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1.
Active tectonics in a basin plays an important role in controlling a fluvial system through the change in channel slope. The Baghmati, an anabranching, foothills-fed river system, draining the plains of north Bihar in eastern India has responded to ongoing tectonic deformation in the basin. The relatively flat alluvial plains are traversed by several active subsurface faults, which divide the area in four tectonic blocks. Each tectonic block is characterized by association of fluvial anomalies viz. compressed meanders, knick point in longitudinal profiles, channel incision, anomalous sinuosity variations, sudden change in river flow direction, river flow against the local gradient and distribution of overbank flooding, lakes, and waterlogged area. Such fluvial anomalies have been identified on the repetitive satellite images and maps and interpreted through DEM and field observations to understand the nature of vertical movements in the area. The sub-surface faults in the Baghmati plains cut across the river channel and also run parallel which have allowed us to observe the effects of longitudinal and lateral tilting manifested in avulsions and morphological changes.  相似文献   

2.
A vast bajada consisting of coalescing low-gradient (< 0.3°) alluvial fans exceeding 100 km in length formed along the southwestern margin of the Oman Mountains. It comprises an old fan sequence of inferred Miocene to Pliocene age termed Barzaman Formation, diagenetically highly altered to dolomitic clays, and a thin veneer of weakly cemented Quaternary gravels. A combination of remote sensing, lithological analyses and luminescence dating is used to interpret the complex aggradation history of the Quaternary alluvial fans from the interior of Oman in the context of independent regional climate records. From satellite imagery and clast analysis four fans can be discerned in the study area. While two early periods of fan formation are tentatively correlated to the Miocene–Pliocene and the Early Pleistocene, luminescence dating allows the distinction of five phases of fan aggradation during the Middle–Late Pleistocene. These phases are correlated with pluvial periods from Marine Isotope Stage (MIS) 11 through 3, when southern Arabia was affected by monsoonal precipitation. It is concluded that the aggradation of the alluvial fans was triggered by the interplay of increased sediment production during arid periods and high rainfall with enhanced erosion of hillslopes and transport rates during strong monsoon phases. However, the lack of fine-grained sediments, bioturbation and organic material implies that although the Quaternary fans are sourced by monsoonal rains they formed in a semi-arid environment. Thus, it appears that, in contrast to the Oman Mountains, the interior was not directly affected by monsoonal precipitation.  相似文献   

3.
The Dien Bien Phu fault zone (DBP), orientated NNE to N, is one of the most seismically active zones in Indochina. In NW Vietnam, this zone is 160 km long and 6–10 km wide, cutting sedimentary and metamorphic rocks of the Late Proterozoic, Palaeozoic and Mesozoic age, as well as Palaeozoic and Late Triassic granitoids. Along the DBP relatively small, narrow pull-apart basins occur, the three largest of which (Chan Nua, Lai Chau and Dien Bien Phu) have been studied in detail. All of them are bounded by sinistral and sinistral-normal faults, responsible for offset and deflected drainage, presence of numerous shutter ridges and displaced terraces and alluvial fans. The normal component of motion is testified to by well-preserved triangular facets on fault scarps, highly elevated straths in river watergaps, overhanging tributary valleys, as well as high and uneven river-bed gradients.Our observations indicate a minimum recent sinistral offset ranging from 6–8 to 150 m for Holocene valleys to 1.2–9.75 km for middle–late Pleistocene valleys in different fault segments. The thickness of Quaternary sediments varies from 5–25 m in the Lai Chau area to some 130 m in the Dien Bien Phu Basin. In the Lai Chau Basin, the middle terrace (23 m) alluvia of Nam Na River at Muong Te bridge have been optically stimulated luminescence/single aliquot regenerative dose technique (OSL-SAR) dated at 23–40 to 13 ka. These sediments were normal-faulted by some 11 m after 13 ka, and mantled by vari-coloured slope loams, 8–12 m thick, containing colluvial wedges composed of angular debris. These wedges were probably formed due to at least three palaeoseismic events postdating 6 ka. In the Dien Bien Phu Basin, in turn, alluvium of the upper Holocene terraces has been OSL-SAR dated to 6.5–7 and 1.7–1.0 ka, whereas the younger (sub-recent) terrace sediments give ages of 0.5–0.2 ka.Displaced terraces and alluvial fans allow us to suppose that the sinistral and sinistral-normal faults bounding narrow pull-apart basins in the southern portion of the DBP fault reveal minimum rates of left-lateral strike-slip ranging from 0.6 to 2 mm/year in Holocene and 0.5–3.8 mm/year in Pleistocene times, whereas rates of Holocene uplift tend to attain 1 mm/year north of Lai Chau and 0.4–0.6 mm/year west of Dien Bien Phu. More precise estimations, however, are difficult to obtain due to poor age control of the displaced drainage. Rates of Quaternary strike-slip are comparable with those of the Red River fault; the sense of movement being, however, opposite. Taking into account the presence of two phases of Late Cenozoic strike-slip of contrasting sense of motion, as well as the geometry of the two fault zones, we hypothesize that the Red River and Dien Bien Phu faults are conjugate faults capable of generating relatively strong earthquakes in the future.  相似文献   

