黄河下游2300年以来沉积速率的变化

采用沉积学方法提取沉积速率的记录,利用历史文献研究的方法提取气候与人类活动变化的信息,对黄河下游2300年以来的沉积速率的变化及其与气候、植被及人类活动的关系进行了研究,揭示黄河流域地貌系统对气候变化及人类活动所导致的土地覆被、土地利用方式变化的响应过程。研究表明,黄河流域地貌系统中的下游沉积带对于中游侵蚀带的响应是灵敏的,流域植被和土地利用方式的变化,是导致下游河道沉积加速的主导因素;这种变化主要取决于人类活动,同时与气候的变化也有一定关系。在沉积加速的过程中,公元7世纪至10世纪以及18世纪中叶以来,表现为两个突变时期。     

Long-term rates of sediment accumulation in the Tonle Sap,Cambodia: a threat to ecosystem health?

The Tonle Sap is an invaluable resource for the people of Cambodia, and is a globally significant ecological site. Much of the literature published on the environmental health and management of the lake suggests that its sustainability is threatened by accelerated rates of infilling related to unfettered land-use in the catchment. However, the evidence for supposedly increased sedimentation rates is confused and contradictory, and there have been very few studies that have attempted to actually measure the rate at which sediment is accumulating on the bed of the lake. This research measures long-term sedimentation rates in the Tonle Sap in an attempt to provide an empirically derived baseline against which more recent and allegedly accelerated sedimentation rates can be compared. Radiometric dating techniques (¹⁴C, ²¹⁰Pb, ¹³⁷Cs) reveal that sediment has been accumulating in the lake at an average long-term rate of less than 1 mm per year. There is a clear and consistent decline in the rate of sediment accumulation in the lake basin over time, with the highest rates recorded in the middle to early Holocene epoch. It is recommended that research be directed toward measuring changes in the morphology of the lake margin over time, rather than focus on changes in the bathymetry of the lake basins themselves, which are here shown to be effectively stable in terms of sediment accumulation.     

A record of accelerated erosion in the recent sediments of Blelham Tarn in the English Lake district

Two frozen cores from Blelham Tarn were subsampled and measured using mineral magnetic, loss-on-ignition (LOI), radiometric, granulometric and diatom analyses. A detailed chronology was established using varves, radioisotopes and diatoms. This has enabled an accurately dated reconstruction of sedimentation over the past forty years. Despite a large increase in lake productivity, evidence suggests that the observed exponential increase in sedimentation rates can be attributed to erosion within the catchment. The predominant sediment source has been identified as surface soil. A comparison between the trend of accelerated sedimentation and the record of increased sheep stocking density for the area within which the most of the catchment lies, as well as observations of contemporary surface processes within the catchment, both suggest that much of the recent erosion is a direct response to increased pressure from sheep grazing.     

PHYSICAL GEOGRAPHY IN SOUTHEAST ASIA: PAST THEMES, CURRENT STATUS AND FUTURE TRENDS

Our knowledge regarding the physical geography of Southeast Asia is incomplete to a surprising extent. This paper reviews the past research, describes the ongoing work, and attempts to identify the future trends. Coverage of such a large area, even for the last 25 years, requires study of publications in several languages and coping with literature which is not easily available. Therefore certain topics. which are prominent in current research and likely to remain so in the future were chosen and reviewed in detail. Such topics include studies related to active plate margin features; the Pleistocene in Southeast Asia; erosion and sedimentation rates; rainforest; river systems; karst in Southeast Asia; coastal geomorphology; and the urban environment. Environmental studies are rapidly gaining importance chiefly because of (a) the destruction of the natural vegetation and the associated loss of biodiversity and (b) the growing problems due to accelerated erosion and sedimentation This trend is likely to continue with accelerated destruction of forests, development of coastal areas, and urbanization of the landscape The direction of research in physical geography of Southeast Asia has been determined by a combination of individual research interest, governmental priorities, and international expectations     

