Terrace Surfaces Research Articles

Latest Quaternary rapid river incision across an inactive fold in the northern Chinese Tian Shan foreland

This work focuses on the incision process over the Tuostai anticline, a fold of the proximal structure Belt I in the northern Chinese Tian Shan foreland, where the Sikeshu River has incised deeply into the alluvial gravels and the fold's underlying bedrock strata. Field investigation and geomorphic mapping define five terraces of the Sikeshu River (designated as T1 to T5 from oldest to youngest) preserved within the Tuostai anticline. 10Be surface exposure dating and optically stimulated luminescence dating constrain stabilization of the highest three terrace surfaces at about 80 ka (T1), 16 ka (T2), and 15 ka (T3), respectively. Around 16 ka, the calculated river incision rates significantly increase from <2 mm/yr to >6 mm/yr. Undeformed longitudinal profiles of terraces T2, T3 and T4 over the Tuostai anticline suggest that this structure may have been tectonically inactive since stabilization of these three terraces. We thus think that the observed rapid river incision over the Tuostai anticline has not been largely forced by tectonic uplift. Instead, the progressively warmer and wetter palaeoclimatic condition within the Tian Shan range and its surrounding area during the period of ∼20–10 ka may have enhanced river incision across the Tuostai anticline. A reduced sediment/water ratio might have lowered the gradient of the Sikeshu River.

Quaternary marine terraces and fault activity in the northern mainland sectors of the Messina Strait (southern Italy)

The Strait of Messina area has been affected by strong uplift, which caused the development of spectacular sequences of Pleistocene coastal marine terraces. A new detailed mapping of the terraced surfaces has been carried out on both sides of the northern sector of the Strait. In the Calabrian side, a complete sequence of ten fluvial-coastal terraces has been recognized at elevations ranging from 40 to 520 m a.s.l. and dated from 60 to 330 ka. The series is partly displaced by normal faults bordering the structural high of Campo Piale and the estimated uplift rates change in time and space in response to the fault activity. They range from 1.5 mm/yr for the period 330-200 ka, on the Campo Piale high, to 0.8 mm/yr for the period 125-60 ka, on the hanging wall of the Scilla Fault that borders the Campo Piale high to the north. The constant elevation of the I order terrace suggests an uniform uplift rate of 1.4 mm/yr along the Villa San Giovanni coastal area and the termination of the western sector of the Scilla Fault, even though the offshore activity of segments belonging to the same system is not excluded. In the Sicilian side, six orders of terraces have been recognized on the Capo Peloro promontory. Their inner edges range in elevation from 30 m to 170 m a.s.l., the age attribution varies from 60 to 240 ka. The series is tilted of ~10-15° southward due to the activity of the Mortelle Fault, bounding the promontory to north. The elevation of inner edges suggests that the uplift process, characterized by rate of 0.8 mm/yr, has undergone an acceleration during the late Pleistocene, probably related to activity of offshore structures.

Emergence and detailed structure of terraced surfaces produced by oblique-incidence ion sputtering

We study the nanoscale terraced topographies that arise when a solid surface is bombarded with a broad ion beam that has a relatively high angle of incidence θ. We find that the surface is not completely flat between the regions in which the surface slope changes rapidly with position: Instead, small-amplitude ripples propagate along the surface. Our analytical work on these ripples yields their propagation velocity as well as the scaling behavior of their amplitude. Our simulations establish that the surfaces exhibit interrupted coarsening, i.e., the characteristic width and height of the surface disturbance grow for a time but ultimately asymptote to finite values as the fully terraced state develops. In addition, as θ is reduced, the surface can undergo a transition from a terraced morphology that changes little with time as it propagates over the surface to an unterraced state that appears to exhibit spatiotemporal chaos. For different ranges of the parameters, our equation of motion produces unterraced topographies that are remarkably similar to those seen in various experiments, including pyramidal structures that are elongated along the projected beam direction and isolated lenticular depressions.