4.
The Rio Negro has responded significantly in the Late Pleistocene and Holocene to lagged environmental changes largely associated with activity during the last glacial in the Amazon basin. On the basis of geological structure, the Rio Negro can be divided into six distinct reaches that each reflects very marked differential processes and geomorphological styles. No deposits of the Upper Pleniglacial were recognized in the field. The oldest recognizable Late Pleistocene alluvial unit is the Upper Terrace of Middle Pleniglacial age (ca. 65–25 ka) (reach I), tentatively correlated with the oldest terrace identified on the left bank of reach III. At that time, the river was mainly an aggradational bed load system carrying abundant quartz sand, a product of more seasonal conditions in the upper catchment. The late glacial (14–10 ka) is represented by a lower finer-grained terrace along the upper basin (reach I), which was recognized in the Tiquié, Curicuriarí, and Vaupes rivers. At that time, the river carried abundant suspended load as a response to climatic changes associated with deglaciation.Since about 14 ka, the river has behaved as a progradational system, infilling in downstream series a sequence of structurally controlled sedimentary basins or ‘compartments,’ creating alluvial floodplains and associated anabranching channel systems. Reach II was the first to be filled, then reach III, both accumulating mainly sand. Fine deposits increase downstream in reach III and become predominant in some anabranch islands of the distal reach. The lowermost reaches of the Negro (V and VI) have been greatly affected by a rising base level and associated backwater effect from aggradation of the Amazon during late glacial and recent times. Reach V has acted almost entirely as a fine sediment trap. The remarkable Anavilhanas archipelago is the product of Holocene deposition in the upper part of this sedimentary basin; however, suspended sediment load declined about 1.5 ka, prior to the lower part of this basin becoming infilled.The progradational behavior of the Rio Negro, filling tectonic basins as successive sediment traps with sand in the upper basins and fines in the downstream ones, illustrates how a large river system responses to profound changes in Late Quaternary base level and sediment supply. The most stable equilibrium conditions have been achieved in the Holocene in reaches IIb and IIIa, where an anabranching channel and erosional–relictual island system relatively efficiently convey water and sediment downstream. Reaches IIIb and V never achieved equilibrium conditions during the Holocene, characterised as they are today with incomplete floodplains and open water.  相似文献   

