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1.
Alpa Sridhar   《Geomorphology》2007,88(3-4):285-297
This paper attempts to quantify contemporary and palaeo-discharges and changes in the hydrologic regime through the mid–late Holocene in the alluvial reach of the arid Mahi River basin in western India. The occurrence of terraces and pointbars high above active river levels and change in the width/depth ratio can be regarded as geomorphic responses to changes in discharge. Discharge estimates are made based on the channel dimensions and established empirical relations for the three types of channels: mid–late Holocene, historic (the channel that deposited extensive pointbars above the present-day average flow level) and the present ones. The bankfull discharge of the mid–late Holocene channel was  55 000 m3 s− 1 and that of the historic channel was  9500 m3 s− 1, some  25 times and  5 times greater than that of the present river (2000 m3 s− 1), respectively. Since the mid–late Holocene, the channel form has changed from wide, large-amplitude meanders to smaller meanders, and decreases in the width/depth ratio, unit stream power and the bed shear stresses have occurred. It can be inferred that there has been a trend of decreasing precipitation since the mid–late Holocene.  相似文献   

2.
Using the Ar–Ar technique, we have obtained the first numerical dates for the Pleistocene volcanism along the valley of the River Ceyhan in the Düziçi area of southern Turkey, in the western foothills of the Amanos Mountains. Our six dates indicate a single abrupt episode of volcanism at  270 ka. We have identified a staircase of 7 fluvial terraces, at altitudes of up to  230 m above the present level of the Ceyhan. Using the disposition of the basalt as an age constraint, we assign these terraces to cold-climate stages between marine oxygen isotope stages 16 and 2, indicating rates of fluvial incision, equated to surface uplift, that increase upstream through the western foothills of this mountain range at between 0.25 and 0.4 mm a− 1. Extrapolation of these uplift rates into the axis of the range suggests that the entire  2300 m of present-day relief has developed since the Mid-Pliocene, a view that we confirm using numerical modelling. Since  3.7 Ma the Amanos Mountains have formed a transpressive stepover along the northern part of the Dead Sea Fault Zone, where crustal shortening is required by the geometry. Using a physics-based technique, we have modelled the overall isostatic response to the combination of processes occurring, including crustal thickening caused by the shortening, erosion caused by orographic precipitation, and the resulting outward flow of mobile lower-crustal material, in order to predict the resulting history of surface uplift. This modelling suggests that the effective viscosity of the lower crust in this region is in the range  1–2 × 1019 Pa s, consistent with a Moho temperature of  590 ± 10 °C, the latter value being in agreement with heat flow data. This modelling shows that the nature of the active crustal deformation is now understood, to first order at least, in this key locality within the boundary zone between the African and Arabian plates, the structure and geomorphology of which have been fundamentally misunderstood in the past.  相似文献   

3.
Rates of sediment supply by landsliding to an alluvial channel in a small catchment in central Switzerland were estimated over an 11-month study period. Fluvial sediment transport in the channel is independently monitored at the upstream and downstream ends of the study reach, yielding a unique opportunity to quantitatively compare the volume of sediment supplied to the channel with the volume in fluvial transport. Landslide-derived sediment discharge to the channel was greatest during the winter and spring months, while most of the fluvial sediment transport occurred during short, intense summer storms. Approximately 98 m3 of sediment was delivered directly to the study reach by landsliding,  80 m3 was transported into the reach from upstream, and  70 m3 was transported out of the reach. Thus, there was a net accumulation of  100 m3 of sediment during the 11-month study. Decadal-scale channel aggradation was independently assessed by comparing channel longitudinal profiles in 1993 and 2004. During this 11-year period, a total of  1500 m3 of sediment has accumulated in the study reach. Aggradation has occurred largely in two broad zones that correspond with both the locations of major landslide complexes and reaches of high channel slope, indicating that hillslope sediment input left an imprint on the morphology of the channel bed that appears to be stable over at least decadal time scales.  相似文献   