Contrasting sedimentation rates in Lake Illawarra and St Georges Basin,two large barrier estuaries on the southeast coast of Australia

Sedimentation rates over the last 100 years within two lagoons on the southeast coast of Australia, Lake Illawarra and St Georges Basin, have been quantified to determine the effects of catchment land use change and native vegetation clearance on infill rates, and spatial variations in the rate at which the estuaries have filled. Both catchments have similar lake and catchment area but have experience different degrees of modification due to land clearing for agriculture practices, urbanisation and industrialisation. Results indicate that in the heavily modified catchment of Lake Illawarra sedimentation rates close to fluvial deltas can be in excess of 16 mm/year, and between 2 and 4 mm/year in the adjacent central basin. This is approximately an order of magnitude greater than the pre-European rates. In contrast, at St Georges Basin, where the catchment has experienced much less modification, sedimentation rates in the central basin appear to have remained close to those prior to European settlement. However, sedimentation rates in the urbanized margin of St Georges Basin are relatively high (up to 4.4 mm/year). This rapid modern sedimentation in the margin of the estuarine embayments has been detected in several other estuaries in the region. However the degree of sedimentation within the bay-head deltas, and more significantly in the central basin appears proportional to the degree clearance of native vegetation (forest) in the catchment, urban expansion and development of heavy industry in the respective catchment areas.     

近期长江北支口门圆陀角附近潮滩地貌动态变化

圆陀角位于长江北支岸线与江苏海岸线的交会处,独特的互花米草潮滩、淤泥质光滩环境和复杂的河海沉积动力,决定了潮滩地貌对海洋环境变化的响应具有敏感性。由于大规模的围垦,圆陀角附近过去40年来海岸线向东推进了6km。根据2006年以来多次的野外调查和室内粒度与钻孔岩芯的¹³⁷Cs分析, 2006年以来圆陀角附近潮滩淤积明显加强,由¹³⁷Cs时标估算的互花米草滩多年平均沉积速率为2.3cm/a, 2006~2008年观测到的互花米草滩淤积速率>4cm/a,粉砂淤泥质光滩的淤积速率更高;圆陀角风景区内互花米草滩前缘陡坎在风暴潮影响下侵蚀后退,并因粉砂淤泥质光滩的快速淤长而消亡,圆陀角附近潮滩地貌动态是对人类围垦活动、风暴潮与潮汐海洋动力的综合响应,互花米草与光滩快速淤积是近期圆陀角附近潮滩地貌演化的主要特点。     

气候变化对中亚天山山区水资源影响研究

本文结合资料分析和文献阅读,对全球气候变化背景下的中亚天山山区水文、水资源变化进行了讨论分析。在全球升温滞缓背景下,中亚天山山区在过去的10余年,气温却一直处于高位态波动状态;气候变暖及持续高位态波动加剧了山区冰川和积雪等固态水体的消融,导致山区降雪率降低,天山山区降雪率从1960-1998年的11%~24%降低到2000年以来的9%~21%,有97.52%的冰川表现为退缩状态,水储量呈明显减少趋势,减小幅度约为-3.72 mm/a;气候变暖直接影响区域水循环和水系统的稳定性,引起径流补给方式和水资源数量的改变,加大了水资源时空分布的不确定性。天山山区在短时期内因冰雪融水增多,会出现径流量增加现象。但在未来气候持续变暖、降水条件维持不变的条件下,河川径流量将会出现减少趋势。     

Historical change in lake sedimentation in Lake Takkobu, Kushiro Mire, northern Japan over the last 300 years

