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1.
Elizabeth B. Oswald  Ellen Wohl   《Geomorphology》2008,100(3-4):549-562
A jökulhlaup burst from the head of Grasshopper Glacier in Wyoming's Wind River Mountains during early September 2003. Five reaches with distinct sedimentation patterns were delineated along the Dinwoody Creek drainage. This paper focuses on a portion of the jökulhlaup route where erosion of the forested banks created 16 large logjams spaced at longitudinal intervals of tens to hundreds of meters. Aggradation within the main channel upstream from each logjam created local sediment wedges, and the jams facilitated overbank deposition during the jökulhlaup. Field surveys during 2004 and 2006 documented logjam characteristics and associated erosional and depositional features, as well as initial modification of the logjams and flood deposits within the normal seasonal high-flow channel. Overbank deposits have not been altered by flows occurring since 2003. Field measurements supported three hypotheses that (i) logjams present along the forested portions of the jökulhlaup route are larger and more closely spaced than those along adjacent, otherwise comparable stream channels that have not recently experienced a jökulhlaup; (ii) logjams are not randomly located along the jökulhlaup route, but instead reflect specific conditions of channel and valley geometry and flood hydraulics; and (iii) the presence of logjams facilitated significant erosional and depositional effects. This paper documents a sequence of events in which outburst floodwaters enhance bank erosion and recruitment of wood into the channel, and thus the formation of large logjams. These logjams sufficiently deflect flow to create substantial overbank deposition in areas of the valley bottom not commonly accessed by normal snowmelt peak discharges, and through this process promote valley-bottom aggradation and sediment storage. Changes in the occurrence of glacier outburst floods thus have the potential to alter the rate and magnitude of valley-bottom dynamics in these environments, which is particularly relevant given predictions of worldwide global warming and glacial retreat. Processes observed at this field site likely occur in other forested catchments with headwater glaciers.  相似文献   

2.
In February/March 2007 extreme rainfall occurred over a four‐day period in the 7000 km2 East Alligator River catchment in Arnhem Land, northern Australia. The resultant large flood caused extensive bank erosion, channel widening, stripping of point bars and floodplain, resulting in large amounts of sand transport. This sand was largely deposited in the downstream river channel as a sand slug, and as deep overbank sand splays where the valley abruptly widened immediately downstream of an island anabranching, bedrock‐confined reach. Interpretation of a time series of aerial photographs and satellite images from 1950 to 2012 showed that there have been considerable channel changes along the study reach. The aerial photographs show that extensive sedimentation in the same reach as in 2007 also occurred in 1975, 1981 and 1984. Each time, the sand slug was reworked rapidly over succeeding years by subsequent smaller floods, and the channel deepened naturally as sand supply declined due to revegetation of the upstream riparian zone and the reformation and stabilisation of point and lateral bars. Sand slug formation at an intermediate floodout is an episodic process dependent on the supply of large volumes of sand by extensive channel erosion during extreme floods. A conceptual geomorphic model was developed to highlight the differential effectiveness of extreme versus moderate floods.  相似文献   

3.
J. Shaw  A. Pugin  R.R. Young   《Geomorphology》2008,102(3-4):364-375
The geomorphology of troughs crossing the Antarctic shelf is described and interpreted in terms of ice-stream hydrology. The scale of tunnel channels on the inner shelf and the absence of sediment at their mouths are taken to infer catastrophic drainage. Drumlins on the inner and outer shelves with pronounced crescentic and hairpin scours are also interpreted as products of catastrophic flow. Gullies and channels on the continental slope and turbidites on the rise and abyssal plain point to abundant meltwater discharge across the shelf. Attempts to explain this morphology and sedimentology in terms of release or discharge of meltwater by pressure melting, strain heating, Darcian flow, or advection in deforming till are shown to be unrealistic. We suggest that meltwater flow across the middle and outer shelves might have been in broad, turbulent floods, which raises the possibility that megascale glacial lineations (MSGL) on the shelf might originate by erosion in turbulent flow. This possibility is explored by use of analogs for MSGL from flood and eolian landscapes and marine environments. An extended discussion reflects on objections that stand in the way of the flood hypothesis.  相似文献   