4.
New estimates on the Quaternary slip rate of the active transform margin of North Anatolia are provided. We investigated the area struck by a Mw 7.1 earthquake on the 12th of November 1999 that ruptured the Düzce Fault segment of the North Anatolian Fault. In order to analyze the spectacular tectonically driven cumulative landforms and the drainage pattern settings, we carried out a 1:25,000-scale geological and geomorphological mapping along the fault trace. We reconstruct and describe, as offset geomorphic markers, right-hand stream deflections and fluvial terraces inset into alluvial fan deposits. Radiocarbon dating indicates that  100 m stream deflections were built up by the last  7000 yrs of fault activity. Conversely, two documented and correlated Late Pleistocene fluvial terraces are horizontally offset by  300 and  900 m, respectively. These were dated by means of Optically Stimulated Luminescence (OSL) to  21 ka BP and 60 ka BP. Assuming a constant rate of deformation for the Düzce Fault, ages and related offsets translate to consistent slip rates that yield an average slip rate of 15.0 ± 3.2 mm/yr for the last 60 ka. Thus, the Düzce Fault importantly contributes to the North Anatolian margin deformation, suggesting a present-day partitioning of displacement rates with the Mudurnu Fault to the south and confirming its important role in the seismic hazard of the area.  相似文献   

5.
Previous studies of chemical weathering rates for soil developed on glacial moraines generally assumed little or no physical erosion of the soil surface. In this study, we investigate the influence of physical erosion on soil profile weathering rate calculations. The calculation of chemical weathering rates is based on the assumption that soil profiles represent the integrated amount of weathering since the time of moraine deposition. The weathering rate of a surface subjected to denudation is the sum of the weathering loss from the existing soil profile added to the weathering loss in the material removed by denudation, divided by the deposition age. In this study, the amount of weathered material removed since moraine deposition is calculated using the denudation rate estimated from cosmogenic nuclide data and the deposition age of the moraine. Weathering rates accounting for denudation since moraine deposition are compared to weathering rates based on the assumption of no physical erosion and on the assumption of steady-state denudation for the Type Pinedale moraine ( 21 ka) and the Bull Lake-age moraine ( 140 ka) in the Fremont Lake Area (Wind River Mountains, Wyoming, USA). The total weathering rates accounting for denudation are 8.15 ± 1.05 g(oxide) m− 2 y− 1 for the Type Pinedale moraine and 4.78 ± 0.89 g(oxide) m− 2 y− 1 for the Bull Lake-age moraine, which are  2 to 4 times higher, respectively, than weathering rates based on the assumption of no physical erosion. The weathering rates based on denudation since moraine deposition are comparable or smaller than weathering rates assuming steady-state denudation. We find the assumption of steady-state denudation is not valid in depositional landscapes with young deposition ages or slow denudation rates. The decrease in weathering rates over time between the Type Pinedale and Bull Lake-age soils that is observed in the case of no physical erosion is decreased when the influence of denudation on the total weathering rates is taken into account. Fresh unweathered material with high reactive mineral surface area is continuously provided to the surface layer by denudation diminishing the effect of decreasing weathering rate over time.  相似文献   

6.
The impact of large twentieth century floods on the riparian vegetation and channel morphology of the relatively wide anabranching and braided Nahal Arava, southern Israel, was documented as part of developing tools to (a) identify recent large floods, (b) determine these flood's respective magnitudes in alluvial ungauged streams, and (c) determine long-term upper bounds to flood stages and magnitudes. Along most of its course Nahal Paran, a major tributary that impacts the morphology, floods and sediments of Nahal Arava at the study reach, is a coarse-gravel, braided ephemeral stream. Downstream of the Arava–Paran confluence, aeolian and fluvial sand delivered from eastern Arava valley alters the channel morphology. The sand has accreted up to 2.5 m above the distinct current channels, facilitating the recording of large floods. This sand enhances the establishment of denser riparian vegetation (mainly Tamarix nilotica and Haloxylon persicum) that interacts with floods and affects stream morphology. A temporal association was found between specific floods recorded upstream and tree-ring ages of re-growth of flood-damaged tamarix trees (‘Sigafoos trees’) in the past 30 years. This association can be utilized for developing a twentieth century flood chronology in hyperarid ungauged basins in the region. The minimum magnitude of the largest flood that covered the entire channel width, estimated from flood deposits, is approximately 1700–1800 m3s− 1. This is a larger magnitude than the largest gauged flood of 1150 m3s− 1 that occurred in 1970 about 30 km upstream in Nahal Paran. Our estimation agrees with flood magnitude estimated from the regional envelope curve of the largest floods. Based on Holocene alluvial stratigraphy and OSL dating in the study reach we also conclude that flood stages did not reach the late Holocene ( 2.2 ka) surface and therefore we estimate a non-exceedance upper bound of  2000 m3s− 1 flood magnitudes for Nahal Arava during that interval. This study indicates that in unfavorable areas the combination of hydrology, fluvial morphology and botanic evidence can increase our understanding of ungauged basins and give information crucial for hydrology planning.  相似文献   

