Response of active tectonics on the alluvial Baghmati River, Himalayan foreland basin, eastern India

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

Catastrophic flood effects in alpine/foothill fluvial system (a case study from the Sudetes Mts, SW Poland)

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.     

甬江河床演变及航道治理

本文分析甬江动力、泥沙、边界及河床演变的基本特征,提出甬江航道整治的原则和措施。     

Railroads, roads and lateral disconnection in the river landscapes of the continental United States

Railroads and roads are ubiquitous features in the river corridors of the United States. However, their impact on hydrologic, geomorphic, and ecological processes in fluvial and riparian landscapes has not been systematically explored at regional or continental extents. This study documents the geographic distribution of roads and railroads in the alluvial floodplains of the continental United States and the regional variability of their potential impacts on lateral connectivity and resultant channel and floodplain structure and function. We use national scale data sets and GIS analysis to derive data on stream–transportation network interactions in two broad categories: (1) crossing impacts, such as bridges and culverts, and (2) impacts where transportation infrastructure acts as a longitudinal dam along the stream channel, causing lateral floodplain disconnection. Potential stream crossing impacts are greatest in regions with long histories of road and railroad development and relatively low relief, such as the Mid-Atlantic, New England, and the Lower Mississippi and Ohio Valleys. Potential lateral disconnections are more prevalent in rugged regions such as the Western U.S. and Appalachians where transportation routes follow river corridors along valley bottoms. Based on these results, we develop a conceptual model that suggests that the area of lateral disconnection due to transportation infrastructure should be most extensive in mid-sized alluvial valleys in relatively rugged settings. The result of this disconnection is the disruption of the long-term, cut-and-fill alluviation and of the shorter-term flood and flow pulse processes that create and maintain ecosystem function in river landscapes. The tremendous extent of transportation infrastructure in alluvial valleys documented in this study suggests a revision to H.B.N. Hynes' statement that the valley rules the stream. Instead, it appears that in modern landscapes of the U.S. the valley rules the transportation network — and the transportation network rules the stream.     

Regional and local controls on the spatial distribution of bedrock reaches in the Upper Guadalupe River, Texas

While studies on gravel mantled and mixed alluvial bedrock rivers have increased in recent decades, few field studies have focused on spatial distributions of bedrock and alluvial reaches and differences between reach types. The objective of this work is to identify the spatial distribution of alluvial and bedrock reaches in the Upper Guadalupe River. We compare reach length, channel and floodplain width, sinuosity, bar length and spacing, bar surface grain size, and slope in alluvial and bedrock reaches to identify whether major differences exist between channel reach types. We find that local disturbances, interaction of the channel and valley sides, variation in lithology, and regional structural control contribute to the distribution of bedrock reaches in the largely alluvial channel. Alluvial and bedrock channel reaches in the Upper Guadalupe River are similar, particularly with respect to the distribution of gravel bars, surface grain size distributions of bars, and channel slope and width. Our observations suggest that the fluvial system has adjusted to changes in base level associated with the Balcones Escarpment Fault Zone by phased incision into alluvial sediment and the underlying bedrock, essentially shifting from a fully alluvial river to a mixed alluvial bedrock river.     

Hydro-geomorphic hazards and impact of man-made structures during the catastrophic flood of June 2000 in the Upper Guil catchment (Queyras, Southern French Alps)