4.
The objectives of this study were: (1) to document spatial and temporal distributions of large woody debris (LWD) at watershed scales and investigate some of the controlling processes; and (2) to judge the potential for mapping LWD accumulations with airborne multispectral imagery. Field surveys were conducted on the Snake River, Soda Butte Creek, and Cache Creek in the Greater Yellowstone Ecosystem, USA. The amount of woody debris per kilometer is highest in 2nd order streams, widely variable in 3rd and 4th order streams, and relatively low in the 6th order system. Floods led to increases in woody debris in 2nd order streams. Floods redistributed the wood in 3rd and 4th order streams, removing it from the channel and stranding it on bars, but appeared to generate little change in the total amount of wood throughout the channel system. The movement of woody debris suggests a system that is the reverse of most sediment transport systems in mountains. In 1st and 2nd order tributaries, the wood is too large to be moved and the system is transport-limited, with floods introducing new material through undercutting, but not removing wood through downstream transport. In the intermediate 3rd and 4th order channels, the system displays characteristics of dynamic equilibrium, where the channel is able remove the debris at approximately the same rate that it is introduced. The spatial distribution and quantity of wood in 3rd and 4th order reaches varies widely, however, as wood is alternatively stranded on gravel bars or moved downstream during periods of bar mobilization. In the 6th order and larger channels, the system becomes supply-limited, where almost all material in the main stream can be transported out of the central channel by normal stream flows and deposition occurs primarily on banks or in eddy pool environments. Attempts to map woody debris with 1-m resolution digital four-band imagery were generally unsuccessful, primarily because the imagery could not distinguish the narrow logs within a pixel from the surrounding sand and gravel background and due to problems in precisely coregistering imagery and field maps.  相似文献   

5.
Diversity theory predicts that species numbers should be highest at intermediate levels of both disturbance and environmental stress. We examined woody and herbaceous plant species richness and cover in the San Pedro River flood plain, along lateral gradients of water availability (ground-water depth), flood disturbance (inundation frequency), and distance from and elevation above the channel, and along longitudinal gradients of water availability (ground-water depth, surface flow permanence, and rainfall) and flood disturbance (total stream power). Herbaceous species were recorded during four sampling periods, and spatial patterns for this group were time-dependent, reflecting temporal variation in limiting factors. During the summer dry season of a dry year, when overall richness was low, richness and cover of herbaceous species declined laterally from the stream channel with increasing ground-water depth, consistent with the idea that low resource levels can limit species richness. Following the summer monsoon rains and floods, when water was less limiting and annuals were seasonally abundant, lateral patterns shifted such that herbaceous species richness and cover increased with increasing plot location above or from the channel. The relationship of herbaceous species richness with tree canopy cover also varied seasonally, shifting from positive (greater richness under canopy) in dry seasons to negative (lesser richness under canopy) in wet seasons. Longitudinally, herbaceous species richness and cover were limited primarily by stream flow and/or ground-water availability during the summer dry season of a dry year. Following the summer monsoon rains and floods, patterns were weighted by the seasonally abundant annuals, and richness increased among sites primarily with distance upstream (and related rainfall gradients). Richness and cover patterns also varied between years with different flood conditions. During the two sampling seasons in the year following a large flood, herbaceous species richness increased with flood disturbance intensity but declined at the few most intensely disturbed sites, consistent with intermediate disturbance theory.For woody species, richness within plant functional groups varied in opposing ways along the lateral gradients: hydromesic pioneer species decreased and hydromesic and xeric competitors increased with distance from or above the channel, with no overall change in species richness. Among sites, woody species richness patterns were related to water availability, but not to flood disturbance. However, richness of woody hydromesic pioneer species increased with both increasing site moisture and flood disturbance. Woody and herbaceous species richness both increased among sites as a function of increasing flood-plain width, likely due to species–area effects. Overall, results indicate that flood disturbance and water availability both influence species richness of riparian plants in the flood plain of this semi-arid region river, with the relative influence of each factor varying among plant groups and over time.  相似文献   