7.
Recent improvements in understanding glacial extents and chronologies in the Wasatch and Uinta Mountains and other mountain ranges in the western U.S. call for a more detailed approach to using glacier reconstructions to infer paleoclimates than commonly applied AAR-ELA-ÄT methods. A coupled 2-D mass balance and ice-flow numerical modeling approach developed by [Plummer, M.A., Phillips, F.M., 2003. A 2-D numerical model of snow/ice energy balance and ice flow for paleoclimatic interpretation of glacial geomorphic features. Quaternary Science Reviews 22, 1389–1406] allows exploration of the combined effects of temperature, precipitation, shortwave radiation and many secondary parameters on past ice extents in alpine settings. We apply this approach to the Little Cottonwood Canyon in the Wasatch Mountains and the Lake Fork and Yellowstone Canyons in the south-central Uinta Mountains. Results of modeling experiments indicate that the Little Cottonwood glacier required more precipitation during the local Last Glacial Maximum (LGM) than glaciers in the Uinta Mountains, assuming lapse rates were similar to modern. Model results suggest that if temperatures in the Wasatch Mountains and Uinta Mountains were  6 °C to 7 °C colder than modern, corresponding precipitation changes were  3 to 2× modern in Little Cottonwood Canyon and  2 to 1× modern in Lake Fork and Yellowstone Canyons. Greater amounts of precipitation in the Little Cottonwood Canyon likely reflect moisture derived from the surface of Lake Bonneville, and the lake may have also affected the mass balance of glaciers in the Uinta Mountains.  相似文献   

8.
At least one large, late Pleistocene flood traveled into the Owyhee River as a result of a rise and subsequent outburst from pluvial Lake Alvord in southeastern Oregon. Lake Alvord breached Big Sand Gap in its eastern rim after reaching an elevation of 1292 m, releasing 11.3 km3 of water into the adjacent Coyote Basin as it eroded the Big Sand Gap outlet channel to an elevation of about 1280 m. The outflow filled and then spilled out of Coyote Basin through two outlets at 1278 m and into Crooked Creek drainage, ultimately flowing into the Owyhee and Snake Rivers. Along Crooked Creek, the resulting flood eroded canyons, stripped bedrock surfaces, and deposited numerous boulder bars containing imbricated clasts up to 4.1 m in diameter, some of which are located over 30 m above the present-day channel.Critical depth calculations at Big Sand Gap show that maximum outflow from a 1292- to 1280-m drop in Lake Alvord was  10,000 m3 s− 1. Flooding became confined to a single channel approximately 40 km downstream of Big Sand Gap, where step-backwater calculations show that a much larger peak discharge of 40,000 m3 s− 1 is required to match the highest geologic evidence of the flood in this channel. This inconsistency can be explained by (1) a single 10,000 m3 s− 1 flood that caused at least 13 m of vertical incision in the channel (hence enlarging the channel cross-section); (2) multiple floods of 10,000 m3 s− 1 or less, each producing some incision of the channel; or (3) an earlier flood of 40,000 m3 s− 1 creating the highest flood deposits and crossed drainage divides observed along Crooked Creek drainage, followed by a later 10,000 m3 s− 1 flood associated with the most recent shorelines in Alvord and Coyote Basins.Well-developed shorelines of Lake Alvord at 1280 m and in Coyote Basin at 1278 m suggest that after the initial flood, postflood overflow persisted for an extended period, connecting Alvord and Coyote Basins with the Owyhee River of the Columbia River drainage. Surficial weathering characteristics and planktonic freshwater diatoms in Lake Alvord sediment stratigraphically below Mt. St. Helens set Sg tephra, suggest deep open-basin conditions at  13–14 ka (14C yr) and that the flood and prominent shorelines date to about this time. But geomorphic and sedimentological evidence also show that Alvord and Coyote Basins held older, higher-elevation lakes that may have released earlier floods down Crooked Creek.  相似文献   