5.
The Quaternary evolution and the morpho-sedimentary features of some of the most important rivers in Spain (Ebro and Tagus rivers among others) have been controlled by subsidence due to alluvial karstification of the evaporitic bedrock. The subsidence mechanism may range from catastrophic collapse to slow sagging of the alluvium by passive bending. In the Ebro Basin, the mechanisms and processes involved in karstic subsidence were studied through the analysis of present-day closed depressions as well as through old subsidence depressions (palaeocollapses and solution-induced basins) and associated deformations recorded in the Quaternary alluvial sediments. The Gállego–Ebro river system is presented as a case study of channel adjustments and geomorphic and sedimentary evolution of fluvial systems in dissolution-induced subsidence areas. In this fluvial system, evaporite dissolution during particular Quaternary time intervals (namely early and middle Pleistocene) have lead to the development of a solution-induced basin, approximately 30 km-long by 8 km-wide, filled by Quaternary deposits with a total thickness in excess of 190 m. The main river response to balance the subsidence in the alluvial plain was aggradation in the central reach of the subsiding area, and degradation both in the upstream reach and in the valley sides where alluvial fans and covered pediments may prograde over the fluvial sediments. The main sinking areas are recognized in the sedimentary record by anomalous thickenings in the alluvial deposits and fine-grained sediments deposited in backswamp and ponded areas.  相似文献   

6.
Anabranching is characteristic of a number of rivers in diverse environmental settings worldwide, but has only infrequently been described from bedrock-influenced rivers. A prime example of a mixed bedrock-alluvial anabranching river is provided by a 150-km long reach of the Orange River above Augrabies Falls, Northern Cape Province, South Africa. Here, the perennial Orange flows through arid terrain consisting mainly of Precambrian granites and gneisses, and the river has preferentially eroded bedrock joints, fractures and foliations to form multiple channels which divide around numerous, large (up to 15 km long and 2 km wide), stable islands formed of alluvium and/or bedrock. Significant local variations in channel-bed gradient occur along the river, which strongly control anabranching style through an influence on local sediment budgets. In relatively long (>10 km), lower gradient reaches (<0.0013) within the anabranching reach, sediment supply exceeds local transport capacity, bedrock usually only crops out in channel beds, and channels divide around alluvial islands which are formed by accretion in the lee of bedrock outcrop or at the junction with ephemeral tributaries. Riparian vegetation probably plays a key role in the survival and growth of these islands by increasing flow roughness, inducing deposition, and stabilising the sediments. Less commonly, channels may form by eroding into once-continuous island or floodplain surfaces. In shorter (<10 km), higher gradient reaches (>0.0013) within the anabranching reach, local transport capacity exceeds sediment supply, bedrock crops out extensively, and channels flow over an irregular bedrock pavement or divide around rocky islands. Channel incision into bedrock probably occurs mainly by abrasion, with the general absence of boulder bedforms suggesting that hydraulic plucking is relatively unimportant in this setting. Mixed bedrock-alluvial anabranching also occurs in a number of other rivers worldwide, and appears to be a stable and often long-lived river pattern adjusted to a number of factors commonly acting in combination: (1) jointed/fractured granitoid rock outcrop; (2) erosion-resistant banks and islands; (3) locally variable channel-bed gradients; (4) variable flow regimes.  相似文献   

7.
Quaternary sediments represent the main constituent which covers an ENE–WSW elongated depression some 25 km long and 10 km wide (Al-Ain area). This depression is encountered between two north and south low fault scarps and is located perpendicular to the Al Jaww plain and Jabel Hafit axes.Four main types of Quaternary surface deposits were identified belonging to: flood plain and braided channels; desert plain; aeolian sand; and sabkha. The first type shows many pedogenic and non-pedogenic features of which are dolocrete, calcrete and gypcrete.An ENE–WSW closely-spaced dip-slip, stepping pattern fault set could be traced in dolocretized-calcretized braided channel deposits, on the south margin of this depression, where the relationship between fault geometry, displacement and geomorphology suggest a model of either graben or half-graben. The role of this system in developing the landscape of Al-Ain is well documented near the surface but a comprehensive study to assess its role in the sub-surface is needed.  相似文献   

