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1.
The middle reaches of the Tsangpo River consist of alternating sections of wide valleys and gorges. The wide valley sections have braided and anastomosing channels, gentle hydraulic gradients, thick alluvial deposits and low terraces. In contrast, the gorge sections exhibit single, straight and deeply entrenched meandering channels with steep hydraulic gradients, bare rock river beds and higher terraces. Several hypotheses have been used to explain these unusual fluvial landforms, but geological, landform and sedimentary analyses along with dating information, suggest that the key could be the active faults across the river valley. All gorge sections are located on the upthrown side of active faults, which mainly occurred in or after the Pliocene, whilst the wide valley sections appear on the downthrown side. The faulting blocked the river and caused the formation of palaeolakes, with thick deposits laid down behind the faults. Therefore, depositional wide valleys were formed and old terraces were buried. On these downthrown sides of the faults, braided and anastomosing channels have developed. On the upthrown sides, strong incision of the river occurred because of the changes of the local base levels and river gradients. As a result, deep gorges and deeply entrenched meandering channels formed in various lithologies. The terraces on the gorge slope indicate different stages of river incision and the related knick points appeared close to the local active faults. Rock resistance is only a minor influence on the alternation of valley forms and river gradients in this area. © 1998 John Wiley & Sons, Ltd.  相似文献   

2.
The main indicators of Quaternary tectonic uplift are the young mountain slopes of the Darjeeling Himalaya, rising straight above the Ganga–Brahmaputra foredeep, fragments of uplifted river terraces and fresh fault scarps. Evidence for the continuation of the uplift includes downcutting of the Tista and other straight rivers in the bedrock, continuing aggradation in the plains and overriding of the metamorphic rocks on the alluvia. Owing to deforestation and extensive land use, the earlier natural tendency of a dominance of channel incision over slope degradation has changed to prevailing aggradation, even in steep valley reaches, caused by intensive slope mass movements and the overloading of the mountain creeks. Aggradation progresses upstream along the rivers dissecting the mountain front.  相似文献   

3.
Mountain rivers respond to strong earthquakes by rapidly aggrading to accommodate excess sediment delivered by co-seismic landslides. Detailed sediment budgets indicate that rivers need several years to decades to recover from seismic disturbances, depending on how recovery is defined. We examine three principal proxies of river recovery after earthquake-induced sediment pulses around Pokhara, Nepal's second largest city. Freshly exhumed cohorts of floodplain trees in growth position indicate rapid and pulsed sedimentation that formed a fan covering 150 km2 in a Lesser Himalayan basin with tens of metres of debris between the 11th and 15th centuries AD. Radiocarbon dates of buried trees are consistent with those of nearby valley deposits linked to major medieval earthquakes, such that we can estimate average rates of re-incision since. We combine high-resolution digital elevation data, geodetic field surveys, aerial photos, and dated tree trunks to reconstruct geomorphic marker surfaces. The volumes of sediment relative to these surfaces require average net sediment yields of up to 4200 t km–2 yr–1 for the 650 years since the last inferred earthquake-triggered sediment pulse. The lithological composition of channel bedload differs from that of local bedrock, confirming that rivers are still mostly evacuating medieval valley fills, locally incising at rates of up to 0.2 m yr–1. Pronounced knickpoints and epigenetic gorges at tributary junctions further illustrate the protracted fluvial response; only the distal portions of the earthquake-derived sediment wedges have been cut to near their base. Our results challenge the notion that mountain rivers recover speedily from earthquakes within years to decades. The valley fills around Pokhara show that even highly erosive Himalayan rivers may need more than several centuries to adjust to catastrophic perturbations. Our results motivate some rethinking of post-seismic hazard appraisals and infrastructural planning in active mountain regions. © 2018 John Wiley & Sons, Ltd.  相似文献   