Environmental degradation, including shallowing, deterioration of aquatic habitat and water pollution, has arisen from the inflow of fine sediment to Lake Takkobu in northern Japan. The lake has experienced gradual environmental degradation due to agricultural development, which has introduced both fine sediment and sediment-associated nutrients into the lake. We have reconstructed the history of sediment yield to Lake Takkobu in Kushiro Mire over the last 300 years and have examined trends with reference to land-use development. Fifteen lake sediment core samples were obtained, and various physical variables of lake sediments were analyzed and dated using ¹³⁷Cs and tephrochronology. The physical variables showed that all points contained mainly silt, except for two points close to the river mouths, where the mean diameter was < 35 μm. The peaks were defined as a “signal” when the physical variables were synchronous in a profile. These were created by floods and engineering works constructing drainage systems. The signal of canal construction in 1898 was detected in all core points. Lake Takkobu core samples contained two tephra layers. From the refractive indices of dehydrated glasses, the lower tephra layer was identified as Ko-c2 (1694) and the upper tephra layer as Ta-a (1739). A clear peak in the ¹³⁷Cs concentration was detected at all the sampling points, except for the site close to the Takkobu River. This site showed two peaks in the ¹³⁷Cs concentration, which was attributed to perturbation from flood events and a drainage project. The maximum ¹³⁷Cs concentration was identified as the sediment surface from 1963, enveloped by the 1962 and 1964 signals. The sediment yield averaged over the last 300 years for Lake Takkobu was reconstructed for four periods using the signal, tephra and ¹³⁷Cs as marker layers. The sediment yield under the natural erosion condition for the first two periods was 226 tons/year from 1694 to 1739 and 196 tons/year from 1739 to 1898. The development of the Takkobu watershed started in 1880s with partial deforestation and channelization in 1898, 1959, and 1962 leading to an increased sedimentation yield of 1016 tons/year from 1898 to 1963. Continued deforestation, channelization works in 1964, road construction in 1980–1990s, as well as agriculture development caused a further increase to 1354 tons/year from 1963 to 2004. Compared to the averaged natural sedimentation yield of 206 tons/year until 1898, initial land-use development in a catchment accelerated lake sedimentation, indicated by the 5-fold sediment yield. With increasing agricultural development since 1960s, sedimentation yields were highest for 1963–2004; a 7-fold increase compared with pre-impact conditions. To reduce sediment yield, riparian buffers along the rivers should be preserved or rebuilt, and sluices may function effectively during short-term periods of flooding. Environmental management policy and laws restricting uncontrolled and inappropriate land-use might help in addition to ensure longer-term environmental health by reducing the sedimentation rate.     

The relationship between glacial activity and sediment production: evidence from a 4450-year varve record of neoglacial sedimentation in Hector Lake, Alberta, Canada

A 4450-year sequence of varves, spanning the entire Neoglacialinterval, has been recovered from Hector Lake, Alberta. The varve record is compared to records of regional glacial history toevaluate therelationship between alpine glacial activity and sediment production. Glacial controls on sediment production vary with the timescale considered. Long-term variations in sedimentation rate, of centuries to millennial duration, reflect changes in ice extent of the same timescale. Superimposed on these long-term changes is decadal-scale variability that is complexly related to upvalley ice extent. Over the short term, high sedimentation rates may be associated with glacier maximum stands, or with periods of glacier advance or recession. Overthe last millennium at least, highest sedimentation rates appear to have been associated with transitional periods, preceding or post-dating maximum ice stands, rather than with times of maximum ice extent.     

Chronology and tectonic controls of Late Tertiary deposition in the southwestern Tian Shan foreland, NW China