8.
Confluence dynamics in the Ganga–Ramganga valley in the western Ganga plains of India has been studied through systematic mapping of channel configuration using multi-date remote sensing images and topographic sheets for a period spanning nearly 100 years (1911–2000). The study has been supplemented with a detailed analysis of the channel morphology, hydrology and sediment transport characteristics of the different rivers. Our study indicates that new confluences have been created during this period and that the confluence points have moved both upstream and downstream on a historical time scale. Apart from major avulsions, other processes that have controlled the confluence movements include river capture, cut-offs and aggradation in the confluence area. River capture occurs through lateral bank erosion and migration, encroachment by the master stream and beheading of smaller rivers resulting in upstream movement of the confluence point. Another process which influences the upstream migration of the confluence is an increase in sinuosity of one of the channels near the confluence and then a cut-off. Aggradation in the confluence area and local avulsions of the primary channel in a multi-channel system seem to be the major process controlling the downstream movement of the confluence point. Analysis of channel morphology, hydrology and sediment budget for the study period supports our interpretations.  相似文献   

9.
The River Ganga in the central Gangetic plain shows the incision of 20 m of Late Quaternary sediments that form a vast upland terrace (T2). The incised Ganga River Valley shows two terraces, namely the river valley (terrace-T1) and the present-day flood plain (terrace-T0). Terrace-T1 shows the presence of meander scars, oxbow lakes, scroll plains, which suggests that a meandering river system prevailed in the past. The present-day river channel flows on terrace-T0 and is braided, sensu stricto. It is thus inferred that the River Ganga experienced at least two phases of tectonic adjustments: (1) incision and (2) channel metamorphosis from meandering to braided.Optical dating of samples from three different terraces has bracketed the phase of incision to be <6 and 4 ka. Different ages of the top of terrace-T2 show that this surface experienced differential erosion due to tectonic upwarping in the region, which also caused the river incision. River metamorphosis occurred some time during 4 and 0.5 ka.  相似文献   

10.
The geomorphology and dynamics of the Mfolozi River floodplain and estuary, located in the subtropical region of northern KwaZulu-Natal, South Africa, were considered with respect to existing models of avulsion and alluvial stratigraphy. The Mfolozi River floodplain may be divided into regions based on longitudinal slope and dominant geomorphic processes. Confinement of the Mfolozi River above the floodplain has led to the development of an alluvial fan at the floodplain head, characterized by a relatively high sedimentation rate and avulsion frequency, at a gradient of 0.10%. The lower floodplain is controlled by sea level, with an average gradient of 0.05%. Between the two lies an extremely flat region with an average gradient of 0.02%, which may be controlled by faulting of the underlying bedrock.Avulsion occurrences on the Mfolozi floodplain are linked to the two main zones of aggradation, the alluvial fan at the floodplain head, and toward the river mouth in the lower floodplain. On the alluvial fan, normal flow conditions result in scour from local steepening. During infrequent, large flood events, the channel becomes overwhelmed with sediment and stream flow, and avulses. The resulting avulsion is regional, and affects the location of the channel from the floodplain head to the river mouth. Deposits resulting from such avulsions contribute significantly to the total volume of sediment stored in the floodplain, and tend to persist for long periods after the avulsion. Contrastingly, on the lower floodplain, reaching of the avulsion threshold is not necessarily linked to large flood events, but rather to long-term aggradation on the channel that decreases the existing channels gradient while increasing its elevation above the surrounding floodplain. Resultant avulsions tend to be local and do not contribute significantly to the overall volume of floodplain alluvium.  相似文献   

11.
In the Lake Guillaume-Delisle area of subarctic Québec, storm-generated alluvial fans have been active sporadically throughout the Holocene. In this study, we propose that the persistence of late-lying snowpatches in fan catchments during Holocene cold episodes promoted alluvial fan activity by lowering the precipitation threshold required to trigger a torrential event. This hypothesis was tested by characterizing the depositional processes responsible for alluvial fan formation below snowpatches, and by reconstructing the Holocene alluvial fan activity. Stratigraphic and sedimentary analyses conducted on seven alluvial fans revealed that they were deposited by torrential activity leading to waterlaid, transient, or hyperconcentrated deposition. The chronology of the storm-generated alluvial fans — based on 22 radiocarbon dates — indicates that torrential activity was enhanced during the cooler Late Holocene (i.e., after ca. 3500 cal. yr BP). Snowier winters and cooler summers were beneficial to nival activity, allowing the persistence of larger snowpatches throughout the summer and fall seasons. Rainfall-induced thaw of such snowpatches during rainstorm events is inferred to have contributed to alluvial fan activity by increasing water availability. Three peaks of alluvial activity occurred during the Late Holocene (2950–2750, 1900–1400, and 800–300 cal. yr BP) and are indicative of increased storminess resulting in higher fan activity. Increased fan activity during cooler episodes was concurrent with increased runoff activity in the immediate pronival area. This stresses the importance of nivation below snowpatches and pinpoints the role of nivation in enhancing geomorphological activity during period of cooler and more humid climate in subarctic environments.  相似文献   