The Guil River Valley (Queyras, Southern French Alps) is prone to catastrophic floods, as the long historical archives and Holocene sedimentary records demonstrate. In June 2000, the upper part of this valley was affected by a “30-year” recurrence interval (R.I.) flood. Although of lower magnitude and somewhat different nature from that of 1957 (>100-year R.I. flood), the 2000 event induced serious damage to infrastructure and buildings on the valley floor. Use of methods including high-resolution aerial photography, multi-date mapping, hydraulic calculations and field observations made possible the characterisation of the geomorphic impacts on the Guil River and its tributaries. The total rainfall (260 mm in four days) and maximum hourly intensity (17.3 mm h⁻¹), aggravated by pre-existing saturated soils, explain the immediate response of the fluvial system and the subsequent destabilisation of slopes. Abundant water and sediment supply (landsliding, bank erosion), particularly from small catchment basins cut into slaty, schist bedrock, resulted in destructive pulses of debris flow and hyperconcentrated flows. The specific stream power of the Guil and its tributaries was greater than the critical stream power, thus explaining the abundant sediment transport. The Guil discharge was estimated as 180 m³ s⁻¹ at Aiguilles, compared to the annual mean discharge of 6 m³ s⁻¹ and a June mean discharge of 18 m³ s⁻¹. The impacts on the Guil valley floor (flooding, aggradation, generalised bank erosion and changes in the river pattern) were widespread and locally influenced by variations in the floodplain slope and/or channel geometry. The stream partially reoccupied former channels abandoned or modified in their geometry by various structures built during the last four decades, as exemplified by the Aiguilles case study, where the worst damage took place. A comparative study of the geomorphic consequences of both the 1957 and 2000 floods shows that, despite their poor maintenance, the flood control structures built after the 1957 event were relatively efficient, in contrast to unprotected places. The comparison also demonstrates the role of land-use changes (conversion from traditional agro-pastoral life to a ski/hiking-based economy, construction of various structures) in reducing the Guil channel capacity and, more generally, in increasing the vulnerability of the human installations. The efficiency of the measures taken after the 2000 flood (narrowing and digging out of the channel) is also assessed. Final evaluation suggests that, in such high mountainous environments, there is a need to keep most of the 1957 flooded zone clear of buildings and other structures (aside from the existing villages and structures of particular economic interest), in order to enable the river to migrate freely and to adjust to exceptional hydro-geomorphic conditions without causing major damage.     

Hydrologic modeling of flood conveyance and impacts of historic overbank sedimentation on West Fork Black's Fork, Uinta Mountains, northeastern Utah, USA

This study assesses historic overbank alluvial sedimentation along a low-gradient reach of West Fork Black's Fork in the northern Uinta Mountains, Utah. In this previously glaciated setting, an alluvial floodplain that is approximately 400 m wide by 1500 m long has been modified by the combined effects of valley morphometry and the recent history of clear-cut logging during the late 19th and early 20th Centuries. To quantify the effects on sedimentation and flow conveyance, three natural streambank exposures were sampled and analyzed for nuclear bomb fallout ¹³⁷Cs. The distribution of ¹³⁷Cs within the three profiles suggests that a remnant outwash terrace exerts a first-order control over the deposition of overbank alluvium. Upstream from a constriction in the floodplain caused by the terrace remnant, as much as 40 cm of overbank alluvium has been deposited since the beginning of clear-cut logging. Immediately downstream of that constriction, no evidence exists for any overbank sedimentation during that same period. Vibracore samples and Oakfield soil probe sampling throughout the study reach quantified the geographic extent and thicknesses of the historic alluvial package. Flood conveyance through the study area was modeled using the U.S. Army Corps of Engineers HEC-RAS modeling program. Model simulations were run for modern conditions (using surveyed topography) and for prehistoric conditions (using the modern topography less the historic alluvial package determined by ¹³⁷Cs analyses). Model results indicate that the floodplain constriction caused a significant impediment to flood conveyance at even modest discharges during prehistoric conditions. This promoted ponding of floodwaters upstream of the constriction and deposition of alluvium. This has increased bank heights upstream of the constriction, to the point that under modern conditions 1- to 5-year recurrence interval floods are largely confined within the channel. These results confirm the validity of this new approach of combining ¹³⁷Cs dating of alluvial sediments with HEC-RAS flow modeling to compare flood conveyance along a single stream reach prior to and since an abrupt change in alluvial sedimentation patterns.     