Magnetostratigraphy from the Kashi foreland basin along the southern margin of the Tian Shan in Western China defines the chronology of both sedimentation and the structural evolution of this collisional mountain belt. Eleven magnetostratigraphic sections representing ～13 km of basin strata provide a two‐ and three‐dimensional record of continuous deposition since ～18 Ma. The distinctive Xiyu conglomerate makes up the uppermost strata in eight of 11 magnetostratigraphic sections within the foreland and forms a wedge that thins southward. The basal age of the conglomerate varies from 15.5±0.5 Ma at the northernmost part of the foreland, to 8.6±0.1 Ma in the central (medial) part of the foreland and to 1.9±0.2, ～1.04 and 0.7±0.1 Ma along the southern deformation front of the foreland basin. These data indicate the Xiyu conglomerate is highly time‐transgressive and has prograded south since just after the initial uplift of the Kashi Basin Thrust (KBT) at 18.9±3.3 Ma. Southward progradation occurred at an average rate of ～3 mm year^?1 between 15.5 and 2 Ma, before accelerating to ～10 mm year^?1. Abrupt changes in sediment‐accumulation rates are observed at 16.3 and 13.5 Ma in the northern part of the foreland and are interpreted to correspond to southward stepping deformation. A subtle decrease in the sedimentation rate above the Keketamu anticline is determined at ～4.0 Ma and was synchronous with an increase in sedimentation rate further south above the Atushi Anticline. Magnetostratigraphy also dates growth strata at <4.0, 1.4±0.1 and 1.4±0.2 Ma on the southern flanks the Keketamu, Atushi and Kashi anticlines, respectively. Together, sedimentation rate changes and growth strata indicate stepped migration of deformation into the Kashi foreland at least at 16.3, 13.5, 4.0 and 1.4 Ma. Progressive reconstruction of a seismically controlled cross‐section through the foreland produces total shortening of 13–21 km and migration of the deformation front at 2.1–3.4 mm year^?1 between 19 and 13.5 Ma, 1.4–1.6 mm year^?1 between 13.5 and 4.0 Ma and 10 mm year^?1 since 4.0 Ma. Migration of deformation into the foreland generally causes (1) uplift and reworking of basin‐capping conglomerate, (2) a local decrease of accommodation space above any active structure where uplift occurs, and hence a decrease in sedimentation rate and (3) an increase in accumulation on the margins of the structure due to increased subsidence and/or ponding of sediment behind the growing folds. Since 5–6 Ma, increased sediment‐accumulation (～0.8 mm year^?1) and gravel progradation (～10 mm year^?1) rates appear linked to higher deformation rates on the Keketamu, Atushi and Kashi anticlines and increased subsidence due to loading from both the Tian Shan and Pamir ranges, and possibly a change in climate causing accelerated erosion. Whereas the rapid (～10 mm year^?1) progradation of the Xiyu conglomerate after 4.0 Ma may be promoted by global climate change, its overall progradation since 15.5 Ma is due to the progressive encroachment of deformation into the foreland.     

Shaping Sydney Harbour: Sedimentation,dredging and reclamation 1788-1990s

Described by Governor Phillip as 'the finest harbour in the world', Sydney Harbour was affected by accelerated sedimentation very soon after settlement in 1788. Dredging began in 1842 to deal with the problems caused by sedimentation and to enhance foreshore amenity. By the end of the nineteenth century, dredging was also excavating the bottom of the Harbour and its bays to accommodate larger ships with deeper draughts, and dredged sediment was increasingly used to reclaim mudflats or marshes deemed unhealthy or unsightly, thereby creating additional useful waterfront land. There was virtually continuous dredging in some part or other of the Harbour and its tributaries for almost 140 years. With increasing costs and mounting environmental concerns, dredging is now intermittent, and carried out largely to facilitate navigation for specific projects, despite continued sedimentation. While there was an early lack of understanding of the Australian environment and the impact of clearing on soils, the causes of increased sedimentation had been documented by the 1860s. However, measures to seriously address causes were not introduced until late in the twentieth century. This paper briefly outlines the development of the Sydney estuary to 1788, considers the impact of settlement in accelerating sedimentation, traces the development of dredging and reclamation, and discusses their significance for a number of areas of current research and environmental management, as well as to social and economic history. Within the limitations of the data available, annual sediment dredged, annual expenditure, cost per tonne, and areas reclaimed by dredging and filling are included.     