6.
Floodplains contain valuable stratigraphic records of past floods, but these records do not always represent flood magnitudes in a straightforward manner. The depositional record generally reflects the magnitude, frequency, and duration of floods, but is also subject to storm-scale hysteresis effects, flood sequencing effects, and decade-scale trends in sediment load. Many of these effects are evident in the recent stratigraphic record of overbank floods along the Upper Mississippi River (UMR), where the floodplain has been aggrading for several thousand years. On low-lying floodplain surfaces in Iowa and Wisconsin, 137Cs profiles suggest average vertical accretion rates of about 10 mm/year since 1954. These rates are slightly less than rates that prevailed earlier in the 20th Century, when agricultural land disturbance was at a maximum, but they are still an order of magnitude greater than long-term average rates for the Holocene. As a result of soil conservation practices, accretion rates have decreased in recent decades despite an increase in the frequency of large floods.The stratigraphic record of the Upper Mississippi River floodplain is dominated by spring snowmelt events, because they are twice as frequent as rainfall floods, last almost twice as long, and are sometimes associated with very high sediment concentrations. The availability of sediment during floods is also influenced by a strong hysteresis effect. Peak sediment concentrations generally precede the peak discharges by 1–4 weeks, and concentrations are usually low (<50 mg/l) during the peak stages of most floods. The lag between peak concentration and peak discharge is especially large during spring floods, when much of the runoff is contributed by snowmelt in the far northern reaches of the valley.The great flood of 1993 on the Mississippi River focused attention on the geomorphic effectiveness and stratigraphic signature of large floods. At McGregor, where the peak discharge had a recurrence interval of 14 years, the flood was most notable for its long duration (168 days above 1600 m3s−1), high sediment concentrations (three episodes >180 mg/l), and large suspended load (1.71 Mt). The flood of 2001, despite its greater magnitude (recurrence interval 70 years), was associated with relatively low sediment concentrations (<60 mg/l). The 1993 and 2001 floods each left 30–80 mm of silty fine sand on most low-lying floodplain surfaces, but the 2001 flood produced sandy levees near the channel while the 1993 flood did not. The stratigraphic signature of these recent floods is more closely related to the duration and total suspended load of the event than to the magnitude of the peak discharge.  相似文献   

7.
Depositional effects of two great floods in the Nysa drainage basin have been studied: the alluvial forms and deposits of channels and floodplains. Three types of bars and one overbank form were found in the mountain streams, and four types of bars and three types of overbank forms have been distinguished in main rivers. A specific spatial succession of depositional forms was recognized along the mountain streams, however, no analogous phenomena were noted within the main river valleys. Several types of bars and alluvial lithofacies have been regarded as characteristic effects of the catastrophic flood. The study indicates that the Nysa river is close to the threshold of metamorphosis to a typical braided fluvial system.  相似文献   

8.
David J. Milan 《Geomorphology》2012,138(1):319-328
Quantitative assessments of the impacts of extreme floods on channel morphology are rare. Real Time Kinematic (RTK) GPS surveys of a 500-m reach of the Thinhope Burn, an upland gravel-bed stream in the UK, taken in 2003 and 2004 permitted an assessment of geomorphic work whilst the channel was at steady-state. A large flood that occurred on 17 July 2007 resulted in a catastrophic impact to the Thinhope Burn valley floor. The reach was re-surveyed after the event in 2007, and again in 2008 and in 2011. Digital elevation models were produced from the survey data, which allowed the spatial patterns of erosion and deposition and volumetric changes between surveys to be established. A total of 5202 m3 of deposition and 2125 m3 of erosion was recorded in the reach following the flood event. Field walking of the catchment and comparison of aerial photographs for 2003 and 2007 suggested that most of the material mobilised had originated from existing sediment held in terraces and paleoberms on the valley floor. Although slope failures were evident, including peat slides in the headwaters, delivery of sediment from coupling zones to the channel was thought to play a secondary role in the geomorphic response shown by the channel. Similarly, large volumes of erosion and deposition were found after resurveys in 2008 and 2011, suggesting that the system was still in its relaxation phase. The results obtained in this investigation coupled with historical information on the flood history of Thinhope Burn dating back to 1766 suggested that rare large floods are the geomorphically effective flows in the catchment.  相似文献   