9.
Studies on the geomorphological evolution of the South American passive margin have been based on the pediplanation model, which predicts that its morphology is a response to regional uniform uplift and concomitant development of erosion surfaces. We combined remote sensing, geological mapping, lithostratigraphic and facies analyses, and luminescence dating in the Cariatá trough, northeastern Brazil, in order to determine how brittle tectonics and climate influenced colluviation and the shaping of local landforms in the Quaternary. Our work indicates that Cariatá is an asymmetrical trough  40 km long,  25 km wide, 250–550 m deep, and delimited by ENE–WSW-trending faults to the north and south. We recognized an ENE–WSW-oriented compression related to a strike-slip faulting regime, which corresponds to the present-day stress field in the region. This faulting event led to the deposition of colluvial fans, shed from adjacent uplifted crustal blocks, in a tectonically controlled depression. The colluvial succession is  45 m thick and presents two facies assemblages that filled the southern and, in particular, the northern borders of the trough: non-cohesive debrisflow and mudflow deposits. Optically stimulated luminescence dates of the sedimentary infill yielded ages at 224–128 ka and 45–28 ka, dominated by debrisflow and mudflow deposits, respectively. These ages may be over-estimated due to poor bleaching of colluvium, but they and our field data suggest that the margins of the trough were tectonically uplifted and eroded twice in the Late Pleistocene. The spasmodic colluvial accretion reflects the occurrence of high-magnitude, low-recurrence episodes probably associated with climate shifts in a semi-arid hillslope system. It follows that the present-day low-lying piedmont in which the Cariatá trough occurs is a juxtaposition of surfaces of various ages. This trough may have counterparts across the region. These conclusions do not corroborate the application of the cyclical pediplanation hypothesis in the area.  相似文献   

10.
In catchments adjacent to the Great Barrier Reef World Heritage Area in Queensland, Australia, there is a growing concern that sediments and nutrients being exported from the land are having a detrimental effect on coral reef communities. There is a need to determine the processes and rates of erosion from the major land use types, so that management intervention can be initiated to reduce sediment yields where required. This paper presents a sediment budget for Weany Creek, a 13.5 km2 grazed semi-arid sub-catchment of the Burdekin River catchment, Australia. A range of field methods was used to measure erosion from hillslopes, gullies and stream banks, as well as identify the amount of sediment being deposited and remobilised on the bed of gullies and the stream network. The data suggests that at least during drought conditions, the primary erosion source in this catchment is gully erosion. However, the largest source of sediment in the budget is actually associated with the remobilisation of in-channel sediment stores. Overall, the sediment budget is comprised of  81% coarse material and 19% fine sediment and an agreement between the fine sediment yield estimated in the sediment budget and the yield measured at the catchment outlet is within 10%. The total sediment yield estimated for this catchment is  4205 t yr− 1 and is much lower than expected for a catchment of this size. This may reflect the drought conditions during the measurement period; however, there is also the possibility that the primary erosion sources have been exhausted, and the rates of sediment loss may be much lower now than they may have been in the past. Nonetheless, the results show that stored sediment, which may have been deposited in the channel many decades ago, is an important contributor to end of catchment sediment yields and warrants further investigation.  相似文献   

11.
Horizontal and vertical zones of influence for root systems of four Mojave Desert shrubs were characterized using 32P as a nutrient tracer. Larrea tridentata's horizontal zone of influence was sparse near the plant's stem base, with a maximum probability of accessing 32P (Pmax) of 41%. However, its horizontal zone of influence extended beyond 5 m, and the distance from the stem base at which the probability of accessing 32P was half Pmax (L503 m) was significantly greater than the other three shrubs. Ambrosia dumosa's zone of influence was dense near the plant's stem base (Pmax78%), but was rare at distances >2 m (L501 m). Zones of influence for Lycium andersonii and Lycium pallidum were intermediate between those of L. tridentata and A. dumosa. For vertical zones of influence, L. tridentata was more likely to obtain 32P from 5 m soil depths than A. dumosa, but L. pallidum was not significantly different from either A. dumosa or L. tridentata. Horizontal zones of influence did not change with treatments that altered soil water and nitrogen availability, but vertical zones of influence increased with a flood irrigation treatment that increased water availability to 5 m soil depth. These differences among species likely reflect compromises between their shoot growth strategies and their need to acquire spatially and temporally limited soil resources, especially through competitive interactions.  相似文献   