Diagenesis in Okavango fan and adjacent dune deposits with implications for the record of palaeo-environmental change in Makgadikgadi–Okavango–Zambezi basin, northern Botswana

This work considers the spatial distribution and ages of western MOZ basin siliclastic sediments prior to providing insights into the diagenesis of degraded dune and alluvial fan sands. Previously published and new TL/OSL ages imply that extensive over-washing of dune sands took place at least 100 ka ago while ages on Okavango floodplains imply that the fan was formed ca. 40 ka and has since undergone periods of higher and lower flood regimes. Sediment analyses indicate that both dune and fan sands contain a diagenetic matrix of clay-enhanced amorphous silica (CEAS) which bonds weakly formed aggregates. The time of formation of diagenetic matrix products is inconclusive but may have been accelerated during or shortly after events dated using OSL/TL techniques. Hence earlier dune over-washing may have led to greater porewater of an acidic to near neutral nature which in turn promoted smectite formation and silicic acid precipitation > 100 000 years ago. The relatively abundant CEAS matrix in floodplain sands implies more recent semi-continuous flood events again of an acidic-near neutral nature leading to the formation of smectite. In this case the floodplain sediments are dated as having been deposited around 40 and 11 ka, when porewater content may have accelerated clay formation and silica dissolution. The dual nature of the CEAS in the islands reflects a changing environment from smectite-dominated flooding events to sepiolite-dominated desiccation events. Flooding may also correspond to TL/OSL ages over the past 40 000 years which contributed to accelerated CEAS formation. The sepiolite is associated with a Ca-rich matrix implying desiccation which may relate to drying events over the 40 000 year period or to riparian tree root pumping and selective salt accumulation. This work shows that sedimentation in incipient rifts is complex and rarely explained totally in terms of primary depositional events. The implications of different stages of sand diagenesis may be significant in enhancing palaeo-environmental interpretations in semi-arid fluvial environments.     

Depositional and tectonic evolution of a supradetachment basin: 40Ar/39Ar geochronology of the Nova Formation, Panamint Range, California

The Nova Basin contains an upper Miocene to Pliocene supradetachment sedimentary succession that records the unroofing of the Panamint metamorphic core complex, west of Death Valley, California. Basin stratigraphy reflects the evolution of sedimentation processes from landslide emplacement during basin initiation to the development of alluvial fans composed of reworked, uplifted sections of the basin fill. ⁴⁰Ar/³⁹Ar geochronology of volcanic units in middle and lower parts of the sequence provide age control on the tectonic and depositional evolution of the basin and, more generally, insights regarding the rate of change of depositional environments in supradetachment basins. Our work, along with earlier research, indicate basin deposition from 11.38 Ma to 3.35 Ma. The data imply sedimentation rates, uncorrected for compaction, of ~100 m Myr⁻¹ in the lower, high-energy part to ~1000 m Myr⁻¹ in the middle part characterized by debris-flow fan deposition. The observed variation in sediment flux rate during basin evolution suggests that supradetachment basins have complex depositional histories involving rapid transitions in both the style and rate of sedimentation.     

新疆孔雀河北岸中更新世湖相沉积物年代学及沉积速率特征

选择位于新疆孔雀河北岸的LX01湖相沉积剖面作为研究对象,根据14个ESR年代结果,建立了塔里木盆地东部中更新世湖泊沉积地层的年代序列。此外,采用ESR测年法得到了剖面的沉积速率,在727～93 ka BP期间,研究区域平均沉积速率介于0.429～3.895 cm·ka^-1,表现为沉积速率不稳定,发生了较大变化,不但有长期缓慢的变化,还有短期的急剧变化,总体呈现波动特征。具体表现为：727～278 ka BP期间,波动幅度逐渐增大;278～93 ka BP期间,沉积速率阶段性降低。在此基础上,通过对比分析罗布泊地区中更新统沉积速率和罗布泊及周边区域中更新世期间的气候变化,发现位于孔雀河北岸LX01剖面沉积物的沉积速率可在一定程度上揭示罗布泊地区中更新世期间的古气候变化特征,表现为：沉积速率速率高的278～200 ka BP、306～325 ka BP、389～410 ka BP、530～568 ka BP、638～679 ka BP为温暖湿润气候;沉积速率低的93～200 ka BP、278～306 ka BP、325～389 ka BP、410～530 ka BP、568～638 ka BP、679～727 ka BP为冷干气候。该研究为中更新世地层沉积速率的空间分布特征和古气候变化研究提供科学依据。     

