 ± 0.5 Ka BP according to TL analysis. • Detailed run out process of the Samaoding paleolandslide was well reproduced by DEM simulation. • The relative incision rate of the study area is at least 250 cm/ka in the past 10,000 years. • The evolution of the Samaoding paleolandslide is proposed. • Failure mechanism of steep-inclined antidip rockslides is summarized. [ABSTRACT FROM AUTHOR]

The morphodynamics of the Lower Yellow River (LYR) has been impacted by intense dam-induced changes over the past few decades. Here we present results on the morphological impacts produced on the LYR by the construction and operation of Xiaolangdi Dam, whose construction was completed in 1999. We used multi-temporal remotely sensed images and geomorphological parameters, and developed a systematic and universal river morphology extraction framework to investigate changes in the planform of the LYR between 1987 and 2017. The morphological changes were assessed by identifying seven sub-reaches (R1 to R7) along ∼860 km of the LYR. The multichannel upper reaches of the LYR exhibited more marked changes in anabranching and braid-channel ratio, sinuosity, and lateral migration rate than the lower reaches, which are characterized by a single channel pattern. Sandbars in the upper reaches (R1–R3) underwent degradation after the beginning of operation of Xiaolangdi Dam, while the reverse was seen in the lower reaches (R6–R7). The area of overlap between the 1987 channel mask and the channel mask at different times throughout the study decreased gradually over the study period, indicating intense channel migration. The operation of Xiaolangdi Dam prevented the river from running dry but also from suffering floods, while extending the duration of the low/medium stage flows. These actions account for the morphological changes in the LYR. Despite the fact that effective discharge did not change significantly after the Xiaolangdi Dam began operating, it became compressed into a shorter range of annual discharge variability (i.e., larger low-level discharges and shorter rising-smaller floods stages). Our integrated assessment shows a significant amount of water flow energy has been consumed by multiple planform adjustments in the upper reaches of LYR, while local siltation is the main issue in the sinuous but laterally stable lower reaches. [ABSTRACT FROM AUTHOR]

• We present an analysis of chute cutoff along a non-vegetated, ephemeral river. • Twenty-two cutoffs are evident, with chute cutoff the dominant mechanism. • Chute cutoffs occur despite low stream powers (<10 W/m2) and cohesive sediments. • Episodic La Niña-driven flood events drive clusters of chute cutoffs. • Findings contribute to improved understanding of dryland river meander dynamics. The origin and development of meandering river planforms has long been a focus of research in the geosciences. Most attention has focused on perennial meandering rivers with well developed riparian vegetation assemblages, and while increasing interest is turning towards the dynamics and sedimentology of ephemeral meandering rivers with sparse to no vegetation, additional field data are required. As a contribution, this study presents a remote-sensing and field-based analysis of chute cutoff-driven abandonment and sedimentation of meander bends along the fine-grained, non-vegetated, ephemeral Río Colorado on the Bolivia Altiplano. Along the 25 km long sinuous study reach, located between 25 and 50 km upstream of the present-day margins of Salar de Uyuni, quasi-regular flood events (typically at least one per year) drive bend cutoffs and wider channel-floodplain dynamics, despite low specific stream power (<10 W/m2) and cohesive (dominantly silt and clay) bed and bank sediments. Along the reach, twenty-two cutoffs are evident, with chute cutoff the dominant mechanism. Three chute cutoffs (CC1, CC2, CC3) occurred between 1996 and 2016; for one bend (CC3), high-resolution (<0.65 m) satellite imagery and field investigations reveal the details of pre-, mid- and post-chute cutoff processes and sedimentary products. Together, the findings suggest that for any given bend, an increase in bend amplitude (mean ratio of meander bend length to chute channel length ∼4) combines with a typically high diversion angle between the channel-belt axis and the upstream limb of the meander bend (mean ∼98°) to result in declining flow efficiencies. This enhances overbank flooding and promotes deposition along the upstream limb of the bend. Overbank flooding promotes the development of chute channels, which commonly initiate as shallow (typically <1 m) headward eroding channels, while sediment derived from overbank flow and headward erosion tends to be deposited in the downstream limb of the bend. By obstructing flow, such deposition reduces sediment transport capacity within the meander bend as a whole, thereby further inducing deposition within the upstream limb. During subsequent floods, deepening and widening of a dominant chute channel and sedimentation and shallowing of the meander bend continues. Along the reach, episodic La Niña-driven flood events drive phases of more rapid bend migration and clusters of chute cutoffs. Our findings contribute to a more comprehensive understanding of meandering river dynamics across the full range of Earth's conditions, and also may help to improve interpretations of Earth's pre-vegetation rivers and meandering fluvial forms on other planetary bodies. [ABSTRACT FROM AUTHOR]