4.
In this study, we present direct field measurements of modern lateral and vertical bedrock erosion during a 2-year study period, and optically stimulated luminescence (OSL) ages of fluvial material capping a flat bedrock surface at Kings Creek located in northeast Kansas, USA. These data provide insight into rates and mechanisms of bedrock erosion and valley-widening in a heterogeneously layered limestone-shale landscape. Lateral bedrock erosion outpaced vertical incision during our 2-year study period. Modern erosion rates, measured at erosion pins in limestone and shale bedrock reveal that shale erosion rate is a function of wetting and drying cycles, while limestone erosion rate is controlled by discharge and fracture spacing. Variability in fracture spacing amongst field sites controls the size of limestone block collapse into the stream, which either allowed continued lateral erosion following rapid detachment and transport of limestone blocks, or inhibited lateral erosion due to limestone blocks that protected the valley wall from further erosion. The OSL ages of fluvial material sourced from the strath terrace were older than any material previously dated at our study site and indicate that Kings Creek was actively aggrading and incising throughout the late Pleistocene. Coupling field measurements and observations with ages of fluvial terraces can be useful to investigate the timing and processes linked to how bedrock rivers erode laterally over time to form wide bedrock valleys.  相似文献   

5.
Geostatistical topographic analysis is widely recognized as a useful tool for the statistical reconstruction of planar geomorphic markers from relict surfaces. This work is aimed at improving the geostatistical approach used in previous works and developing a method for evaluating the incision rates of rivers in their lower catchments during the Late Quaternary. We chose the major valleys of the Adriatic foothills (central Italy), affected since Late Miocene by a differential tectonic uplift which is still active. In particular, (i) we applied the geostatistical analysis to reconstruct the original top‐surfaces of fluvial‐to‐coastal terrace bodies at the Metauro River and Cesano River mouths; (ii) we performed correlations between the height distribution of the alluvial terrace sequences and the Quaternary climatic curve to estimate the average long‐term fluvial incision rates in the lowermost reaches of the Metauro, Cesano, Misa and Esino Rivers. The obtained averaged incision rates have been interpreted also in the light of the Stream‐Length Gradient Index (SL Index), Steepness Index (Ks), and Concavity Index (θ) as proxies of the stream‐power per unit length. Results confirm that geostatistical and terrain analysis of topographic and geometric arrangements of fluvial and coastal terraces is an effective tool in detecting geomorphic and tectonic factors inducing perturbations on planar geomorphic markers. In particular, we better delineated the surface geometry and boundaries of well‐developed coastal fans at the mouths of the Metauro and Cesano Rivers, already recognized in previous works through sedimentological, morphostratigraphic, and chronological data. Moreover, we found evidence for cut‐and‐fill phases that took place during and immediately after the river aggradation of the late Quaternary glacial periods. Despite the Slope–Area analysis evidenced a widespread influence of the regional differential uplift on single river basin configuration, we observed some space and time variability of averaged incision rates for adjacent valleys, mainly explained by physiographic configuration and dynamics of drainage network. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

6.
Takahiro  Yamamoto 《Island Arc》2005,14(2):199-212
Abstract River incision into bedrock results in the decrease of burial depths, which can be of critical importance, for example, in the safe long‐term storage of high‐level radioactive waste. For the long‐term prediction of river erosion, it is essential to know the rate of incision during the Late Quaternary period. In the Abukuma Mountains on the forearc side of northeast Japan, a low‐relief peneplain that was uplifted in the Pliocene period is widely developed. Degradational fluvial terraces that are discontinuously distributed along draining rivers are scattered in the study area. The fluvial sediments were mainly transported from the summit regions as debris flows or hyperconcentrated flows. The terrace deposits are capped by a series of eolian veneers containing key tephra beds. From the oldest to the youngest, these tephra beds are the 150–125 ka Iizuna–Kamitaru tephra, the 135–125 ka Hiuchigatake–Tagashira tephra, the 120 ka Adatara–Dake tephra, the 70–80 ka Bandai–Hayama‐2 tephra, the 45 ka Numazawa–Mizunuma tephra and the 42 ka Bandai–Hayama‐1 tephra. Using tephrochronological data, the terraces are divided into three groups: higher, middle and lower. The ages of formation of the higher, middle and lower terraces are estimated to be within marine isotope stage (MIS) 6, MIS 5.4–5.2 and MIS 3 to MIS 2, respectively. The incision rate, calculated from the relative height between the terrace surface and present‐day valley floor fill, is 1.5–0.8 m/10 000 years in the elevations from 350 to 700 m. The calculated rate does not show significant differences between the higher, middle and lower terraces. All the relative heights decrease with increasing elevation, because the erosional rates of streams in the upper reaches are lower than those in downstream reaches where the discharge rates are higher. This value can be regarded as an estimate of the rate of incision in granitic mountains where there is no volcanic or distinct tectonic activity.  相似文献   