Lifetime and migration length of B-related admolecules on diamond {1 0 0}-surface: Comparative study of hot-filament and microwave plasma-enhanced chemical vapor deposition

The growth of heavily B-doped low-resistivity diamond films will facilitate the development of novel semiconductor applications. To discuss the key factors that increase B solubility into single-crystal {100} diamond, the misorientation-angle (θmis) dependences of B incorporation were compared between hot filament (HF) and microwave plasma (MW)-enhanced chemical vapor deposition. Based on the model that considers the step-flow motion, the lifetime of B-related admolecules (τ) on terrace surface was evaluated. We found that τ can be extended more than ∼13 times by utilizing HF growth. As a result, the longer migration length of B-related admolecules (χB) was evidenced. Conversely, shorter τ and χB were revealed for MW growth which limit the B incorporation (probably due to etching). This study will provide an important insight to increase the B solubility.

Highly conductive Ge-doped GaN epitaxial layers prepared by pulsed sputtering

Highly conductive Ge-doped GaN epitaxial layers were grown by low-temperature pulsed sputtering, and their fundamental structural and electrical properties were investigated. The room-temperature (RT) electron concentration was increased to 5.1 × 1020 cm−3 by the Ge doping, and the atomically flat stepped and terraced surface and the crystalline quality of the layers were maintained. Consequently, the RT resistivity was reduced to 0.20 mΩ·cm, which is comparable to that for typical transparent conductive oxides such as indium tin oxide. The contact resistance of Ge-doped GaN with a Ti/Al/Ti/Au metal stack prepared without annealing was as low as 0.087 Ω·mm. Furthermore, the selective formation of a Ge-doped region using an SiO2 mask was demonstrated. The results clearly indicate the strong potential of pulsed sputtering Ge-doped GaN growth for forming low-parasitic-resistance contact layers of various electrical and optical devices.

Growth of AlGaN under the conditions of significant gallium evaporation: Phase separation and enhanced lateral growth

The growth kinetics of AlGaN in NH3 MBE under significant Ga desorption was studied. It was found that the addition of gallium stimulates 2D growth and provides better morphology of films compared to pure AlN. The effect was experimentally observed at up to 98% desorption of the impinging gallium. We found that under the conditions of significant thermal desorption, larger amounts of gallium were retained at lateral boundaries of 3D surface features than at flat terraces because of the higher binding energy of Ga atoms at specific surface defects. The selective accumulation of gallium resulted in an increase in the lateral growth component through the formation of the Ga-enriched AlGaN phase at boundaries of 3D surface features. We studied the temperature dependence of AlGaN growth rate and developed a kinetic model analytically describing this dependence. As the model was in good agreement with the experimental data, we used it to estimate the increase in the binding energy of Ga atoms at surface defects compared to terrace surface sites using data on the Ga content in different AlGaN phases. We also applied first-principles calculations to the thermodynamic analysis of stable configurations on the AlN surface and then used these surface configurations to compare the binding energy of Ga atoms at terraces and steps. Both first-principles calculations and analytical estimations of the experimental results gave similar values of difference in binding energies; this value is 0.3 eV. Finally, it was studied experimentally whether gallium can act as a surfactant in AlN growth by NH3 MBE at elevated temperatures. Gallium application has allowed us to grow a 300 nm thick AlN film with a RMS surface roughness of 2.2 Å over an area of 10 × 10 μm and a reduced density of screw dislocations.

Refining fault slip rates using multiple displaced terrace risers—An example from the Honey Lake fault, NE California, USA

Faulted terrace risers are semi-planar features commonly used to constrain Quaternary slip rates along strike-slip faults. These landforms are difficult to date directly and therefore their ages are commonly bracketed by age estimates of the adjacent upper and lower terrace surfaces. However, substantial differences in the ages of the upper and lower terrace surfaces (a factor of 2.4 difference observed globally) produce large uncertainties in the slip-rate estimate. In this investigation, we explore how the full range of displacements and bounding ages from multiple faulted terrace risers can be combined to yield a more accurate fault slip rate. We use 0.25-m cell size digital terrain models derived from airborne lidar data to analyze three sites where terrace risers are offset right-laterally by the Honey Lake fault in NE California, USA. We use ages for locally extensive subhorizontal surfaces to bracket the time of riser formation: an upper surface is the bed of abandoned Lake Lahontan having an age of 15.8 ± 0.6 ka and a lower surface is a fluvial terrace abandoned at 4.7 ± 0.1 ka. We estimate lateral offsets of the risers ranging between 6.6 and 28.3 m (median values), a greater than fourfold difference in values. The amount of offset corresponds to the riser's position relative to modern stream meanders: the smallest offset is in a meander cutbank position, whereas the larger offsets are in straight channel or meander point-bar positions. Taken in isolation, the individual terrace-riser offsets yield slip rates ranging from 0.3 to 7.1 mm/a. However, when the offset values are collectively assessed in a probabilistic framework, we find that a uniform (linear) slip rate of 1.6 mm/a (1.4–1.9 mm/a at 95% confidence) can satisfy the data, within their respective uncertainties. This investigation demonstrates that integrating observations of multiple offset elements (crest, midpoint, and base) from numerous faulted and dated terrace risers at closely spaced sites can refine slip-rate estimates on strike-slip faults.