• We provide definite evidence for prehistoric great Yellow River floods. • Regional-scale Yellow River floods may be ascribed to prolonged weak ENSO conditions. • Climate dominated the physical landscape evolution in China's Central Plains. Understanding the societal impacts of past climate changes may deepen our insight into human adaptation and resilience against potential climate changes in the future. However, the degree and nature of climate-civilization link are still a matter of debate. Fluvial deposits preserved within a cultural stratigraphical context at archaeological sites in the lower Yellow River area may offer an exceptional framework for evaluating the role of climate changes in the rise and fall of ancient civilizations. Here we show evidence for great lower Yellow River floods during the Neolithic-Bronze Age transition about 4000–3500 years ago. Comparing with existing terrestrial and marine records suggests that they were a downstream manifestation of flooding in the middle reaches of the Yellow River, most likely triggered by excessive summer precipitation while a weak El Niño condition prevailed. The floods substantially modified the preexisting landscape and altered the agricultural conditions, thereby driving people dwelling on the higher mounds to move upward. Our results provide a past analogue of the linked climatic, environmental, and societal changes at a time when large and rapid climate changes occurred. [ABSTRACT FROM AUTHOR]

• OSL/CRN dated fluvial terraces described from the Qingtong Gorge (NE Tibetan Plateau). • Fill, erosion and strath terraces are found in the terrace sequence. • The age of the topmost Yellow River terrace in Qingtong Gorge is restricted to ∼ 0.6 Ma. • Incision rate over past 350 ka was ∼200 m/Ma and increased significantly in the past ∼60 ka. • The Yellow River can be divided into two parts as the formation of the Qingtong Gorge. The formation and abandonment of fluvial terraces provide key records of past climatic changes and regional tectonics that reflect the evolution of rivers. At the northeastern periphery of the Tibetan Plateau, the Yellow River flows through the Qingtong Gorge, a deep canyon incised through the Niushou Shan. A flight of 8 fluvial terraces preserved along the canyon, four of which (T2, T3, T6 and T8) were dated using OSL and cosmogenic-nuclide exposure dating. The results show that the four terraces were formed at 51.6 ± 2.9 ka, 60.7(+14.4/-8.3) ka, 346.1(+30/-27.3) ka, and 580–640 ka, respectively. Precise measurement of terrace heights suggests that the average incision rate of the Yellow River over the past 350 ka was ∼200 m/Ma and increased significantly in the past ∼60 ka. Analysis of the relationship between the observed incision histories with regional climatic records and local tectonics, suggests that climatic variations since the late Quaternary drove cutting and abandonment of the terraces during progressive tectonic growth of the Niushou Shan. Our results provide constraints on the timing of incision and linkage of various segments of the modern Yellow River fluvial network. [ABSTRACT FROM AUTHOR]