9.
This study examined vertical, lateral, and downstream variations in the grain-size characteristics of historical (post-1830) overbank deposits in a watershed that has experienced high rates of accelerated flood plain sedimentation. More than 800 samples were collected from 53 cores along nine flood plain transects. Overbank deposits exhibit a coarsening-upward sequence attributed to historical changes in the sand content of source materials. The erosion of loess-capped soils increased the exposure, erosion, and transport of sandy parent materials. The average sand content of near-channel cores increases moderately downstream along two of the reaches because sandy source materials are increasingly exposed in larger main valleys in the northern part of the watershed. The two northernmost reaches are coarser overall, but do not display significant downstream trends. The sand content of surface and early historical overbank deposits generally decreases laterally as an exponential function of distance from the channel, suggesting transport by turbulent diffusion. The presence of sand throughout the transects and lateral coarsening at two of the transects, however, suggests that sediment transport by convection is also important.  相似文献   

10.
We present data from a proglacial river in Iceland that exhibits very different sedimentological characteristics when compared to its alpine counterparts. The braidplain is characterised by coarse outburst gravels that inhibit sediment transport and channel change. Bedload transport is restricted to the movement of fine-grained gravels that pass through the channel system without promoting significant changes in channel geometry. Bar forms are erosional features, inherited from the last major peak flow, rather than depositional in nature. On the basis of our observations we conclude that braidplain morphology is controlled by low frequency, high magnitude flow events, possibly associated with glacial outburst floods. This is in marked contrast to process-form relationships in more dynamic alpine proglacial channels that are characterised by high rates of sediment transport and channel change.  相似文献   

11.
Glacial Lake Hind was a 4000 km2 ice-marginal lake which formed in southwestern Manitoba during the last deglaciation. It received meltwater from western Manitoba, Saskatchewan, and North Dakota via at least 10 channels, and discharged into glacial Lake Agassiz through the Pembina Spillway. During the early stage of deglaciation in southwestern Manitoba, part of the glacial Lake Hind basin was occupied by glacial Lake Souris which extended into the area from North Dakota. Sediments in the Lake Hind basin consist of deltaic gravels, lacustrine sand, and clayey silt. Much of the uppermost lacustrine sand in the central part of the basin has been reworked into aeolian dunes. No beaches have been recognized in the basin. Around the margins, clayey silt occurs up to a modern elevation of 457 m, and fluvio-deltaic gravels occur at 434–462 m. There are a total of 12 deltas, which can be divided into 3 groups based on elevation of their surfaces: (1) above 450 m along the eastern edge of the basin and in the narrow southern end; (2) between 450 and 442 m at the western edge of the basin; and (3) below 442 m. The earliest stage of glacial Lake Hind began shortly after 12 ka, as a small lake formed between the Souris and Red River lobes in southwestern Manitoba. Two deltas at an elevation of 450 were formed in this lake. At the same time, the Souris Lobe retreated far enough to allow glacial Lake Souris to expand farther north along the western side of the basin from North Dakota into what was to become glacial Lake Hind. Three deltas were built at an elevation above 460 m in the Canadian part of this proglacial lake. Continued ice retreat allowed the merger of glacial Lake Souris with the interlobate glacial Lake Hind to the east. Subsequent erosion of the outlet to the Pembina Spillway allowed waters in the glacial Lake Hind basin to become isolated from glacial Lake Souris, and a new level of glacial Lake Hind was established at 442 m, with 5 deltas built at this level by meltwater runoff from the west. Next, a catastrophic flood from the Moose Mountain uplands in southeastern Saskatchewan flowed through the Souris River valley to glacial Lake Souris, spilling into Lake Hind and depositing another delta. This resulted in further incision of the outlet (Pembina Spillway). A second flood through the Souris Spillway from glacial Lake Regina further eroded the outlet; most of glacial Lake Hind was drained at this time except for the deeper northern part. Coarse gravel was deposited by this flood, which differs from previous flood gravel because it is massive and contains less shale.  相似文献   