7.
Weichselian geomorphological and fluvial evolution has been recorded in the valley fill of the Reusel, a small tributary of the lower Maas river. It has been studied by means of borehole sections. A first deep incision into the substratum occurred during the Early Weichselian-Lower Pleniglacial. This was followed by aggradation, at first by a braided river, at a later date by a meandering river, dating respectively from the Lower and Middle Pleniglacial. The Upper Pleniglacial aggradation again points to a braided river system. The end of the Pleniglacial is characterized by aeolian infilling of the valley and decreasing carrying capacity of the river.  相似文献   

8.
Floodplain wetlands are common features of rivers in southern Africa, but they have been little studied from a geological or geomorphological perspective. Study of the upper Klip River, eastern Free State, South Africa, indicates strong geological controls on the formation of alluvial meanders and associated floodplain wetlands. Along this river, pronounced and abrupt changes in valley width are strongly linked to lithological variations. Where weakly cemented sandstone crops out, the Klip has laterally eroded bedrock and carved valleys up to 1500 m wide. In these valleys, the river meanders (sinuosity up to ～1·75) on moderate gradients (<0·001) within extensive floodplains marked by numerous oxbow lakes, backswamps and abandoned channels, many of which host substantial wetlands. In contrast, where highly resistant dolerite crops out, lateral erosion of bedrock is restricted, with the Klip tending instead to erode vertically along joints or fractures. Here, valleys are narrower (<200 m), channel‐bed gradients are steeper (>0·003), the river follows a much straighter course (sinuosity ～1·10–1·34), and floodplains are restricted in width. Long‐term landscape development in the Klip and numerous similar catchments depends on the interaction between fluvial processes in the sandstone and dolerite valleys. In the sandstone valleys, vertical erosion rates are controlled by erosion rates of the more resistant dolerites downstream. Hence, in the short‐ to medium‐term (decades to tens of thousands of years), lateral erosion dominates over vertical erosion, with the river concomitantly planing sandstone in the channel floor and reworking floodplain sediments. The thickness of alluvial fill in the sandstone valleys is limited (<4 m), but the resultant meanders are naturally dynamic, with processes such as point bar deposition, cutoff formation and channel avulsion resulting in an assemblage of fluvial landforms. In the longer term (greater than tens of thousands of years), however, vertical erosion will occur in the sandstone valleys as the downstream dolerites are lowered by erosion, resulting in channel incision, floodplain abandonment, and desiccation of the wetlands. Identification of the geological controls on meander and wetland formation provides information vital for the design of effective management guidelines for these ecologically rich habitats, and also contributes to a better understanding of rivers that are intermediate between fully alluvial and fully bedrock. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

9.
Much research has been devoted to the development of numerical models of river incision. In settings where bedrock channel erosion prevails, numerous studies have used field data to calibrate the widely acknowledged stream power model of incision and to discuss the impact of variables that do not appear explicitly in the model's simplest form. However, most studies have been conducted in areas of active tectonics, displaying a clear geomorphic response to the tectonic signal. Here, we analyze the traces left in the drainage network 0.7 My after the Ardennes region (western Europe) underwent a moderate 100–150 m uplift. We identify a set of knickpoints that have traveled far upstream in the Ourthe catchment, following this tectonic perturbation. Using a misfit function based on time residuals, our best fit of the stream power model parameters yields m = 0.75 and K = 4.63 × 10‐8 m‐0.5y‐1. Linear regression of the model time residuals against quantitative expressions of bedrock resistance to erosion shows that this variable does not correlate significantly with the residuals. By contrast, proxies for position in the drainage system prove to be able to explain 76% of the residual variance. High time residuals correlate with knickpoint position in small tributaries located in the downstream part of the Ourthe catchment, where some threshold was reached very early in the catchment's incision history. Removing the knickpoints stopped at such thresholds from the data set, we calculate an improved m = 0.68 and derive a scaling exponent of channel width against drainage area of 0.32, consistent with the average value compiled by Lague for steady state incising bedrock rivers. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