12.
Nicola J. Litchfield   《Geomorphology》2008,99(1-4):369-386
In order to make robust predictions of future coastal processes and hazards, historical rates of coastal processes such as coastal erosion need to be put into a long-term (Holocene) context. In this study a methodology is proposed that uses fluvial terraces to construct longitudinal profiles which can be projected offshore to infer paleo-coastline positions. From these positions, an average Holocene coastal erosion rate can be calculated. This study also shows how constraints can be placed on sea level changes and Late Pleistocene uplift rates using fluvial terraces, and by assuming the latter has been constant since  55–37 ka, these constraints feedback into the coastal erosion rate calculations. For the northwestern Hawke Bay (North Island, New Zealand) coastline, Late Pleistocene uplift rates of 0.6 ± 0.2, 0.6 ± 0.2, and − 0.1 ± 0.1 (i.e., stable or subsiding) mm/yr have been determined for the Waikari, Mohaka, and Waihua River mouths, respectively. These rates are consistent with previous interpretations of subsidence to the northeast and uplift being the result of regional, subduction-related processes. A Holocene coastal erosion rate of 0.5 ± 0.1 m/yr was determined for the Waikari River mouth, which is at the higher end of the calculated historical ( 1880–1980) rates (0.02–0.5 m/yr). If this difference is significant, then two possible reasons for this difference are: (i) the historical rate is affected by events such as the 1931 Napier earthquake, and (ii) the Holocene rate is the average of a steadily declining rate over the last 7.3 ka.  相似文献   

13.
We have investigated secondary influences on the noble gas budget in rim and interior pairs of three Martian meteorites from Antarctica: the lherzolitic shergottites Y000027 and Y000097, and the nakhlite Y000593. Three factors have been found to influence the original Martian noble gas budget: shock metamorphic overprint, cosmic irradiation, and terrestrial weathering. The 3He/4He ratio of the shergottites is between 0.189 and 0.217, which indicates almost complete loss of radiogenic 4He. This is expected from the high shock pressure observed in the shergottite samples. The concentration of 4He in these shergottite samples ranges from 33.8 to 39.4 × 10−8 ccSTP/g. 22Ne in the shergottites is on the order of 14 × 10−9 ccSTP/g. The nakhlite has 800 × 10−8 ccSTP/g 4He and 26 × 10−9 ccSTP/g 22Ne. An indication for solar cosmic ray contribution to the neon budget can be found in the shergottites. As Y000027 and Y000097 are reported to be paired we conclude the cosmic ray exposure (CRE) age T(3+21) of this shergottite to be 4.41 ± 0.54 Ma. For the nakhlite Y000593 T(3+21) is 11.8 ± 0.3 Ma. Heavy noble gas concentrations show large differences between rim and interior samples with the rim samples having 1.3–2.9, 1.7–38, and 1.4–20 times as much 36Ar, 84Kr, and 132Xe, respectively. The enrichment of heavy noble gases in the rim samples indicates severe terrestrial contamination. The relation between 129Xe/132Xe and 84Kr/132Xe in the rim samples shows that the incorporation mechanism caused elemental fractionation of Kr and Xe to the extent that in the Y000027 shergottite samples any Martian signature is completely masked by terrestrial contamination, if the total is taken. Only the 1400 °C steps show clear evidence for Martian atmosphere. The Y000593 nakhlite interior sample, on the other hand, shows low 84Kr/132Xe in relation to 129Xe/132Xe, which is characteristic for fractionated Martian atmosphere observed in nakhlites.  相似文献   

14.
We analyze remarkable examples of the large ( 10,000 km2) and local-scale ( 100 km2) landscape forms related to Late Cenozoic geomorphologic evolution of the Andean forearc region in the Southern Atacama Desert. We also consider the continental sedimentary deposits, so-called “Atacama Gravels”, which are related to the degradation of the landscape during the Neogene. Our analysis integrates 1:50,000 field cartography, Landsat TM images observations,  1:1000 sedimentary logging data, and 50 m horizontal resolution topographic data to reconstruct the Late Cenozoic geomorphologic evolution of this region and discuss the factors that control it, i.e., Miocene aridification of the climate and Neogene Central Andean uplift. We determine that the Precordillera was already formed in the Oligocene and most of the present-day altitude of the Precordillera was reached before that time. Afterward, five episodes of geomorphologic evolution can be differentiated: (1) the development of an Oligocene deep incised drainage system cutting the uplifted Precordillera (up to 2000 m of vertical incision) and connecting it to the Ocean; followed by (2) the infilling of deep incised valleys by up to 400 m of Atacama Gravels. This infill started in the Early Miocene with the development of fluvial deposition and finished in the Middle Miocene with playa and playa lake depositions. We propose that playa-related deposition occurs in an endorheic context related to tectonic activity of the Atacama Fault System and Coastal Cordillera uplift. However, the upward sedimentologic variation in the Atacama Gravels evidences a progressive aridification of the climate. Subsequently, we have identified the effects of the Middle–Upper Miocene slow tectonic deformation: the Neogene Andean uplift is accommodated by a tilting or flexuring of the inner-forearc (Central Depression and Precordillera) related to some hundreds of meters of uplift in the Precordillera. This tilting or flexuring results in (3) the Middle Miocene re-opening of the valley network to the Pacific Ocean. Upper Miocene aridification, from arid to hyperarid, induces alluvial fans backfilling in the Central Depression (4) resulting in up to 50 m of Atacama Gravel deposition. Finally, in response to an increase in the rate of tilting, a new phase of vertical incision (up to 800 m in the Precordillera) allows the development of the canyon that crosses the forearc (5).  相似文献   