12.
黄河内蒙古不同河型段对洪水过程的响应特征   总被引：7，自引：0，他引：7  
黄河内蒙古河段的河道淤积近年来有所加重,一些人寄望于人造洪峰冲沙来降低河床。本文根据典型年份的最大洪水的流量—水位过程线变化特征,揭示黄河内蒙古不同河型段对这些洪水的响应机制,得出黄河内蒙古河段洪水的流量—水位过程线类型有线形、顺时针环线、逆时针环线3类单一关系和线形+逆时针环线、逆时针环线+线形、"8"字形+线形、嵌套状逆时针环线以及交叉线形5类复合关系。这些关系总体上可以反映洪水过程中河床是侵蚀还是沉积、何时侵蚀何时沉积、以及冲淤是否具有反复性。黄河内蒙古冲积性河段的河道对同一次洪水过程的响应沿程表现出分化现象,其中辫状河段以河床侵蚀下降为特征,弯曲河段以河床明显沉积升高为特征,而顺直河段以少量沉积或冲淤平衡为特征。不同河型段的河道对洪水的响应趋势与洪水动能的沿程减小相适应,洪峰含沙量沿程减小是该趋势的具体体现。人造洪峰冲沙难以逆转上述淤积趋势,对防治洪灾帮助有限。  相似文献   

13.
Northwestern California is prone to regional, high magnitude winter rainstorms, which repeatedly produce catastrophic floods in the basins of the northern Coast Ranges. Major floods on the Eel River in 1955 and 1964 resulted in substantial geomorphic changes to the channel, adjacent terraces, and tributaries. This study evaluated the changes and the effects of a moderate flood in 1997 through field observations and examination of aerial photographs that spanned from 1954 to 1996. The purpose was to document the nature and magnitude of geomorphic responses to these three floods and assess the rates and controls on the recovery of the Eel River and its tributaries. Channel widening from extensive bank erosion was the dominant geomorphic change along the lower Eel River during major floods. As a result of the 1964 flood, the largest amount of widening was 195 m and represented an 80% change in channel width. Channel narrowing characterized the periods after the 1955 and 1964 floods. More than 30 years after the 1964 flood, however, the river had not returned to pre-flood width, which suggests that channel recovery required decades to complete. A long recovery time is unusual given that the Eel River is located in an area with a “superhumid” climate and has an exceptionally high sediment yield. This long recovery time may reflect highly seasonal precipitation and runoff, which are concentrated in 3–5 months each winter. In contrast to the main stem of the Eel River, the dominant effects of floods on the tributaries of the Eel River were rapid aggradation of channel bed and valley floor followed by immediate downcutting. Dendrogeomorphic data, aerial photographs, and field observations indicate that thick wedges of gravel, derived largely from hillslope failures in upper reaches of the tributaries, are deposited at and immediately upstream of the mouths of tributaries as the stage of the Eel River exceeded that of the tributaries during major floods. In the waning stages of the flood, the tributaries cut through the gravel at a rate equal to the lowering of the Eel and generated unpaired terraces and nickpoints. The complete process of deposition and incision can occur within a few days of peak discharge. Although reworking of some sediment on the valley floor may continue for years after large floods, channel morphology in the tributaries appears to be a product of infrequent, high magnitude events. The morphology of the tributary channel also appears to be greatly influenced by the frequency and magnitude of mass wasting in headwater areas of small basins.  相似文献   

14.
Fluvial response to tectonic deformation is dependent on the amount and style of surface deformation and the relative size of the stream. Active folding in the New Madrid seismic zone (NMSZ) forms the Tiptonville dome, a 15-km long and 5-km wide surface fold with up to 11 m of late Holocene structural relief. The fold is crossed by streams of varying size, from the Mississippi River to small flood-plain streams. Fluvial response of these streams to repeated coseismic folding has only been preserved for the past 2.3 ka, since the Tiptonville meander of the Mississippi River migrated across the area forming the present flood plain. This surface comprises a sandy point-bar deposit locally overlain by clayey overbank and silty sand crevasse-splay deposits, an abandoned chute channel infilled with laminated sandy silt and silty clay, and an abandoned neck cutoff filled with a sandy cutoff bar and silty clay oxbow lake deposits.Dating various stream responses to coseismic folding has more tightly constrained the timing of earthquake events in the central NMSZ and provides a means of partitioning the deformation amount into individual seismic events. Three earthquakes have been dated in the Reelfoot Lake area, ca. A.D. 900, 1470, and 1812. The latter two earthquakes had large local coseismic deformation. Both of these events were responsible for numerous stream responses such as shifting depocenters, modification of Mississippi River channel geometry, and derangement of small streams. Overbank sedimentation ceased on the dome as it was uplifted above the normal flood stage, and sedimentation of crevasse-splay deposits from the Mississippi River, colluvium from the scarp, and lacustrine sediment accumulated in the adjacent Reelfoot basin. The much larger Mississippi River channel responded to uplift by increasing its sinuosity across the uplift relative to both upstream and downstream, increasing its width/depth ratio across and downstream of the uplift, and decreasing the width/depth ratio upstream of the uplift. Despite the size of the Mississippi River, it has not yet attained equilibrium since the latest uplift 190 years ago. Small channels that could not downcut through the uplift were filled, locally reversed flow direction, or formed a lake where they were dammed.Uplift and stream response to folding along the Tiptonville dome is less dramatic between 2.3 and 0.53 ka. During this interval, abandoned channel fill and overbank deposition across the dome suggests that it was not a high-relief feature. One earthquake event occurred during this interval (ca. A.D. 900), but coseismic stream response was probably limited to a slight aggradation of a small flood-plain stream.  相似文献   