10.
The northeast margin of the Tibetan Plateau, a particularly important area to understand the mechanism of plateau formation, is characterized by large transpressional arcuate faults. There is debate on the amount of Quaternary sinistral displacement on the major Haiyuan Fault. Previously unrecognized systemic asymmetrical valleys have developed between the Haiyuan and Xiangshan faults. Southeast tilting and sinistral displacement on the northeast side of the Haiyuan Fault resulted in southeast migration of large rivers and asymmetrical widening of their valleys, leaving a systematic distribution of tilted strath terraces along their northwest sides. Where asymmetrical widening created by tilting kept pace with sinistral displacement, rivers have not been deflected, and the increase in valley width downstream from the fault should equate to total lateral displacement since river formation (e.g. Yuan River, a 7 km asymmetrical valley with a c. 2.2 Ma paleomagnetic age). Where river deflection and asymmetrical valley growth are coeval, valley width is less than total horizontal displacement (e.g. Hebao River, a c. 2.1 km asymmetrical valley with c. 2 km deflection). All rivers north of the Haiyuan Fault converge to cut across the Xiangshan Mountains as a gorge. Northeast thrusting of the upthrown side of the Xiangshan Fault has resulted in degradation and related strath terrace formation as the valleys asymmetrically widened. A probable earthquake‐induced landslide caused by movement on the Xiangshan Fault in latest Pleistocene blocked the gorge causing aggradation along all rivers and their tributaries. Deposition terraces were formed after the landslide dam was breached. Together with previous research on the Xiangshan Fault, it is concluded that there has been c. 7 km of Quaternary sinistral displacement on the Haiyuan and Xiangshan faults along the northeast margin of the Tibetan Plateau since the formation of rivers that intersect them. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

11.
Anticipating geomorphic tipping points requires that we learn from the past. Major geomorphic changes in coastal plain rivers of Texas resulting in river metamorphosis or regime shifts were identified and the major driving factors determined. Eleven such transformations – possible tipping points – were identified from contemporary observations, historical records, and Quaternary reconstructions. Two of the tipping points (between general aggrading and degrading valley states) are associated with reversals in a fundamental system control (sea‐level). One (stable or aggrading versus degrading channels) is associated with an abrupt change in sediment supply due to dam construction, and two others (changes from meandering to anastomosing channel patterns, and different anastomosis styles) are similarly related to changes in sediment supply and/or transport capacity, but with additional elements of historical contingency. Three tipping points are related to avulsions. One, from a regime dominated by re‐occupation of former channels to one dominated by progradation into flood basins, is driven by progressive long‐term filling of incised valleys. Another, nodal avulsions, is triggered by disturbances associated with tectonic uplift or listric faults. The third, avulsions and related valley metamorphosis in unfilled incised valleys, is due to fundamental dynamical instabilities within the fluvial system. This synthesis and analysis suggests that geomorphic tipping points are sometimes associated with general extrinsic or intrinsic (to the fluvial system) environmental change, independent of any disturbances or instabilities. Others are associated with natural (e.g. tectonic) or human (dams) disturbances, and still others with intrinsic geomorphic instabilities. This suggests future tipping points will be equally diverse with respect to their drivers and dynamics. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