Environmental conditions as a key factor in the functioning of wells at a settlement from the Roman period of the Iron Age

The article deals with geological and geomorphological conditions of the construction and functioning of wells from the Roman period of the Iron Age in a settlement on the Amber Road. This settlement is located in Central Poland (in the vicinity of Kwiatków village), on a flat surface of the lower terrace of the Warta River valley, among the dry channels of numerous streams cutting the terrace, which developed in three generations, from the Younger Dryas onwards. In the Holocene, the area was characterised by an abundance of flowing water in a multichannel system and the stability of the groundwater table. Among the archaeological features discovered there, the most interesting is a large complex of well-preserved wells dated to the early Roman period. The structures of the wells were based or dug in an organic series which constituted the terrace profile at a depth of between 2.0 and 2.5 m, dated to the late Alleröd and the Younger Dryas. In the study, the authors focus on the purpose of locating a large number of wells within a relatively small area abundant with numerous streams. The authors used a range of methods, including lithological and textural analyses, a geochronological analysis and a geochemical examination to describe the properties of the deposits and the quality of the waters. The characteristics of the wells were provided from dendrochronological studies. The archaeological approach included an analysis of the features morphology and the stage of technological advancement, as well as the diversity of the wells’ casings. The spatial and functional relationships of the locations of the wells were also given special attention.

Crossover from compact to branched films in electrodeposition with surface diffusion.

We study a model for thin film electrodeposition in which instability development by preferential adsorption and reduction of cations at surface peaks competes with surface relaxation by diffusion of the adsorbates. The model considers cations moving in a supported electrolyte, adsorption and reduction when they reach the film surface, and consequent production of mobile particles that execute activated surface diffusion, which is represented by a sequence of random hops to neighboring lattice sites with a maximum of G hop attempts (G≫1), a detachment probability ε<1 per neighboring particle, and a no-desorption condition. Computer simulations show the formation of a compact wetting layer followed by the growth of branched deposits. The maximal thickness z_{c} of that layer increases with G but is weakly affected by ε. A scaling approach describes the crossover from smooth film growth to unstable growth and predicts z_{c}∼G^{γ}, with γ=1/[2(1-ν)]≈0.43, where ν≈0.30 is the inverse of the dynamical exponent of the Villain-Lai-Das Sarma equation that describes the initial roughening. Using previous results for related deposition models, the thickness z_{c} can be predicted as a function of an activation energy for terrace surface diffusion and the temperature, and the small effects of the parameter ε are justified. These predictions are confirmed by the numerical results with good accuracy. We discuss possible applications, with a particular focus on the growth of multifuncional structures with stacking layers of different porosity.

Late Holocene cliff-top dune evolution in the Hengchun Peninsula of Taiwan: Implications for palaeoenvironmental reconstruction