15.
Over the past 150 years, major land use changes have occurred in the Stemple Creek Watershed in northern California that have caused erosion to move soils from the upland to the flood plain, stream channels, and the bay. The purpose of this study is to document the recent (1954 to present) sediment deposition patterns in the flood plain area adjacent to Stemple Creek using the 137Cesium technique. Sediment deposition ranged from 0.26 to 1.84 cm year−1 for the period from 1964 to 2002 with an average of 0.85±0.41 cm year−1. Sediment deposition rates were higher for the 1954 to 1964 period with a range of 0.31–3.50 cm year−1 and an average of 1.29±1.04 cm year−1. These data indicate that sediment deposition in the flood plain has decreased since the middle 1950s, probably related to reduction in row crop agriculture and an increase in pasturelands. This study shows that the flood plains in the Stemple Creek Watershed are a significant sink for the soils being eroded from the upland area. Given the significance of the flood plain for trapping eroded materials before they reach the stream channels or the bay, efforts need to be made to manage these flood plain areas to insure that they do not change and become a source rather than a sink for eroded materials as improved management practices on the upland areas reduce sediment input to the flood plain.  相似文献   

16.
新疆洪旱灾害与大尺度气候强迫因子的联系   总被引：13，自引：6，他引：7  
通过对新疆洪灾灾害历史资料和太阳黑子、ENSO事件年数据以及北大西洋涛动指数的分析,表明在太阳黑子极低值年或不活跃年,新疆易发生重大洪灾,而在太阳黑子低值年或相对不活跃年,新疆也易发生重大旱灾。近200a来,新疆重大洪旱灾害绝大部分都发生在太阳黑子的低谷时期。ENSO事件对新疆夏季降水的影响效应明显。因而该事件对新疆的洪旱灾害也产生了影响。对近50年来灾害统计资料分析显示,与拉尼娜年相比,在厄尔尼诺年新疆更易发生洪旱灾害。20世纪后半段新疆洪旱灾害指数与北大西洋涛动指数进行对比可以发现．夏季NAO指数与新疆洪水灾害之间存在大致上的反相关系。而冬季NAO指数与新疆干旱灾害之间存在比较明显的反相关系。  相似文献   

17.
Channel responses to flow depletions in the lower Duchesne River over the past 100 years have been highly complex and variable in space and time. In general, sand-bed reaches adjusted to all perturbations with bed-level changes, whereas the gravel-bed reaches adjusted primarily through width changes. Gravel-bed reaches aggraded only when gravel was supplied to the channel through local bank erosion and degraded only during extreme flood events.A 50% reduction in stream flow and an increase in fine sediment supply to the study area occurred in the first third of the 20th century. The gravel-bed reach responded primarily with channel narrowing, whereas bed aggradation and four large-scale avulsions occurred in the sand-bed reaches. These avulsions almost completely replaced a section of sinuous channel about 14 km long with a straighter section about 7 km long. The most upstream avulsion, located near a break in valley slope and the transition from a gravel bed upstream and a sand bed downstream, transformed a sinuous sand-bed reach into a braided gravel-bed reach and eventually into a meandering gravel-bed reach over a 30-year period. Later, an increase in flood magnitudes and durations caused widening and secondary bed aggradation in the gravel-bed reaches, whereas the sand-bed reaches incised and narrowed. Water diversions since the 1950s have progressively eliminated moderate flood events, whereas larger floods have been less affected. The loss of frequent flooding has increased the duration and severity of drought periods during which riparian vegetation can establish along the channel margins. As a result, the channel has gradually narrowed throughout the study area since the late 1960s, despite the occasional occurrence of large floods. No tributaries enter the Duchesne River within the study area, so all reaches have experienced identical changes in stream flow and upstream sediment supply.  相似文献   

