共查询到20条相似文献，搜索用时 25 毫秒
1.
The Mekong from satellite imagery: A quick look at a large river 总被引：1，自引：1，他引：1
Physical datasets on rivers, thousands of kilometres long, can be efficiently and rapidly acquired through satellite imagery. As large rivers commonly cross several tectonic and climatic zones, their channels are composed of a number of units, each with characteristic morphology and behaviour. Separating a large river into such units provides a framework for understanding the geomorphology of the river concerned, allowing acquisition of information for environmental impact analysis and river management.About 2500 km of the Mekong River, from the China border to the sea, has been examined with SPOT satellite images with a selection of MODIS and IKONOS scenes in support. Hydrological data of the Mekong River Commission, hydrographic maps of the Commission, and field observations have been used to verify and extend the findings from the satellite imagery. The technique provides a rapid and holistic conceptualisation of forms and functions along the derived eight-unit classification of the Mekong River. Such a framework is valuable for (1) determining selected aspects of the geomorphological behaviour of a large river system, (2) rapidly analysing project-related environmental impacts, and (3) examining the geological evolution of the river. Investigating large rivers is difficult and resource consuming, but satellite imagery provides an easy and rapid tool. The cost of the images, however, may be high as a number of them are required for this type of analysis. 相似文献
2.
Engineered channel controls limiting spawning habitat rehabilitation success on regulated gravel-bed rivers 总被引：1，自引：0，他引：1
In efforts to rehabilitate regulated rivers for ecological benefits, the flow regime has been one of the primary focal points of management strategies. However, channel engineering can impact channel geometry such that hydraulic and geomorphic responses to flow reregulation do not yield the sought for benefits. To illustrate and assess the impacts of structural channel controls and flow reregulation on channel processes and fish habitat quality in multiple life stages, a highly detailed digital elevation model was collected and analyzed for a river reach right below a dam using a suite of hydrologic, hydraulic, geomorphic, and ecological methods. Results showed that, despite flow reregulation to produce a scaled-down natural hydrograph, anthropogenic boundary controls have severely altered geomorphic processes associated with geomorphic self-sustainability and instream habitat availability in the case study. Given the similarity of this stream to many others, we concluded that the potential utility of natural flow regime reinstatement in regulated gravel-bed rivers is conditional on concomitant channel rehabilitation. 相似文献
3.
The hydrology and geomorphology of large rivers in America reflect the pervasive influence of an extensive water control infrastructure including more than 75,000 dams. One hundred thirty-seven of the very large dams, each storing 1.2 km3 (106 acre feet) of water or more, alter the flows of every large river in the country. The hydrologic effects of these very large dams emerge from an analysis of the stream gage records of 72 river reaches organized into 36 pairs. One member of each pair is an unregulated reach above a dam, whereas the other is a regulated reach downstream from the same structure. Comparison of the regulated and unregulated reaches shows that very large dams, on average, reduce annual peak discharges 67% (in some individual cases up to 90%), decrease the ratio of annual maximum/mean flow 60%, decrease the range of daily discharges 64%, increase the number of reversals in discharge by 34%, and reduce the daily rates of ramping as much as 60%. Dams alter the timing of high and low flows and change the timing of the yearly maximum and minimum flows, in some cases by as much as half a year. Regional variation in rivers, dams, and responses are substantial: rivers in the Great Plains and Ozark/Ouachita regions have annual maximum/mean flow ratios that are 7 times greater than ratios for rivers in the Pacific Northwest. At the same time, the ratio of storage capacity/mean annual water yield for dams is greatest for Interior Western, Ozark/Ouachita and Great Plains rivers and least for Pacific Northwest streams. Thus, in many cases those rivers with the highest annual variability have the greatest potential impact from dams because structures can exert substantial control over downstream hydrology. The hydrologic changes by dams have fostered dramatic geomorphic differences between regulated and unregulated reaches. When compared to similar unregulated reaches, regulated reaches have 32% larger low flow channels, 50% smaller high flow channels, 79% less active flood plain area, and 3.6 times more inactive flood plain area. Dams also affect the area of active areas, the functional surfaces that are functionally connected to the present regime of the river. Regulated reaches have active areas that are 72 smaller than the active areas of similar unregulated reaches. The geomorphic complexity (number of separate functional surfaces per unit of channel length) is 37% less in regulated reaches. Reductions in the size of hydrologically active functional surfaces are greatest in rivers in the Great Plains and least in Eastern streams. The largest differences in geomorphic complexity are in interior western rivers. The shrunken, simplified geomorphology of regulated large rivers has had direct effects on riparian ecology, producing spatially smaller, less diverse riparian ecosystems compared to the larger, more complex ecosystems along unregulated reaches of rivers. 相似文献
4.