12.
Effective river management strategies require an understanding of how fluvial processes vary both spatially and temporally. Here, we examine the natural range of variability in the Conejos River Valley, southern Colorado, through documentation of terrace morphostratigraphic and sedimentological characteristics as well as through investigation of sediment contributions from headwaters, hillslopes and tributary streams. Additionally, soil development and radiocarbon ages, together with local and regional paleoclimate reconstructions, were used to infer the range of processes acting in this system. Since de‐glaciation, the Conejos River has fluctuated between episodes of bedrock strath formation, aggradation and vertical incision. Morphostratigraphic relationships, soil development and radiocarbon ages enable us to propose a chronology for periods of alluvial deposition (around 8·9–7·6 ka, 5·5 ka and from 3·5 to 1·1 ka), separated by intervals of fluvial incision. We infer potential forcing mechanisms by utilizing multiple working hypotheses. Specifically, we discuss the potential for increases in sediment supply during periods of (1) para‐glacial adjustment, (2) climatic cooling, (3) increased frequency of climate change and (4) increased fire frequency or severity. We also consider the effects of changes in stream discharge and extreme storm occurrence. We conclude that combinations of these processes, operating at different times, have contributed to sediment mobilization since de‐glaciation. Stream and landform morphology also varies longitudinally due to the influence of remnant glacial topography. In particular, valley bottom overdeepening at tributary junctions has resulted in incision and strath formation into unlithified glacial deposits (i.e. fill‐cut terraces) rather than bedrock in some reaches. Overall, the Conejos fluvial system has varied significantly both temporally and spatially since de‐glaciation and appears to be sensitive to changes in sediment supply related to Holocene scale climate fluctuations. This natural range of variability must therefore be a key consideration in any future stream management policies. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

13.
The large river systems are the major transfer of continental masses to the ocean and basin, playing significant roles in global geochemical cycles. The Tibetan plateau is the birthplace of many huge rivers flowing through eastern and southern Asia, in which the fluvial deposits kept not only closely relate to the geological evolution information from the source areas, but also record the river itself building process. The low-temperature thermochronology method of detrital minerals (zircon and apatite, etc.) can be used to constrain the river's source areas, establishing its source-sink system. It can also combine regional tectonic deformation analysis to determine the potential source region of the river and the formation time of the plateau geomorphology, which is a focused issue in recent years. In this study, we have summarized the research results from the large rivers in the Tibetan plateau in recent years, suggesting that the low-temperature thermochronology analysis of the detrital minerals should be focused on the river's key locations in the upstream, midstream and downstream, respectively, combining the small tributaries analysis which can give a more detailed thermal evolution history in the whole drainage basin. On the conditions of the bedrock, it is shown that in the same river's different place we should use the same low-temperature thermochronology, while in different river's places we should take several low-temperature thermochronology methods (apatite and zircon, etc.)at a same position, so we get a complete time series related to the river incision. Combining the valley bedrock and detrital river minerals with the low-temperature thermalchronology on the Tibetan plateau, together with the chronology, structure analysis and other sedimentary studies, we can obtain detailed structures and river's evolution processes.  相似文献   

14.
《国际泥沙研究》2020,35(6):609-620
The fluvial geomorphology in tectonically active (particularly rapid uplift) regions often undergoes continuous change. The rapid uplift is coincident with high erosion rates; consequently, incised valleys are formed. Mass flows (for example, avalanches, landslides, and debris flows) in incised valleys can markedly influence fluvial processes and even reshape valley geomorphology. However, these processes and long-term evolution corresponding to mass flows require further clarification. Field campaigns were carried out in the region near the Yigong Tsangpo and Palong Tsangpo Rivers (hereafter the Yigong and Palong Rivers), the two largest tributaries of the lower Yarlung Tsangpo River, to examine the feedback between fluvial processes and mass flows. Remote sensing images from recent decades were used to compare the channel morphology before and after typical mass flows (particularly catastrophic ones). The morphology of the lower Yigong River has evidently been impacted by landslides, while that of the Palong River has mainly been shaped by glacial processes and debris flows. At present, the morphology of the latter consists of alternating sections of gorges and wide valleys, with a staircase-like longitudinal profile. The gorge sections exhibit single and deeply incised channels with a high-gradient channel bed and terraces. In contrast, the wide valley sections consist of lakes, braided or anabranching channels, gentle bed gradients, and thick alluvial deposits. Debris flows occur more frequently in gullies in the reaches of the gorge sections and rarely in gullies along the wide valley sections. The occurrence of mass flow events has resulted in an imbalance of the previous (quasi-)equilibrium in the river morphology; however, this has triggered negative feedback that is driving the transient river morphology to a new state of (quasi-)equilibrium.  相似文献   