New constraints on the late Cenozoic incision history of the New River, Virginia

The New River crosses three physiogeologic provinces of the ancient, tectonically quiescent Appalachian orogen and is ideally situated to record variability in fluvial erosion rates over the late Cenozoic. Active erosion features on resistant bedrock that floors the river at prominent knickpoints demonstrate that the river is currently incising toward base level. However, thick sequences of alluvial fill and fluvial terraces cut into this fill record an incision history for the river that includes several periods of stalled downcutting and aggradation. We used cosmogenic ¹⁰Be exposure dating, aided by mapping and sedimentological examination of terrace deposits, to constrain the timing of events in this history. ¹⁰Be concentration depth profiles were used to help account for variables such as cosmogenic inheritance and terrace bioturbation. Fill-cut and strath terraces at elevations 10, 20, and 50 m above the modern river yield model cosmogenic exposure ages of 130, 600, and 600–950 ka, respectively, but uncertainties on these ages are not well constrained. These results provide the first direct constraint on the history of alluvial aggradation and incision events recorded by New River terrace deposits. The exposure ages yield a long-term average incision rate of 43 m/my, which is comparable to rates measured elsewhere in the Appalachians. During specific intervals over the last 1 Ma, however, the New River's incision rate reached 100 m/my. Modern erosion rates on bedrock at a prominent knickpoint are between 28 and 87 m/my, in good agreement with rates calculated between terrace abandonment events and significantly faster than 2 m/my rates of surface erosion from ancient terrace remnants. Fluctuations between aggradation and rapid incision operate on timescales of 10⁴− 10⁵ year, similar to those of late Cenozoic climate variations, though uncertainties in model ages preclude direct correlation of these fluctuations to specific climate change events. These second-order fluctuations appear within a longer-term signal of dominant aggradation (until 2 Ma) followed by dominant incision. A similar signal is observed on other Appalachian rivers and may be the result of sediment supply fluctuations driven by the increased frequency of climate changes in the late Cenozoic.     

水患与黄河流域古代交通道路的变迁——以黄河中、下游分界地区(郑、汴间)为对象

黄河流域是古代中国河患较为频繁的地区。由于河患的频繁,致使这一地区古代环境发生了巨大的变化,交通道路的变迁即是其中一例。黄河郑州—开封河段于黄河中、下游的分界处,对于我们研究水患与古代交通道路的形成具有典型意义。由于水患的加剧,改变了这一地区原有的地形地貌条件,在此基础上形成的城市、聚落和交通道路的格局与黄河水患有着极为密切的关系,今天这一区域内城市、聚落和交通道路是受明清黄河水患影响而形成的。     

Comparison of flood hazard assessments on desert piedmonts and playas: A case study in Ivanpah Valley, Nevada

Accurate and realistic characterizations of flood hazards on desert piedmonts and playas are increasingly important given the rapid urbanization of arid regions. Flood behavior in arid fluvial systems differs greatly from that of the perennial rivers upon which most conventional flood hazard assessment methods are based. Additionally, hazard assessments may vary widely between studies or even contradict other maps. This study's chief objective was to compare and evaluate landscape interpretation and hazard assessment between types of maps depicting assessments of flood risk in Ivanpah Valley, NV, as a case study. As a secondary goal, we explain likely causes of discrepancy between data sets to ameliorate confusion for map users. Four maps, including three different flood hazard assessments of Ivanpah Valley, NV, were compared: (i) a regulatory map prepared by FEMA, (ii) a soil survey map prepared by NRCS, (iii) a surficial geologic map, and (iv) a flood hazard map derived from the surficial geologic map, both of which were prepared by NBMG. GIS comparisons revealed that only 3.4% (33.9 km²) of Ivanpah Valley was found to lie within a FEMA floodplain, while the geologic flood hazard map indicated that ~ 44% of Ivanpah Valley runs some risk of flooding (Fig. 2D). Due to differences in mapping methodology and scale, NRCS data could not be quantitatively compared, and other comparisons were complicated by differences in flood hazard class criteria and terminology between maps. Owing to its scale and scope of attribute data, the surficial geologic map provides the most useful information on flood hazards for land-use planning. This research has implications for future soil geomorphic mapping and flood risk mitigation on desert piedmonts and playas. The Ivanpah Valley study area also includes the location of a planned new international airport, thus this study has immediate implications for urban development and land-use planning near Las Vegas, NV.     