In-channel geomorphic complexity: The key to the dynamics of organic matter in large dryland rivers? 总被引：1，自引：0，他引：1
Large rivers are often considered to retain less organic material than smaller streams primarily because of a decrease in retentive structures. From our observations on the Barwon–Darling River, a semi-arid river in southeastern Australia, we suggest that geomorphic complexity plays a fundamental role in the retention of organic matter. The Barwon–Darling River has a ‘complex’ river channel cross-section with large inset benches being a prominent morphological feature within the channel. The importance of geomorphic complexity for retaining organic material is likely to be significant in dryland rivers. These rivers spend extended periods at low flow with infrequent large floods that inundate the floodplain. They do, however, experience more frequent within channel floods that inundate in-channel ‘bench’ features. In-channel geomorphic complexity and its ability to retain organic material, therefore, means that although the dominant lateral movements of organic material will still occur during large overbank flows, smaller ‘pulse’ inputs will occur with each in-channel rise and fall in water level. In dryland rivers, where large overbank flows may only occur every seven or more years, these small ‘pulse’ inputs of organic material may well be vital for the integrity of the system.This paper describes the contemporary complexity of a channel in a regulated and an unregulated reach of the Barwon–Darling and compares this with cross-sections surveyed in 1886. We show that flow regulation has greatly reduced channel complexity. We estimate the potential organic matter input to each bench level within the channel (using data collected under near natural riparian conditions) and measure the contemporary organic loads within the channel of the regulated and unregulated reach. This modelling suggests that the development of water resources has reduced the complexity of the channel in the regulated reach, resulting in a potential decrease in the retention of organic matter in this region of the river. The importance of this organic matter to the aquatic food web of the Barwon–Darling River is also demonstrated. 相似文献
5.
The Colville basin drains the North Slope of Alaska and is one of several large Arctic river systems located within permafrost. The timing and style of fluvial processes in the earth's permafrost regions differ from those occurring in midlatitude settings. Moreover, in comparison to temperate-zone systems, rivers located entirely within permafrost perform most of their work during relatively short periods of time. This paper examines river ice hydrology and the resulting geomorphic processes that occur within the Colville delta, Alaska. Fluvial processes and landform development within the Colville delta occur after the flood-pulse is initiated by the breakup of river ice. During this 4-month period, the geomorphic processes are largely influenced by the movement of ice. The flood-pulse and accompanying river ice influences erosional and depositional processes and results in unique styles of sediment transport, deposition, and riverbank erosion. 相似文献
6.
Impacts of colonial settlement upon catchment-scale fluvial geomorphic relationships are reported for a relatively small catchment in northern Rio de Janeiro State, Brazil. Structural controls have induced the type and patterns of rivers in Macaé Basin. Fault block activity has resulted in steep, incised headwater streams above the escarpment. Confined and partly confined rivers in mid-catchment reaches of the rounded foothills have limited potential for geomorphic adjustment. Fluvial, estuarine and marine sediments in low relief landscapes of the lowland plain have supported the development of meandering sand bed rivers, with many cut-and-fill (intact valley fill) deposits in tributary systems. Indigenous people exerted relatively minor, localized impacts upon the geomorphology of this river system. Portuguese settlement since the sixteenth century brought about clearance of much of the Atlantic Forest of lowland reaches, and subsequent establishment of sugar cane and coffee plantations. Lowland reaches were channelized from the 1940s-1980s for flood protection and to support the expansion of pastoral agriculture. Significant adjustments have occurred to these geomorphologically sensitive reaches. In contrast, although rivers in the rounded foothills were impacted by forest clearance, the limited availability of sediment stores along these reaches has limited the extent of geomorphic responses to human disturbance. Relatively inaccessible upland reaches were even less impacted, and are now major conservation areas. Building on principles of the River Styles framework, catchment-scale evolutionary trajectories of rivers in the Macaé Basin are assessed based on analysis of patterns of river types, their capacity for adjustment and connectivity relationships, and responses to disturbance events. From this, three future scenarios of prospective evolutionary traits are developed: a ‘steady as she goes’ scenario, an optimistic (effective, proactive management) scenario, and a ‘doomsday’ scenario. 相似文献
7.