12.
The extensive Gangetic alluvial plains are drained by rivers which differ strongly in terms of hydrological and sediment transport characteristics. These differences are manifested in the geomorphic diversity of the plains. The Western Gangetic Plains (WGP) are marked by a degradational topography with incised channels and extensive badland development in some parts, while the Eastern Gangetic Plains (EGP) are characterized by shallow, aggrading channels with frequent avulsions and extensive flooding. We interpret such geomorphic diversity in terms of differences in stream power and sediment supply from the catchment areas. The rivers draining the western plains are marked by higher stream power and lower sediment yield that result in degradation. In comparison, the rivers draining the eastern Gangetic Plains have lower stream power and higher sediment yield that result in aggradation. The variation of stream power, a function of channel slope and high sediment yield, is attributed to differences in rainfall and rate of uplift in the hinterland. It is suggested that such differences have resulted in a marked geomorphic diversity across the plains. It is also suggested that such diversity has existed for a fairly long time because of climatic and tectonic variance.  相似文献   

13.
Varnish microlamination (VML) dating is a correlative age determination technique that can be used to date and correlate various geomorphic features in deserts. In this study, we establish a generalized late Quaternary (i.e., 0–300 ka) varnish layering sequence for the drylands of western USA and tentatively correlate it with the SPECMAP oxygen isotope record. We then use this climatically correlated varnish layering sequence as a correlative dating tool to determine surface exposure ages for late Quaternary geomorphic features in the study region. VML dating of alluvial fan deposits in Death Valley of eastern California indicates that, during the mid to late Pleistocene, 5–15 ky long aggradation events occurred during either wet or dry climatic periods and that major climate shifts between glacial and interglacial conditions may be the pacemaker for alteration of major episodes of fan aggradation. During the Holocene interglacial time, however, 0.5–1 ky long brief episodes of fan deposition may be linked to short periods of relatively wet climate. VML dating of alluvial desert pavements in Death Valley and the Mojave Desert reveals that pavements can be developed rapidly (< 10 ky) during the Holocene (and probably late Pleistocene) in the arid lowlands (< 800 m msl) of these regions; but once formed, they may survive for 74–85 ky or even longer without being significantly disturbed by geomorphic processes operative at the pavement surface. Data from this study also support the currently accepted, “being born at the surface” model of desert pavement formation. VML dating of colluvial boulder deposits on the west slope of Yucca Mountain, southern Nevada, yields a minimum age of 46 ka for the emplacement of these deposits on the slope, suggesting that they were probably formed during the early phase of the last glaciation or before. These results, combined with those from our previous studies, demonstrate that VML dating has great potential to yield numerical age estimates for various late Quaternary geomorphic features in the western USA drylands.  相似文献   

14.
The Late Glacial and Holocene geomorphology of the Manx uplands has received scant attention in previous researches. Solifluction deposits and terraces provide the earliest evidence for geomorphic activity after deglaciation. Fluvial incision into drift-choked valleys is correlated with the formation of the large mountain front alluvial fans that flank the Manx uplands. Formation of these alluvial fans is constrained to 15,000–10,500 cal. years BP by 14C dates on organic deposits beneath and above the alluvial fan gravels. Alluvial fan and river terraces along four valleys postdate this incision. Optically Stimulated Luminescence (OSL) and 14C dating provide a tentative chronology for these landforms. The higher terraces are Late Glacial fluvial surfaces that were probably occupied by rivers into the Holocene. Incision during the Late Holocene led to the abandonment of the higher surfaces, producing a suite of younger river terraces and alluvial fan surfaces. Independent dating constrains this fluvial activity to post-Bronze Age (3500–2800 cal. years BP). Increased human activity and climatic change during the Late Holocene are possible causes for this increased geomorphic activity.  相似文献   