The Fung-Chuei-Sha (FCS) cliff-top dunes, the only coastal cliff-top dunes known in Taiwan, are situated on a 70m high, tectonically uplifted Quaternary marine terrace surface in the Hengchun Peninsula of southern Taiwan. The development of the FCS dune and its relationship with palaeoenvironmental changes are still unknown. The optically stimulated luminescence (OSL) dating approach utilizing a pIRIR150 (post infrared, infrared stimulated luminescence at 150°C) single aliquot regenerative (SAR) dose protocol for K-rich feldspar, in combination with radiocarbon dates and high resolution sedimentological analyzes, have proven to be a powerful tool for reconstructing the depositional history of the FCS dune and the related palaeoenvironmental conditions during the late Holocene. This study identified three major aeolian sediment accumulation periods at ∼4ka to ∼2.3ka, ∼1.8ka to ∼1.1ka, and ∼0.7ka to ∼0.4ka, which are consistent with the cold/cooling periods identified from the palaeoclimate record of the Dongyuan Lake in southern Taiwan. Two palaeosurfaces and two short interruptions of sand accumulation indicate periods of surface stabilization at the FCS site. The OSL ages and radiocarbon ages from the dune, and the palaeoclimate data from Dongyuan Lake suggest that these surfaces developed at ∼4.3ka, ∼2ka, ∼1ka and ∼0.3ka. The palaeoclimatic conditions inferred from the FCS dunes are largely consistent with those reconstructed for the coastal dunes of the Fulong Beach area in northeastern Taiwan. This may indicate an over-regional pattern of dune formation primarily driven by strong winds of the Asian Winter Monsoon as the primary forcing factor for coastal dune development in Taiwan during the late Holocene.

Linear and non-linear responses of vegetation and soils to glacial-interglacial climate change in a Mediterranean refuge

The impact of past global climate change on local terrestrial ecosystems and their vegetation and soil organic matter (OM) pools is often non-linear and poorly constrained. To address this, we investigated the response of a temperate habitat influenced by global climate change in a key glacial refuge, Lake Ohrid (Albania, Macedonia). We applied independent geochemical and palynological proxies to a sedimentary archive from the lake over the penultimate glacial-interglacial transition (MIS 6–5) and the following interglacial (MIS 5e-c), targeting lake surface temperature as an indicator of regional climatic development and the supply of pollen and biomarkers from the vegetation and soil OM pools to determine local habitat response. Climate fluctuations strongly influenced the ecosystem, however, lake level controls the extent of terrace surfaces between the shoreline and mountain slopes and hence local vegetation, soil development and OM export to the lake sediments. There were two phases of transgressional soil erosion from terrace surfaces during lake-level rise in the MIS 6–5 transition that led to habitat loss for the locally dominant pine vegetation as the terraces drowned. Our observations confirm that catchment morphology plays a key role in providing refuges with low groundwater depth and stable soils during variable climate.

Geomorphometric delineation of floodplains and terraces from objectively defined topographic thresholds

Abstract. Floodplain and terrace features can provide information about current and past fluvial processes, including channel response to varying discharge and sediment flux, sediment storage, and the climatic or tectonic history of a catchment. Previous methods of identifying floodplain and terraces from digital elevation models (DEMs) tend to be semi-automated, requiring the input of independent datasets or manual editing by the user. In this study we present a new method of identifying floodplain and terrace features based on two thresholds: local gradient, and elevation compared to the nearest channel. These thresholds are calculated statistically from the DEM using quantile–quantile plots and do not need to be set manually for each landscape in question. We test our method against field-mapped floodplain initiation points, published flood hazard maps, and digitised terrace surfaces from seven field sites from the US and one field site from the UK. For each site, we use high-resolution DEMs derived from light detection and ranging (lidar) where available, as well as coarser resolution national datasets to test the sensitivity of our method to grid resolution. We find that our method is successful in extracting floodplain and terrace features compared to the field-mapped data from the range of landscapes and grid resolutions tested. The method is most accurate in areas where there is a contrast in slope and elevation between the feature of interest and the surrounding landscape, such as confined valley settings. Our method provides a new tool for rapidly and objectively identifying floodplain and terrace features on a landscape scale, with applications including flood risk mapping, reconstruction of landscape evolution, and quantification of sediment storage and routing.