15.
Fluvial systems in uplifting terrain respond to tectonic, climatic, eustatic and local base‐level controls modified by specific local factors, such as river capture. The Rio Alias in southeast Spain is an ephemeral, transverse‐to‐structure fluvial system. The river drains two interconnected Neogene sedimentary basins, the Sorbas and Almeria basins, and crosses two major geological structures, the Sierras de Alhamilla/Cabrera and the Carboneras Fault Zone. Regional epeirogenic uplift resulted in sustained fluvial incision during the Quaternary, punctuated by major climatically driven periods of aggradation and dissection, which created a suite of five river terraces. The river terrace sequence was radically modified in the late Pleistocene by a major river capture (itself a response to regional tectonics), localized tectonic activity and eustatic base‐level change. The Rio Alias is defined by four reaches; within each the climatically‐generated, region‐wide, fluvial response was modified by tectonics, base‐level change or river capture to varying degrees. In the upper part of the basin (Lucainena reach), climate was the dominant control on river development, with limited modification of the sequence by uplift of the Sierra Alhamilla and local drainage reorganization by a local river capture. Downstream of the Sierra Alhamilla in the Polopus reach, the climatic signal is dominant, but its expression is radically modified by the response to a major river capture whereby the Alias system lost up to 70% of its pre‐capture drainage area. In the reach adjacent to the Carboneras Fault Zone (Argamason reach), modification of the terrace sequence by local tectonic activity and a resultant local base‐level fall led to a major local incisional event (propagating c. 3–4 km upstream from the area of tectonic disturbance). At the seaward end of the system (El Saltador reach) Quaternary sea‐level changes modified the patterns of erosion and incision and have resulted in steep incisional terrace profiles. The signals generated by regional tectonics and the Quaternary climate change can be identified throughout the basin but those generated by ongoing local tectonics, river capture and sea‐level change are spatially restricted and define the four reaches. The connectivity of the system from the headwaters to the coast decreased through time as incision progressed, resulting in changes in local coupling characteristics. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

16.
We provide field evidence for the role of bedload in driving fluvial incision and knickpoint propagation. Using aerial photographs, field surveys, and hydrological data, we constrain the incision history of a bedrock gorge 1200 m long and up to 20 m deep cut by Da'an River in western Taiwan. This reach of the river experienced 10 m of uplift during the 1999 Chi‐Chi earthquake. For five years following the earthquake, bedload was prevented from entering the uplift zone, the knickpoint was static and little incision took place. Bedload transport across the uplift zone resumed in 2004, initiating extremely rapid incision, with 620 m of knickpoint propagation and up to 20 m of downcutting by 2008. This change highlights the relative inefficiency of suspended sediment and the dominant role of bedload as a tool for fluvial erosion and knickpoint propagation. Once bedload tools became available, knickpoint propagation was influenced by geological structure, lithology, and drainage organization. In particular, a change in dip of the sandstone beds at the site caused a decrease of knickpoint propagation velocity. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

17.
The stream power incision model (SPIM) is a cornerstone of quantitative geomorphology. It states that river incision rate is the product of drainage area and channel slope raised to the power exponents m and n, respectively. It is widely used to predict patterns of deformation from channel long profile inversion or to model knickpoint migration and landscape evolution. Numerous studies have attempted to test its applicability with mixed results prompting the question of its validity. This paper synthesizes these results, highlights the SPIM deficiencies, and offers new insights into the role of incision thresholds and channel width. By reviewing quantitative data on incising rivers, I first propose six sets of field evidence that any long‐term incision model should be able to predict. This analysis highlights several inconsistencies of the standard SPIM. Next, I discuss the methods used to construct physics‐based long‐term incision laws. I demonstrate that all published incising river datasets away from knickpoints or knickzones are in a regime dominated by threshold effects requiring an explicit upscaling of flood stochasticity neglected in the standard SPIM and other incision models. Using threshold‐stochastic simulations with dynamic width, I document the existence of composite transient dynamics where knickpoint propagation locally obeys a linear SPIM (n=1) while other part of the river obey a non‐linear SPIM (n>1). The threshold‐stochastic SPIM resolves some inconsistencies of the standard SPIM and matches steady‐state field evidence when width is not sensitive to incision rate. However it fails to predict the scaling of slope with incision rate for cases where width decreases with incision rate. Recent proposed models of dynamic width cannot resolve these deficiencies. An explicit upscaling of sediment flux and threshold‐stochastic effects combined with dynamic width should take us beyond the SPIM which is shown here to have a narrow range of validity. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