15.
We present a study on the impact of litho-structural setting and neotectonic activity on meso- and macro-scale relief production in Alpine areas. The topography of the high alpine Triglav Lakes Valley, NW Slovenia, was studied by means of detailed mapping and stratigraphic study of the valley. The Triglav Lakes Valley is characterised by a generally asymmetric transverse (E–W) profile: a very steep eastern slope, a relatively flat valley and a relatively gentle western slope. On the transverse profile the valley floor is essentially flat, gently dipping towards the east. In the longitudinal cross-section, however, the valley floor is marked by sharply-defined fault blocks extending in a W–E to NW–SE direction. Additionally, the highest block (elevations  2100 m) is in the northern part of the valley, the lowest (elevations  1600 m) in the southern part of the valley. Our research shows that the Triglav Lakes Valley directly represents the topographic expression of Paleogene–Neogene thrusting and faulting, having recorded the following geomorphologic evolutionary stages: 1. an Oligocene to early Miocene W-vergent thrusting phase, with steep W-facing slopes of the eastern part of the valley directly representing the thrusting front; and 2. a Neogene-to-present strike–slip faulting in NNE–SSW direction with two bifurcating right-lateral strike–slip systems. We show that the Triglav Lakes Valley almost perfectly mimics the wedge-shaped damage zone located between these faults.  相似文献   

16.
Flux and fate of Yangtze River sediment delivered to the East China Sea   总被引：57，自引：0，他引：57  
Numerous cores and dating show the Yangtze River has accumulated about 1.16 × 1012 t sediment in its delta plain and proximal subaqueous delta during Holocene. High-resolution seismic profiling and coring in the southern East China Sea during 2003 and 2004 cruises has revealed an elongated ( 800 km) distal subaqueous mud wedge extending from the Yangtze River mouth southward off the Zhejiang and Fujian coasts into the Taiwan Strait. Overlying what appears to be a transgressive sand layer, this distal clinoform thins offshore, from  40 m thickness between the 20 and 30 m water depth to < 1–2 m between 60 and 90 m water depth, corresponding to an across shelf distance of less than 100 km. Total volume of this distal mud wedge is about 4.5 × 1011 m3, equivalent to  5.4 × 1011 t of sediment. Most of the sediment in this mud wedge comes from the Yangtze River, with some input presumably coming from local smaller rivers. Thus, the total Yangtze-derived sediments accumulated in its deltaic system and East China Sea inner shelf have amounted to about 1.7 × 1012 t. Preliminary analyses suggest this longshore and across-shelf transported clinoform mainly formed in the past 7000 yrs after postglacial sea level reached its mid-Holocene highstand, and after re-intensification of the Chinese longshore current system. Sedimentation accumulation apparently increased around 2000 yrs BP, reflecting the evolution of the Yangtze estuary and increased land erosion due to human activities, such as farming and deforestation. The southward-flowing China Coastal Current, the northward-flowing Taiwan Warm Current, and the Kuroshio Current appear to have played critical roles in transporting and trapping most of Yangtze-derived materials in the inner shelf, and hence preventing the sediment escape into the deep ocean.  相似文献   