Eustatic and regional tectonic controls on late Pleistocene paleovalley morphology in the central Kanto Plain, Japan

This study analyzed the morphology of three late Pleistocene buried paleovalleys in the central Kanto Plain, Japan, by using data from sedimentary cores and boreholes. The paleo-Arakawa valley and the paleo-Nakagawa valley merge to form the paleo-Tokyo valley, which extends into Tokyo Bay. Although the last glacial sea-level fall controlled the formation of these paleovalleys, fluvial terraces developed differently in these paleovalleys, reflecting regional differences in tectonic activity. The paleo-Tokyo valley beneath the Tokyo Lowland, and the paleo-Arakawa valley beneath the Arakawa Lowland, are in relatively stable or uplifting regions, and both paleovalleys have several distinct fluvial terraces forming stepped cross-sectional profiles. In contrast, in the paleo-Nakagawa and the paleo-Arakawa valleys beneath the subsiding Menuma Lowland, terrace surfaces are obscure and there is a smooth gradient from the valley margins to the valley axis. The structure of the paleovalleys in the central Kanto Plain can thus be explained by the combination of eustatic sea-level fall and concurrent regional tectonic movement.

Dating of river terraces along Lefthand Creek, western High Plains, Colorado, reveals punctuated incision

The response of erosional landscapes to Quaternary climate oscillations is recorded in fluvial terraces whose quantitative interpretation requires numerical ages. We investigate gravel-capped strath terraces along the western edge of Colorado's High Plains to constrain the incision history of this shale-dominated landscape. We use 10Be and 26Al cosmogenic radionuclides (CRNs), optically stimulated luminescence (OSL), and thermally transferred OSL (TT-OSL) to date three strath terraces, all beveled in shale bedrock and then deposited upon by Lefthand Creek, which drains the crystalline core of the Front Range. Our study reveals: (i) a long history (hundreds of thousands of years) of fluvial occupation of the second highest terrace, T2 (Table Mountain), with fluvial abandonment at 92±3ka; (ii) a brief occupation of a narrow and spatially confined terrace, T3, at 98±7ka; and (iii) a 10–25 thousand year period of cutting and fluvial occupation of a lower terrace, T4, marked by the deposition of a lower alluvial unit between 59 and 68ka, followed by deposition of an upper alluvial package at 40±3ka. In conjunction with other recent CRN studies of strath terraces along the Colorado Front Range (Riihimaki et al., 2006; Dühnforth et al., 2012), our data reveal that long periods of lateral planation and fluvial occupation of strath terraces, sometimes lasting several glacial-interglacial cycles, are punctuated by brief episodes of rapid vertical bedrock incision. These data call into question what a singular terrace age represents, as the strath may be cut at one time (its cutting-age) and the terrace surface may be abandoned at a much later time (its abandonment age), and challenge models of strath terraces that appeal to simple pacing by the glacial-interglacial cycles.

Extraction of Terraces on the Loess Plateau from High-Resolution DEMs and Imagery Utilizing Object-Based Image Analysis

Abstract: Terraces are typical artificial landforms on the Loess Plateau, with ecological functions in water and soil conservation, agricultural production, and biodiversity. Recording the spatial distribution of terraces is the basis of monitoring their extent and understanding their ecological effects. The current terrace extraction method mainly relies on high-resolution imagery, but its accuracy is limited due to vegetation coverage distorting the features of terraces in imagery. High-resolution topographic data reflecting the morphology of true terrace surfaces are needed. Terraces extraction on the Loess Plateau is challenging because of the complex terrain and diverse vegetation after the implementation of “vegetation recovery”. This study presents an automatic method of extracting terraces based on 1 m resolution digital elevation models (DEMs) and 0.3 m resolution Worldview-3 imagery as auxiliary information used for object-based image analysis (OBIA). A multi-resolution segmentation method was used where slope, positive and negative terrain index (PN), accumulative curvature slope (AC), and slope of slope (SOS) were determined as input layers for image segmentation by correlation analysis and Sheffield entropy method. The main classification features based on DEMs were chosen from the terrain features derived from terrain factors and texture features by gray-level co-occurrence matrix (GLCM) analysis; subsequently, these features were determined by the importance analysis on classification and regression tree (CART) analysis. Extraction rules based on DEMs were generated from the classification features with a total classification accuracy of 89.96%. The red band and near-infrared band of images were used to exclude construction land, which is easily confused with small-size terraces. As a result, the total classification accuracy was increased to 94%. The proposed method ensures comprehensive consideration of terrain, texture, shape, and spectrum characteristics, demonstrating huge potential in hilly-gully loess region with similarly complex terrain and diverse vegetation covers.