Geomorphological and sedimentological features in Quaternary fluvial systems affected by solution-induced subsidence (Ebro Basin, NE-Spain)

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

EFFECT OF DAMS ON MOUNTAINOUS BEDROCK RIVERS

This study examines the adjustment of river systems in the Golan Heights to reservoir construction in small basaltic mountainous catchments where total water flow is impounded, and discusses comparisons with the effects of large dams on alluvial rivers. Annual rainfall ranges from 500 mm to 900 mm, and no major springs are found in the area. About 3/4 of total runoff is in the form of floods caused by rainstorms. The 11 dams receive drainage from about 20% of the total catchment, and a third of the total runoff. Changes in the river channels were studied at 28 field stations. Flood marks provided peak stage levels for largest season discharge, and 8 nearby hydrometric stations provided a continuous hydrological record. Bed material was sampled at each cross-section site. The main effect of river impoundment is a sharp decrease in flood peaks below the reservoirs to about one-third of their previous natural regime. The channel cross-sectional area decreased, but no temporal adjustment can be established, as most of the reservoirs were built in the last 5–8 years. Channel-flow hydraulics are competent enough to transport the suspended material, but evidence of aggradation was found in the channel reaches close to the dams. Bedload transport and the size of transported cobbles and boulders decreased downstream from the dams. Increased vegetation cover downstream from the dams was the most pronounced effect. Adjustment of the fluvial system to dam building in mountainous streams seems to be less sensitive than in alluvial channels. [Key words: fluvial geomorphology, dams, water reservoirs, environmental changes, Israel.]     

网状河流体系的基本特征及其影响因素

网状河流近年来受到较多关注,无论从沉积特征还是从地貌及水力学特征看,网状河流都是一种典型的河型,但目前对网状河流的认识还存在一定的局限性,从沉积特征,地貌及沉积微环境特征及形成条件等方面的对网状河流进行了系统的论述和总结。     

GIS-based dynamic modelling and analysis of flash floods considering land-use planning

Flood inundation is a common natural disaster and a growing development challenge for many cities and thousands of small towns around the world. Soil features have frequently altered with the rapid development of urbanised regions, which has led to more frequent and longer duration of flooding in urban flood-prone regions. Thus, this paper presents a geographic information system (GIS)-based methodology for measuring and visualising the effects on urban flash floods generated by land-use changes over time. The measurement is formulated with a time series in order to perform a dynamic analysis. A catchment mesh is introduced into a hydrological model for reflecting the spatial layouts of infrastructure and structures over different construction periods. The Geelong Waurn Ponds campus of Deakin University is then selected as a case study. Based on GIS simulation and mapping technologies, this research illustrates the evolutionary process of flash floods. The paper then describes flood inundation for different built environments and presents a comparison by quantifying the flooding extents for infrastructure and structures. The results reveal that the GIS-based estimation model can examine urban flash floods in different development phases and identify the change of flooding extents in terms of land-use planning. This study will bring benefits to urban planners in raising awareness of flood impact and the approach proposed here could be used for flood mitigation through future urban planning.     

Geomorphic hazard assessment of landslide dams in South Westland, New Zealand: fundamental problems and approaches

Quantitative assessments of landslide hazard usually employ empirical, heuristic, deterministic, or statistical methods to derive estimates of magnitude–frequency distributions of landsliding. The formation and failure of landslide dams are common geomorphic processes in mountain regions throughout the world, causing a series of consequential off-site hazards such as catastrophic outburst floods, debris flows, backwater ponding, up- and downstream aggradation, and channel instability.Conceptual and methodological problems of quantifying geomorphic hazard from landslide dams result from (a) aspects of defining “landslide-dam magnitude”, (b) scaling effects, i.e. the geomorphic long-range and long-term implications of river blockage, and (c) paucity of empirical data. Geomorphic hazard from a landslide dam-break flood on the basis of conditional probabilities is being analysed for the alpine South Westland region of New Zealand, where formation and failure of landslide dams is frequent. Quantification of the annual probability of landsliding and subsequent dam formation in the area is limited by historical and only partially representative empirical data on slope instability. Since landslide-dam stability is a major control governing the potential of catastrophic outburst flooding, the ensuing hazard is best assessed on a recurring basis. GIS-based modelling of virtual landslide dams is a simple and cost-effective approach to approximate site-specific landslide dam and lake dimensions, reservoir infill times, and scaled magnitude of potential outburst floods. Although crude, these order-of-magnitude results provide information critical to natural hazard planning, mitigation, or emergency management decisions.     