15.
The regional topography of much of the New South Wales Southern Highlands was long attributed to Quaternary erosion, but it has been recently demonstrated that the region was, in fact, well developed by the Eocene. The hillsides at Inverary Park are mantled by the Late‐Eocene Reevesdale Basalt, thus indicating the extent of landscape development by the mid‐Tertiary. Pre‐, intra‐, and post‐basaltic alluvial and lacustrine sediments are compared with the current extent of the basalt, clearly demonstrating the age of this landscape and the paucity of modification since the Late Eocene. Stratigraphic techniques, U/Th, TL and 14C dating aid reconstruction of much of the Cainozoic history of Inverary Park. Major changes in sediment type and source have long been common, but Late Quaternary changes do not correlate well with similar observations elsewhere, suggesting that changes in hydrologic regime, notably climate change, are not necessarily responsible for changes in the stratigraphic record. This landscape development and preservation is also at odds with most models of landscape development.  相似文献   

16.
Ostracode analysis was carried out on samples from ice-rich permafrost deposits obtained on the Bykovsky Peninsula (Laptev Sea). A composite profile was investigated that covers most of a 38-m thick permafrost sequence and corresponds to the last ca. 60 kyr of the Late Quaternary. The ostracode assemblages are similar to those known from European Quaternary lake deposits during cold stages. The ostracode habitats were small, shallow, cold, oligotrophic pools located in low centred ice wedge polygons or in small thermokarst depressions. In total, 15 taxa, representing 7 genera, were identified from 65 samples. The studied section is subdivided into six ostracode zones that correspond to Late Quaternary climatic and environmental stadial-interstadial variations established by other paleoenvironmental proxies: (1) cold and dry Zyrianian stadial (58–53 kyr BP); (2) warm and dry Karginian interstadial (48–34 kyr BP); (3) transition from the Karginian interstadial to the cold and dry Sartanian stadial (34–21 kyr BP); (4) transition from the Sartanian stadial to the warm and dry Late Pleistocene period, the Allerød (21–14 kyr BP); (5) transition from the Allerød to the warm and wet Middle Holocene (14–7 kyr BP); and (6) cool and wet Late Holocene (ca. 3 kyr BP). The abundance and diversity of the ostracodes will be used as an additional bioindicator for paleoenvironmental reconstructions of the Siberian Arctic.  相似文献   

17.
Abandoned channel belts, ponds and point bar deposits of palaeochannels in the interfluve regions of the central Ganga Plain suggest changes in the morphohydrologic conditions during the Latest Pleistocene–Holocene. Stratigraphy of these ponds comprises channel sand at the base overlain by shell-bearing clayey silt. The contact of the two facies marks the phase when channels converted into standing water bodies. Point bar deposits of some palaeochannels are overlain by oxidised aeolian sand, indicating that the channel abandonment possibly occurred due to the desiccation and aridity in the region.Optically stimulated luminescence (OSL) chronometry of the pond sediments suggests that the deposition of the basal channel sand started before 13 ka and continued up to 8 ka. The ponds formed around 8–6 ka when the channel activity ceased. Evidence from the point bar deposits also indicates that the fluvial activity in the region ended sometime during 7–5 ka. This was followed by aeolian aggradation. The present study thus suggests that the hydrologic conditions in the Gangetic plains, i.e. initiation of channels and their abandonment, formation of microgeomorphologic features such as ponds and their eventual siltation, were controlled largely by climatic changes (i.e. monsoon changes) supported by tectonic activity. For the past 2 ka, increasing human and related agricultural activity has substantially accentuated the natural siltation rate of ponds.  相似文献   