Late Pleistocene slip rate of the northern Qilian Shan frontal thrust, western Hexi Corridor, China

AbstractWe derive a slip rate for a frontal thrust in the western Hexi Corridor along the northern Qilian Shan by combining topographic profiling and 10Be exposure dating. The active Yumen‐Beidahe thrust fault offsets late Pleistocene alluvial‐fan deposits, and a prominent north‐facing scarp is well preserved. To quantify the slip rate, we surveyed the uplifted terraces and sampled quartz‐rich pebbles on terrace surfaces and river channels to determine surface exposure ages and pre‐depositional inheritance. The minimum vertical slip rate of the fault is 0.73 ± 0.09 mm a−1. This represents a horizontal shortening rate of 1.26 ± 0.31 mm a−1 for a fault dip of 30 ± 5°. This estimated slip rate supports the inference made from previous geological and GPS constraints that NNE‐directed shortening across the western Qilian Shan and the Hexi Corridor is distributed on several active faults with a total shortening rate of 4–10 mm a−1.

Modern and buried soils of kurgans in the forest-steppe zone of the Middle Volga region (by the example of Komintern I kurgan)

The construction of the Volga–Kama cascade of water reservoirs and hydroelectric power stations in the middle of the 20th century resulted in the inundation of vast areas and the development of abrasion along the shores that threatens many monuments of the archaeological and cultural heritage. The soils buried under northernmost kurgans of the Lugovskaya culture dating back to the 15th–14th centuries BC (kurgan Komintern I) were studied on the surface of the second terrace of the Kama River near its confluence with the Volga River. Burial sites of kurgan Komintern III were subjected to destruction in 1981, and archaeological excavations of kurgan Komintern II were performed in 2008. The danger of complete disappearance of these archaeological sites necessitated their thorough study with the use of multiple methods, including special paleosol studies. The soils buried under the kurgans ceased to be active components of the soil cover about 3500 years ago. They preserve information on the paleoenvironmental conditions before their burying. The analysis of morphological features and physical, physicochemical, and chemical properties of the buried soils attests to their chernozemic nature. Background surface soils that have passed through the entire cycle of the Holocene pedogenesis have evolved since that time into gray forest soils (Luvic Greyzemic Phaeozems) under forest vegetation. These soils are characterized by the increased acidity of the surface horizons (pHКСl 4.3) and the development of lessivage. Data on the coefficients of mineral weathering in the buried paleosols and background surface soils attest to the identical precipitation both for the surface and buried soils in the Middle and Late Holocene.

Folded fluvial terraces in a young, actively deforming intramontane basin between the Yumu Shan and the Qilian Shan mountains, NE Tibet

Intramontane basins in actively deforming regions contain significant information about the evolution of orogenic belts. We explored the tectonic characteristics and evolution of an intramontane basin between the Qilian Shan and Yumu Shan mountains on the NE Tibetan Plateau. We utilized the deformation of fluvial terraces along the Dahe River to constrain the rate and pattern of Quaternary deformation across the basin. Fluvial landforms include a widespread Mid-Pleistocene alluvial-fluvial fan surface and five terrace levels inset below this surface. We dated deposits associated with these landforms by optically stimulated luminescence (OSL) analyses on eolian loess and fluvial sediments. Our results yield ages of 142.8 ± 11.3 ka (Fs), 103–125 ka (T5), 96–115 ka (T4), 87–104 ka (T3), and 13–15 ka (T2), which we interpret to reflect the abandonment of fluvial terrace surfaces. Elevation surveys indicate that the terrace surfaces are folded along the Dahe anticline and are gently tilted northward across the basin. Analysis of terrace deformation suggests that the Dahe anticline grew by limb rotation and accommodated upper-crustal shortening at a rate of 0.14 +0.14/–0.03 mm/a. We determined the onset of deformation to have occurred between 0.3 and 0.9 Ma, based on the rotation rate of the southern limb of the anticline. This age is significantly younger than the onset ages of the thrust faults along the Qilian Shan (ca. 10 Ma) and the Yumu Shan (ca. 3.7 Ma). The character of the terrace deformation suggests that the Dahe anticline was growing from a south-dipping decollement, which also induced surface tilting across the entire basin. The Late Quaternary activation of the Dahe anticline folding and the detachment slipping provide evidence that the deformation of the intramontane basin occurred much later than that of the surrounding mountain ranges and accommodated part of the crustal shortening through basin narrowing.