Basin filling evolution of the central basins of Mallorca since the Pliocene

A new compilation of data from 436 drill cores using decompaction and backstripping techniques was used to reconstruct the basin filling history from the Pliocene until the present day in the Palma, Inca and Sa Pobla Basins on the island of Mallorca (Spain). Calcareous rocks dominate the source area and provide a limited amount of clastic input to the basins that has resulted in an average accumulation rate of between 5 and 20 m/Ma during the last 5.3 Ma. Carbonate sediment production dominated the basin filling history during early‐mid Pliocene, but during the Quaternary, the sedimentation processes in the Palma Basin were probably enhanced by an evolution in the drainage network that increased the sediment supply and the accumulated thickness caused by stream capture. However, the maximum sedimentation rate filling the depocentres of the three basins has been decreasing since the Pliocene, showing that not only the catchment transport efficiency but also the relative sea level have been controlling the sediment accumulation in these carbonate basins. The isopach cross‐sections support the idea that a palaeorelief was generated during the Messinian sea level drop and that heterogeneities were filled in from the Pliocene to the Quaternary. We conclude that the central basins of Mallorca were filled heterogeneously due to tectonic and geomorphic processes that controlled sediment transport and production, resulting in different average sedimentation thicknesses that decreased since the Pliocene as the accommodation space became filled and the relative sea level dropped.     

Geomorphological characteristics and slope processes associated with different basins: Mallorca (Western Mediterranean)

We compare the different geomorphological processes which occurred in Pliocene–Quaternary times on two very similar slopes in Mallorca, one located at the Tramuntana Range and the other in the Llevant Ranges. Both slopes have the same geological structure, the same stratigraphic and lithological levels, and the same altitude and orientation. The different slope processes are due to the relationship between the accommodation space and the sedimentation rate in the adjacent basins: In the Valencia Trough (located N of the Tramuntana Range), the sedimentation rate has not been sufficient to fill the accommodation space, whereas in the Alcudia Basin (adjacent to the Llevant Ranges), the sedimentation rate has been sufficient to fill the accommodation space. This difference has resulted in major landslides on the Tramuntana slope, whereas the Llevant slope is characterized mainly by alluvial fans and debris screes.     

Holocene glacial-marine sedimentation in the Prydz Bay,Antarctica

吴时国，罗又郎，王有强，陆钧，郑范，孙绍先Ｈｏｌｏｃｅｎｅｇｌａｃｉａｌ－ｍａｒｉｎｅｓｅｄｉｍｅｎｔａｔｉｏｎｉｎｔｈｅＰｒｙｄｚＢａｙ，Ａｎｔａｒｃｔｉｃａ￥ＷｕＳｈｉｇｕｏ；ＬｕｏＹｏｕｌａｎｇ；ＷａｎｇＹｏｕｑｉａｎｇ；ＬｕＪｕｎ；Ｚｈｅｎｇ...     

Stratigraphic development of an Upper Jurassic deep marine syn‐rift succession,Inner Moray Firth Basin,Scotland