Predicting changes in alluvial channel patterns in North-European Russia under conditions of global warming

Global climate change may have a noticeable impact on the northern environment, leading to changes in permafrost, vegetation and fluvial morphology. In this paper we compare the results from three geomorphological models and study the potential effects of changing climatic factors on the river channel types in North-European Russia. Two of the selected models by Romashin [Romashin, V.V., 1968. Variations of the river channel types under governing factors, Annals of the Hydrological Institute, vol. 155. Hydrometeoizdat, Leningrad, pp. 56–63.] and Leopold and Wolman [Leopold, L.B., Wolman, M.G., 1957. River channel pattern: braided, meandering and straight, Physiographic and hydraulic studies of rivers. USA Geological Survey Professional Paper 252, pp. 85–98.] are conventional QS-type models, which predict the existence of either multi-thread or single-tread channel types using data on discharge and channel slope. The more advanced model by Van den Berg [Van den Berg, J.H., 1995. Prediction of alluvial channel pattern of perennial rivers. Geomorphology 12, 259–270.] takes into account the size of the sediment material.We used data from 16 runoff gauges to validate the models and predict the channel types at selected locations under modern and predicted for the future climatic conditions. Two of the three models successfully replicated the currently existing channel types in all but one of the studied sites. Predictive calculations under the hypothetical scenarios of 10%, 15%, 20% and 35% runoff increase gave different results. Van den Berg's model predicted potential transformation of the channel types, from single- to multi-thread, at 4 of 16 selected locations in the next few decades, and at 5 locations by the middle of the 21st century. Each of the QS-type models predicted such transformation at one site only.Results of the study indicate that climatic warming in combination with other environmental changes may lead to transformation of the river channel types at selected locations in north-western Russia. Further efforts are needed to improve the performance of the fluvial geomorphological models and their ability to predict such changes.     

Coupling relationships: Hillslope–fluvial linkages in the Hodder catchment, NW England

Variations in the coupling of sediment transfer between different parts of a fluvial catchment, e.g., hillslope to axial stream, can hamper understanding but are an integral part of the geomorphological record. Depositional environments respond to a combination of land use, climate, storms (floods), and autogenic conditioning. The distribution of sediment in the upland landscapes of NW England is out of equilibrium with contemporary climate and geomorphological processes; more a function of peri- and paraglacial mobilisation of glacigenic deposits. Soil and vegetation development after deglaciation have interrupted any progression toward sediment exhaustion with sediment release controlled largely by extrinsic perturbation, with late Holocene anthropogenic activity, climate and extreme hydrological events the likely candidates. This paper presents a new radiocarbon-dated Holocene geomorphological succession for the River Hodder (NW England), alongside evaluating new palaeoecological and geoarchaeological data to discern the impacts of human activity. These data show a late Holocene expansion in human occupation and use of the landscape since the Iron Age (700–0 cal. B.C.), with more substantial changes in the character and intensity of upland land use in the last 1300 years. The geomorphological responses in the uplands were the onset of considerable and widespread hillslope erosion (gullying) and associated alluvial fan development. Interpretation of the regional radiocarbon chronology limits gullying to four, more extensive and aggressive phases after 500 cal. B.C. The downstream alluvial system has responded with considerable valley floor deposition and lateral channel migration that augmented sediment supply by remobilising the existing floodplain terraces and led to the aggradation of a series of inset alluvial terraces. The timing of these changes between states of aggradation and incision in alluvial reaches reflects the increased connectivity between the hillslope and alluvial systems. Aspects of both the regional climate and land use histories are conducive to increasing discharge and sediment flux, but the region wide lowering of erosion thresholds appears a key driver conditioning these sediment-rich conditions and producing a landscape that was more susceptible to erosion under lower magnitude flows.     