18.
分布在华北北部山顶面上的“冰臼”,无论从地理环境、地貌演化,还是从海拔高度方面分析,都不是第四纪大冰盖的证据,而是古地貌面上流水侵蚀的遗迹。它分布在3个海拔高度不同的山地夷平面上和现代河流谷地内,与当时的地理环境和山地的地貌演化密切相关。未构成山地夷平面前准平原上的河流以河网密集的曲流河为主。随着准平原的抬升,河流也开始下切,下切的河流以溯源侵蚀为主。在源头地区的顺直型谷地中,一是河流流速较急,且挟有数量不多、颗粒较粗的砂砾石;二是源头地区大块石较多,容易形成涡流;三是河水不深,丰水期刚刚没过块石,枯水期块石露出水面。在质地不均的岩体(特别是花岗岩)中,挟带砂砾石的涡流在岩面上形成了壶穴。准平原及壶穴最后被抬升到山地的顶部,构成了山地夷平面。  相似文献   

19.
Holocene and post‐European settlement alluvial histories of three nested drainage basins were reconstructed from detailed litho‐ and chronostratigraphy of cut and fill terraces and flood‐plains in the upper Wollombi Brook catchment. Fernances Creek (13.8 km2) valley fill consisted of intercalated thin mud sheets deposited in ephemeral swamps and thick sand sheets deposited by discontinuous channels. Dairy Arm (39.8 km2) valley fill was more complex, with inset alluvial fills in the upper basin and overlapping vertically stacked fills in the lower basin. However, correlative lithostrati‐graphic units were not found on all tributaries. Furthermore, basal radiocarbon dates on the last inset fill of four tributaries did not overlap, allowing for plus or minus twice the standard deviation of the reported ages. Wollombi Brook (341 km2) valley fill was also complex, with longitudinally discontinuous units, most of which were not found in the two tributaries. Upstream late Holocene channel incision was coeval with downstream chain of ponds because sediment generated by incision was stored in the intervening valley. Historical channel incision occurred between 1838 and 1867 on Fernances Creek at a locally steeper section of valley floor during the period of peak population and frequent floods immediately after a road crossing was constructed, but coincided with a catastrophic flood on Dairy Arm (June 1949) and on Wollombi Brook (1927). Lack of correlative litho‐ and chronostrati‐graphic units plus out‐of‐phase post‐European incision indicate that stratigraphic complexity is a function of geomorphic complexity due to the operation of geomorphic thresholds and complex response.  相似文献   

20.
An 8 m core from the central plain of the Petit Lac d'Annecy, France, two floodplain cores, river bedload sediments and several hundred soil samples from the catchment have been studied using magnetic techniques. The soils, mainly developed on limestones and local glacial tills, show widespread magnetic enhancement with higher ferrimagnetic concentrations and contents of SP grains than found in the lake sediments. Some soils show significant concentrations of canted antiferromagnetic minerals (mainly haematite). Using magnetic quotient parameters the surface soils are classified into four mineralogical types. The lake and floodplain sediment properties over the past 6000 yrs can largely be explained by the erosion and deposition of these sources, with a smaller superimposed biogenic (magnetosomes) signal. Derived sediment-source linkages allow the construction of several hypotheses about geomorphological changes in the catchment system: (i) the long-term erosion of high altitude unweathered substrates has gradually increased towards the present day; (ii) the erosion of high altitude soils has increased within the last 1000 yrs, possibly during the period of the 'Little Ice Age'; (iii) shifts towards an increased erosion of surface lowland soil occurred ~2000 and 1000 yrs ago and may be linked to an accelerated accretion of floodplain overbank deposits; (iv) there has been a significant storage of surface soil within floodplains, which leads to an underestimation of the importance of soil erosion in the lake sediment records; (v) the sediment transported by high magnitude, low frequency flood events has shifted in source from high altitude soils before ~1000 cal. yr BP to lowland and mid-altitude free draining soils after ~1000 cal. yr BP.  相似文献   

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