• Larger-sized wood pieces are most likely to perform sediment-related functions. • Wood-keyed steps trap more sediment than clast- or root-keyed steps. • Pool spacing is related to LW frequency by a negative exponential function. • Large and small headwater channels differ in step formation and sediment storage. Wood influences channel morphology in headwater streams by creating steps and pools in the longitudinal profile and trapping sediment. We used field data from 32 headwater stream reaches in the northwestern Cascade Mountains to test the hypotheses that 1) larger diameter classes of large wood (LW, defined as pieces with diameter > 10 cm and length > 0.5 m) are critical step-keying materials in headwater channels despite narrow channel widths (1–4 m), 2) wood-keyed steps trap more sediment than clast- and root-keyed steps, and 3) the negative relationship between LW frequency and the distance between pools observed elsewhere in large streams extends to headwater streams. We found that the frequency of step key pieces peaked in the 20–40 cm diameter class. Similarly, 40–100 cm diameter pieces were disproportionately associated with key-piece function compared to their overall association with step formation (wood with diameter < 10 cm was an important step-keying material in channels of width < 2 m). Steps keyed by wood were significantly more likely to store sediment than clast- or root-keyed steps. In contrast to previous work that did not detect a relationship between wood loading and pool habitat in step-pool channels, pool spacing ranged between ∼17 and 1.5 channel widths with an apparent relationship described by a negative exponential function of LW frequency (RMSE = 2.85 channel widths/pool), although the range of LW frequency and the functional relationship were different than those of lower gradient channels. Additionally, linear and nonparametric models demonstrate that sediment storage and step and pool characteristics are related to wood loading and stand history with important distinctions based on channel width. These results confirm the importance of maintaining riparian buffers sufficient to provide functional wood to headwater streams. [ABSTRACT FROM AUTHOR]

The upper Colorado River basin drains the western slope of the Rocky Mountains province of North America and hosts a rich record of fluvial terraces and volcanic deposits that provide an archive of bedrock river incision since the Late Miocene. Here we present new geochronology from fluvial deposits and associated volcanic units using multiple methods (luminescence, cosmogenic-burial, 40Ar/39Ar basalt and detrital sanidine, and U/Pb detrital zircon) to reconstruct a detailed record of bedrock incision in the upper Colorado River over timescales ranging from Miocene to Late Pleistocene. Late Miocene (ca. 11–8 Ma) basalt flows perched 800–1700 m above the Colorado River, and the oldest known ancestral Colorado River deposits preserved beneath basalt flows on Grand Mesa, provide the basis for average, long-term (∼107 yr) incision rates. Rates range from ∼110–160 m/Ma between Grand Junction (downstream) and Glenwood Canyon (upstream), with rates decreasing upvalley. Intermediate-scale (∼105–106 yr) incision rates are ∼200–320 m/m.y. over the past 1–3 Ma, and these rates are faster than the average over the past 8–11 Ma. Short-term (<100 ka) bedrock incision rates are highly variable, and range from ∼250–725 m/m.y. The large variation in rates is interpreted to reflect the inherently unsteady nature of climate-driven incision when measured over time intervals (103–104 yr) that are shorter than the duration of ∼100 ky climate cycles. Finally, episodes of anomalously rapid (∼2000–3700 m/m.y.) short-term bedrock incision appear to be the consequence of stream piracy and drainage reorganization and are confined to specific reaches of the system. Collectively, these data provide a substantially refined picture of the history of incision through space and time that is consistent with headward erosion of the Colorado River into the uplifting flanks of the Rocky Mountains since at least the Late Miocene. We argue that the long-term pace of incision is not completely accounted for by isostatic rebound in response to exhumation, and that ongoing differential uplift driven by mantle buoyancy sets the background rates of fluvial incision (100–150 m/m.y.). Pulses of more rapid incision are a consequence of non-steady incision related to climatically-driven changes in erosion efficiency and/or drainage reorganization. • The ancestral upper Colorado River existed by ca. 11 Ma. • Basalt flows support ∼800–1700 m of Neogene fluvial bedrock incision. • Incision was paced by mantle-driven uplift (∼0.5–1.0 km). • The Colorado River eroded headward into the uplifting Rockies. • Climatic episodes and stream-piracy events are superimposed on mantle-driven uplift. [ABSTRACT FROM AUTHOR]