Application of the Stream Evolution Model to a Volcanically Disturbed River: The North Fork Toutle River, Washington State, USA

AbstractIn this study, a recently revised version of the channel evolution model, named the Stream Evolution Model (SEM), was applied to the upper North Fork Toutle River disrupted by the deposition of a 2.5‐km3 debris avalanche during the catastrophic eruption of Mount St. Helens in 1980. The results show that, in the first few years following the eruption, upstream channel reaches generally incised, evolving in SEM Stage 4 (i.e. degradation and widening), while downstream reaches aggraded, evolving in Stage 5 (i.e. aggradation and widening). However, starting in the late‐1980s, this simple pattern was disrupted by incision in the downstream reaches, which seemed to propagate upstream. Since the 1990s, lateral channel adjustments have become predominant as rates of vertical adjustment have slowed and river valley top widths relaxed to asymptotic values. Spatial and temporal sequences of channel evolution have tended to follow the sequences of stages expected according to the SEM, although these sequences have been frequently disrupted by renewed incision, secondary cycles of adjustment and the impacts of local geologic, geomorphic and hydraulic conditions. Within the quasi‐full SEM cycles, stages 4 and 5 were sometimes repeated, while stage 6 (quasi‐equilibrium) was sometimes omitted, and stage 8 (anabranching) only occurred in the downstream braided/anabranching reaches. According to the SEM, degradation, widening and lateral activity (stages 4 and 7) are forecast to continue until transverse valley profiles and channel planforms stabilize and floodplain and terrace surfaces are fully colonized by vegetation. Copyright © 2017 John Wiley &amp; Sons, Ltd.

The linking of the upper-middle and lower reaches of the Yellow River as a result of fluvial entrenchment

The upper–middle Yellow River flows through the Fenwei graben, a structure resulting from extensional tectonism that was formed and repeatedly extended during the Cenozoic. The drainage system within this graben was formerly isolated from the lower reaches of the Yellow River system by the Xiaoshan mountains, an actively growing ∼ NW–SE trending range. The modern course of the Yellow River takes it through this range along the Sanmen gorge, the formation of which was of great significance in that it initiated through-going drainage between the upper–middle and lower reaches of the system. The timing of this event, which was clearly a critical point in the evolution of the Yellow River, can be established by dating the terraces in the gorge. Intermittent deepening of this gorge by the Yellow River from a high-level planation surface capping the mountain range has resulted in the formation of five terraces. Magnetostratigraphic records from aeolian deposits accumulated on these surfaces provide a geochronological sequence for this geomorphic archive, in which the ages of the planation surface and of terraces T5, T4, T3, T2, and T1 have been determined as ∼3.63 Ma, ∼1.24 Ma, ∼0.86 Ma, ∼0.62 Ma, ∼129 ka, and ∼12 ka, respectively.Under the constraint of this chronological framework, a model for landscape evolution is proposed here. Uplift of the inner Fenwei graben and of the surrounding mountain ranges led to dissection of the 3.63 Ma old planation surface in conjunction with the formation of the Sanmen gorge. Drainage of the lake previously occupying the basin would have promoted incision into the fluvio-lacustrine graben sediments; indeed, gorge formation through the Xiaoshan may have been initiated or intensified by lake overflow. The ages obtained for the planation surface and uppermost terrace suggest that the formation of the Sanmen gorge and the initiation of the through-going eastward drainage of the Yellow River occurred between 3.63 and 1.24 Ma. Before the start of gorge entrenchment, the products of erosion in the modern upper catchment of the Yellow River were unable to reach the sea. The dramatic increase in deposition rates in the Bohai Gulf (at the mouth of the modern Yellow River in the East China Sea), ∼1.0 Ma ago, thus resulted from the initiation of an integral (enlarged) Yellow River catchment drainage through the Sanmen gorge; it does not imply an increase in erosion rates at that time.