18.
Since the end of the post‐glacial sea level rise 6800 years ago, progradation of river mouths into estuaries has been a global phenomenon. The responses of upstream alluvial river reaches to this progradation have received little attention. Here, the links between river mouth progradation and Holocene valley aggradation are examined for the Macdonald and Tuross Rivers in south‐eastern Australia. Optical and radiocarbon dating of floodplain sediments indicates that since the mid‐Holocene sea level highstand 6800 years ago vertical floodplain aggradation along the two valleys has generally been consistent with the rate at which each river prograded into its estuary. This link between river mouth progradation and alluvial aggradation drove floodplain aggradation for many tens of kilometres upstream of the estuarine limits. Both rivers have abandoned their main Holocene floodplains over the last 2000 years and their channels have contracted. A regional shift to smaller floods is inferred to be responsible for this change, though a greater relative sea level fall experienced by the Macdonald River since the mid‐Holocene sea level highstand appears to have been an additional influence upon floodplain evolution in this valley. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

19.
Numerical models have not yet systematically been used to predict properties of fluvial terrace records in order to guide fieldwork and sampling. This paper explores the potential of the longitudinal profile model FLUVER2 to predict testable field properties of the relatively well‐studied, Late Quaternary Allier system in France. For the Allier terraces an overlapping 14C and U‐series chronology as well as a record of 10Be erosion rates exist. The FLUVER2 modelling exercise is focused on the last 50 ka of the upper Allier reach because for this location and period the constraints of the available dating techniques are tightest. A systematic calibration based on terrace occurrence and thicknesses was done using three internal parameters related to (1) the sediment erodibility; (2) the sediment transport distance; and (3) the sediment supply derived from the surrounding landscape. As external model inputs, the best available, reconstructed, tectonic, climatic and base‐level data were used. Calibrated model outputs demonstrate a plausible match with the existing fluvial record. Validation of model output was done by comparing the modelled and measured timing of aggradation and incision phases for the three locations. The modelled range of landscape erosion rates showed a reasonably good match with existing erosion rate estimates derived from 10Be measurements of fluvial sands. The quasi‐validated model simulation was subsequently used to make new testable predictions about the timing and location of aggradation and erosion phases for three locations along the Allier river. The validated simulations predict that along the Allier, reach‐specific dynamics of incision and aggradation, related to the variations in sediment supply by major tributaries, cause relevant differences in the local fluvial terrace stratigraphy. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

20.
An inventory of 846 mass movements, mainly landslides, in two alpine regions of southwest New Zealand was created to explore the geomorphic impacts of slope‐failure processes on river channels and valley floors. In total, 213 (i.e. 27 per cent) of the slope failures descended to valley floors, affecting the geomorphology of trunk channels (catchment area AC > 10 km2) and valley floors in recurring patterns. A nominal classification system is introduced for characterizing (a) the physical contact nature between landslides and river channels, and (b) the resulting geomorphic consequences for drainage. Although landslide area A is useful for estimating the length of channel directly impacted by debris, it does not necessarily predict the direction of fluvial response or type of impact. Dominant persistent geomorphic imprints of bedrock landslides include channel occlusions and landslide dams in South Westland and Fiordland, respectively. Differences in size distribution and geomorphic effects on river systems between the two study regions are attributed to bedrock geology, tectonics and sediment flux. Although South Westland rivers are more frequently affected by landslides, disrupting long‐term effects such as blockage are more persistent in Fiordland. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

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