The stratigraphic development of an Upper Jurassic syn‐rift succession exposed at outcrop in the Inner Moray Firth Basin has been investigated using high‐resolution biostratigraphy and sedimentology. A continuous 970 m thick section, exposed in the hangingwall of the Helmsdale Fault was logged in detail. The succession spans 8 Ma and contains eight lithofacies types, which indicate deposition in a deep marine setting. Boulder beds contain large, angular clasts, with bed thicknesses typically >2 m and poor sorting suggesting deposition by debris flows. An inverse clast stratigraphy is observed; the oldest boulder beds contain sandstone clasts of Upper Old Red Sandstone (ORS) with younger debris flows containing clasts of Middle ORS calcareous siltstone. A marked change from siliciclastic to carbonate dominated sedimentation occurred during the Early Tithonian, interpreted primarily as a result of change in lithologies in the footwall catchment from sandstone to calcareous siltstone, which reduced supply of siliciclastic sediment. Secondary factors are identified as increased aridity in the Early Tithonian, which reduced sand supply from the hinterland and a third‐order Early Tithonian eustatic sea‐level rise, which trapped coarser clastic sediment within the hinterland. Biostratigraphy allows calculation of variations in sedimentation rates with recognition of: (1) an early rift phase characterised by sandy turbidite deposition, when sedimentation rates averaged 0.08 m/ky, (2) a rift climax phase from the Early Kimmeridgian where sedimentation rates increased steadily to a maximum of 0.64 m/ky in the Early Tithonian, with strata dominated by boulder scale clast‐supported debris flows and (3) a late stage of rifting from the mid Tithonian, where sedimentation rates decreased to 0.07 m/ky. Overall sedimentation rates are comparable to those of other deep marine rift basins. Unroofing a resistant lithology on the footwall of a rift has important implications for siliciclastic sediment supply in rift basins.     

The timing of alluvial sedimentation and floodplain formation in the lowland humid tropics of Ghana, Sierra Leone and western Kalimantan (Indonesian Borneo)

Temporal patterns in floodplain genesis and alluvial sedimentation in lowlands tropical rain forest zones of Ghana, Sierra Leone and western Kalimantan (Indonesian Borneo) based upon ¹⁴C age determinations are described.Alluvial low terraces or buried sediments in West Africa yielded ages of 36-21 ka. In west Kalimantan a widespread episode of alluviation has yielded dates of 54-51 ka. The 20-13 ka interval was characterised by channel incision with valley floor erosion and neither region records sedimentation. Holocene alluvial sedimentation and floodplain construction in West Africa occurred during two temporal intervals: 10-7 ka and 4 ka to present and in western Kalimantan in response to early Holocene sea level rise followed by late Holocene regression and coastal outgrowth.The clustering of ¹⁴C dates closely corresponds to regional lake level fluctuations and vegetational changes and to global indications of climatic change. We propose that periods of more frequent episodes of accelerated floodplain erosion and reconstitution, channel morpho-sedimentary activity and alluvial accumulation (1) are responses to interstadial and interglacial periods of higher precipitation following intervening periods of cooler and drier conditions; and (2) may be synchronous during the last 60 ka throughout the African and Asian inner humid lowland tropics.     

Relation between the erosion and sedimentation zones in the Yellow River, China

Sedimentation in the lower reaches of the Yellow River is a major problem requiring implementation of large-scale control measures in the upper and middle drainage basin. For maximum benefit, major sediment generation areas must be delimited. For this purpose, the upper and middle drainage basin of the Yellow River has been divided into four major sediment and water source areas. A series of databases of runoff and sediment yields from these source areas and for the corresponding quantities of sedimentation in the lower Yellow River channel have been established. On this basis, a set of multiple-regression equations has been established that define the relationships between sedimentation in the lower Yellow River and the yearly or event-based runoff and sediment yields from the four source areas or subsystems. Based on the regression equations obtained, the contribution of the four major source areas to the sedimentation in the lower Yellow River channel can be estimated. The results obtained indicate that, given other factors, for each ton of sediment reduced from the coarse-sediment producing area (CSA), the sediment deposited in the lower Yellow River would be reduced by 0.455 ton; for each ton of sediment reduced from the fine-sediment producing area (FSA), the sediment deposited in the lower Yellow River would be reduced by 0.154 ton only. Therefore, if limited erosion control measures are applied to the coarse-sediment producing area, the benefits for sedimentation reduction in the lower Yellow River will be much larger than if similar resources are applied to the fine-sediment producing area.