18.
Results from analyses of the Arles‐Piton sediment core, retrieved from the apex of the Rhône Delta, highlight processes of Holocene deltaic construction controlled mainly by hydrosedimentary variability and channel avulsions. The alluvial suite was investigated for grain size, sedimentary structures, CaCO3, organic matter, heavy minerals and chrono‐stratigraphy (14C and archaeological/historical dates). The study shows the succession of six facies associations: a distributary channel (before 6157‐5843 BC), a swamp (5719‐5530/4796‐4463 BC), a distal flood plain (5719‐5530/4796‐4463 BC), a distributary channel (4796‐4463/2900‐2503 BC), a proximal flood plain (2900‐2503 BC/AD 270‐290), and a crevasse splay (after AD 270‐290). Substantial changes in hydrodynamics are strongly linked to three channel avulsions (before 6157‐5843 BC, after 4796‐4463 BC and after 2900‐2503 BC). A correlation with the whole channel avulsion history of the Rhône Delta allowed us to propose an average rhythm of channel avulsion of c. 1450 years. From 5719‐5530 BC to AD 270‐290, the flood plain aggraded at the average rate of 2.5 mm/a. The aggradation rates were higher both in the proximal and distal flood plains, where sedimentation process is continuous. They were lower both in the active distributary channels, because of frequent truncation of the alluvial suite, and the abandoned channels where detritic inputs are minimum. The sediment supply arriving to the upper Rhône Delta was derived mainly from proximal source areas (Massif Central, Southern Alps) during the last 8000 years, except during the hydrological changes of Roman antiquity during which detritic inputs were derived firstly from the Northern Alps and Southern Alps, and secondly from the Massif Central.  相似文献   

19.
Knowledge of the long-term geomorphological dynamics of wetlands is limited, so currently there is an inadequate scientific basis for assessing anthropogenically induced changes and for developing conservation, remediation, and/or sustainable management guidelines for these fragile ecosystems. Along the upper Klip River, eastern South Africa, geomorphological and sedimentological investigations, geochronology, and remote sensing have been used to establish the late Quaternary dynamics of some internationally important floodplain wetlands, thus providing a reference condition against which to assess the extent of recent human impacts. Optically stimulated luminescence dating reveals that the wetlands have developed over at least the last 30 ky as a result of slow meander migration (< 0.2 m y− 1), irregular cutoff events, and infrequent avulsions (approximately one every 3–6 ky) that have occurred autogenically as a natural part of meander-belt development. Following European settlement in the Klip valley (late nineteenth century), however, modifications to local flora and fauna, as well as the initiation of local wetland drainage schemes, have had major impacts. In particular, proliferation of exotic willows and associated debris jams, and the artificial excavation of a 1.2-km-long channel section across the wetlands have initiated an ongoing avulsion that is characterised by failure (gradual abandonment) of the main channel and rapid incision of a headcutting channel. Compared to the pre-settlement condition, little change in lateral migration activity has occurred, but this avulsion provides a clear example of anthropogenically accelerated change, occurring only ~ 1 ky after the last natural avulsion and in a part of the wetlands where avulsions have not occurred previously. Subsequent human interventions have included installing weirs in an attempt to control the resulting erosion and promote reflooding, but ongoing maintenance has been required. In areas that were not glaciated during the Quaternary, many other floodplain wetlands may be of similar antiquity, but the Klip River illustrates their sensitivity to direct and indirect human impacts.  相似文献   

20.
通过对澧阳平原野外调查,多剖面地层对比分析及岩板垱（YBD）剖面样品的粒度、地球化学元素分析和年代测定,揭示该区晚更新世晚期至全新世早中期环境演变,并探讨其对人类活动的影响。研究结果表明,30~6 ka B.P.间,澧阳平原由冲沟发育的黄土台地向河湖发育的平原丘岗地貌过渡;气候环境经历了弱暖湿-气候变差-凉湿-波动回暖-暖湿的变化;更新世末至全新世早期稻作农业的出现与该区的环境条件和文化基础密切相关;环境演变也推动了古人类活动范围和聚落特征的变化。  相似文献   

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