17.
Increases in runoff and erosion after wildfires are often attributed to the development of hydrophobic soils. The potential for increased overland flow depends on the spatial contiguity of the hydrophobicity as well as its overall strength, but there is limited information on the spatial variability of soil hydrophobicity. We conducted spatially intensive hydrophobicity measurements in 225 m2 and 1 m2 plots in forested areas of Montana and Colorado burned at moderate to high severity, and in unburned control plots. Both the burned and unburned 225 m2 plots contained 10–23 hydrophobic soil patches in which hydrophobicity was strongest at the surface and declined rapidly with depth. The hydrophobic patches were closer together and up to 3 times larger in the burned plots. Consequently, 19% to 76% of the burned plots were hydrophobic compared to just 11% of the unburned plots. In five of the six burned plots, the patches were not laterally connected, suggesting that in most cases Hortonian overland flow generated from hydrophobic patches will infiltrate near its point of origin. The 1 m2 plots were smaller than most of the hydrophobic patches, so they did not capture the spatial characteristics of soil hydrophobicity. Characterization of the spatial variability of soil hydrophobicity should be based on measurements conducted at  1 m intervals across areas of > 100 m2. Due to the patchiness of soil hydrophobicity at the 100 to 101 meter scale, overland flow measurements in small ( 1 m2) plots may overestimate the magnitude and variability of runoff from burned catchments.  相似文献   

18.
The Lake Lahontan basin has been the site of numerous pluvial lakes during the Pleistocene. We address the question of whether or not the highest remnant shoreline features around the perimeter of the lake were produced during the most recent Sehoo highstand (13 ka), the penultimate Eetza highstand (140–280 ka), or both. To do so, we document surficial characteristics, morphologic preservation, and soil development on multiple Sehoo beach barriers in the Jessup embayment to define the range in characteristics displayed by latest Pleistocene beach features. Sehoo barriers generally exhibit original constructional morphology that has been little modified by erosion. Soils developed on Sehoo barriers are generally thin and weakly developed and are strongly influenced by the introduction of eolian fines into the predominately clast-supported coarse beach gravels. Similar observations from 13 other highstand barriers and from seven older-than-latest Pleistocene paleosols located around the basin form the basis for a regional comparison. Based on similar characteristics, including the degree of morphologic preservation and weak soil development, we conclude that the widespread and nearly continuous high shoreline around the perimeter of Lake Lahontan dates from the most recent major lake cycle in all areas except in the Walker Lake subbasin. In the Walker Lake subbasin, isolated early to middle Pleistocene lacustrine outcrops and landforms are elevated as much as 70 m above the late Pleistocene limit, but are differentiated by their degraded form and lack of continuity around the subbasin. Similar unambiguous landforms were not observed elsewhere and at similar elevations in the northern subbasins of Lake Lahontan.  相似文献   

19.
The near-surface layer of peatlands of the Peak District, southern Pennines, UK, is severely contaminated with atmospherically deposited Pb. Contemporary catchment soil Pb inventories at Upper North Grain and Torside Clough reveal that  23% and  54%, respectively, of the potential store of Pb in each catchment has been lost through erosion of the contaminated near-surface peat layer. Soil Pb inventories and the Pb content of suspended sediments reveal that, in both catchments, the main mechanism for contemporary particulate Pb export is gully erosion. Historical sheet erosion on bare peat flats at Torside Clough has released significant quantities of Pb into the fluvial system, triggered by the exposure of the near-surface peat during an accidental wildfire in 1970. Up to 32% of the total Pb export from the catchment may have been released during a discrete erosion event soon after the wildfire. Accidental wildfires and the subsequent release of highly contaminated peat into the southern Pennine fluvial system may increase under predicted climate change scenarios.  相似文献   

20.
Earthworks of assumed age and their initial and current morphologies provide an ideal basis for developing and testing models for long-term landform erosion. Inca agricultural terraces abandoned at  1532 A.D in the drylands of southern Peru may be used to document morphological changes since the abandonment. The objective of this research is to determine the erosion pattern and process to estimate the erosion rate.The development of rills and channels on the Inca agricultural terraces is evidence for erosion by wash processes on slopes where the anchoring effect of vegetation is absent and loose material is available for removal. The pattern and amount of erosion from 1532–2005 A.D. is estimated by comparing elevation models of the observed morphology and reconstructed models of the original morphology of the Inca terraces. The results show that in areas of sediment accumulation surface elevation increased up to 0.5 m. Elevation lowering on the terrace treads was 0.7 m at maximum, and a temporally and spatially averaged lowering rate was 0.094 mm yr− 1. This gives insights about how the rate of erosion occurs on currently disturbed lands in arid environments where soil resources are scarce and lands are prone to desertification.  相似文献   

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