Spatial patterns of hydrology, geomorphology, and vegetation on the floodplain of the Amazon river in Brazil from a remote sensing perspective

The spatial heterogeneity of hydrology and vegetation during high-water periods in geomorphically distinct reaches of the Amazon River in Brazil was determined based on semivariance statistics. The spatial statistics were derived from three classified Landsat Thematic Mapper images representing upstream to downstream geomorphic characteristics. In the upstream river reach, scroll-bar topography on the floodplain tends to channelize floodwater into floodplain drainage channels, thus reducing the diversity of water types by reducing opportunities for mixing of flooding river water with locally derived floodplain water. The highest diversity of vegetation types is along floodplain drainage channels, while the rest of the floodplain has a more homogeneous cover. In the middle reach of the river the diversity of wetland classes as measured by semivariance is higher than both upstream and downstream, perhaps because of exposure to more water types and landforms. The diversity of water types is high, because flooding river water flows onto the floodplain as diffuse, non-channelized overbank flow, as well as through drainage channels. The non-channelized overbank flow readily mixes with locally derived floodplain water. Floodplain landforms available for colonization by vegetation include scroll bars, swales, lake shores, lake deltas, and floodplain drainage channels. In the downstream reach where the floodplain is wide, relatively flat, and covered with huge lakes, the floodplain supports a moderately heterogeneous mix of vegetation communities. Where landforms are similar, the spatial distribution of the vegetation is similar to that of the middle reach of the river. In the downstream reach flooded forest comprised only 37% of the wetland vegetation. In contrast, in both the upstream and middle reaches, over 70% of the wetland vegetation was flooded forest. Agricultural clearing of the floodplain is more. common in downstream reaches and may account for the smaller percent of floodplain forest cover.     

清代嘉道时期（1796—1850年）黄河下游决溢时空格局与河工治理响应

历史时期黄河河患及其治理属于全球环境变化与区域响应研究的重要问题之一。基于多源史料建立清代嘉道时期（1796—1850年）黄河下游决溢基础数据库,运用交叉谱、核密度估计、标准差椭圆等分析方法,测算了黄河下游决溢与河工治理的相关性及其滞后性特征,进一步辨识二者在不同阶段的时空特征、响应过程及驱动机制。结果显示：清嘉道时期黄河下游决溢时空变化特征显著,以1825年为节点分为先增后减的两个主要发展阶段;空间分布具有较强聚集性特征,两个阶段累计高发区集中在河南怀庆府、开封府及江苏淮安府附近,黄河下游决溢重心始终位于东河。决溢与河工治理序列年际变化具有明显正相关关系,其显著振动周期为11a,且二者的响应间隔为1a左右。清嘉庆二十三年（1818年）后,黄河下游决溢与河工治理的时空响应存在明显“错位”的独特现象,即决溢重心向“东河”推移,而河工治理重心则向“南河”扩展。二者之间时空响应关系从“一致”到“错位”的转变,不仅显示清廷治黄理念由“积极革新”转向“消极防御”,也说明黄河下游已丧失清初以来“治河保漕”的重要政治经济地位,展现了其作为黄运之交重要地理区位的解构与重塑过程;而这一过程又孕育了咸丰五...     

赣江入湖三角洲上的网状河流体系研究

中国南方的冲积河流有许多属于分汊河流，这已被许多研究者进行过比较深入的分析研究，但赣江在其入湖三角洲上的多河道体系与分汊河道有着明显的不同，它的形成是河流自发调整的结果，而不象分汊河流那样由节点控制。它具有网状河流所具有的地貌和沉积物特征，属于典型的网状河流体系。虽然是低含沙河流，但由于其水动力较弱及汛期基准面的上升，洪泛频繁，可输入河间地以大量的泥沙并在低能环境中发生沉积，使河道及河间地能够协调加积升高，并维持多河道体系的稳定性。