The Late Glacial and Holocene geomorphology of the Manx uplands has received scant attention in previous researches. Solifluction deposits and terraces provide the earliest evidence for geomorphic activity after deglaciation. Fluvial incision into drift-choked valleys is correlated with the formation of the large mountain front alluvial fans that flank the Manx uplands. Formation of these alluvial fans is constrained to 15,000–10,500 cal. years BP by 14C dates on organic deposits beneath and above the alluvial fan gravels. Alluvial fan and river terraces along four valleys postdate this incision. Optically Stimulated Luminescence (OSL) and 14C dating provide a tentative chronology for these landforms. The higher terraces are Late Glacial fluvial surfaces that were probably occupied by rivers into the Holocene. Incision during the Late Holocene led to the abandonment of the higher surfaces, producing a suite of younger river terraces and alluvial fan surfaces. Independent dating constrains this fluvial activity to post-Bronze Age (3500–2800 cal. years BP). Increased human activity and climatic change during the Late Holocene are possible causes for this increased geomorphic activity. 相似文献
8.
河型成因的各种理论及其间关系 总被引：14，自引：1，他引：14
本文较为系统地将河型成因的各种研究方法按其所依据的理论分类,并指出了各种理论之间的内在联系。文中讨论了各种研究方法所具有的优缺点、各研究方法之间的互补作用以及各种研究方法在表面处理方式不同的背后所隐藏的使河流趋向某一极值状态(或稳定状态)的共同倾向,并据此指出了进一步研究河型成因问题时的侧重面。 相似文献
9.
The “perfect landscape” concept is based on the notion that any specific geomorphic system represents the combined, interacting effects of a set of generally applicable global laws and a set of geographically and historically contingent local controls. Because the joint probability of any specific combination of local and global controls is low, and the local controls are inherently idiosyncratic, the probability of existence of any given landscape is vanishingly small. A perfect landscape approach to geomorphic complexity views landscapes as circumstantial, contingent outcomes of deterministic laws operating in a specific environmental and historical context. Thus, explaining evolution of complex landscapes requires the integration of global and local approaches. Because perfection in this sense is the most important and pervasive form of complexity, the study of geomorphic complexity is not restricted to nonlinear dynamics, self-organization, or any other aspects of complexity theory. Beyond what can be achieved via complexity theory, the details of historical and geographic contexts must be addressed. One way to approach this is via synoptic analyses, where the relevant global laws are applied in specific situational contexts. A study of non-acute tributary junctions in the lower Brazos River, Texas illustrates this strategy. The application of generalizations about tributary junction angles, and of relevant theories, does not explain the unexpectedly high occurrence or the specific instances of barbed or straight junctions in the study area. At least five different causes for the development of straight or obtuse junction angles are evident in the lower Brazos. The dominant mechanism, however, is associated with river bank erosion and lateral channel migration which encroaches on upstream-oriented reaches of meandering tributaries. Because the tributaries are generally strongly incised in response to Holocene incision of the Brazos, the junctions are not readily reoriented to the expected acute angle. The findings are interpreted in the context of nonlinear divergent evolution, geographical and historical contingency, synoptic frameworks for generalizing results, and applicability of the dominant processes concept in geomorphology. 相似文献
10.
干旱区沙漠与河流复合地貌过程研究进展 总被引：1，自引：1，他引：0
沙漠-河流过渡带是独特的地理单元,在世界主要沙漠中普遍存在,河岸沙丘地貌发育过程是其独特性的突出表现。一方面,河流通过提供物源和场所控制河岸沙丘地貌的发育过程与格局,另一方面,沙丘发育过程改变河道。本文回顾了沙漠与河流复合地貌过程研究取得的重要进展,包括沙漠与河流交互关系、沙漠和河流交互尺度以及河岸沙丘地貌形成与发育过程等。但水系变迁与沙漠演化、沙丘复杂性与河谷形态之间的关系,以及如何建构沙漠河岸风水复合地貌过程模型等问题,将是未来需要突破的难题,这有助于促进风沙地貌与流水地貌的高度融合,丰富沙漠科学和干旱区水文的理论体系。 相似文献
11.
基于生态功能的河流等级划分及应用——以浙江省河流为例 总被引：1，自引：0，他引：1
河流等级划分是认识河流规律的一项基础性工作,对河流的治理和保护管理具有现实意义。现有的河流等级划分难以反映河流的时空特性,有的人为影响较大,有的不具备河流地貌特征,有的仅具备物理特征,有的仅方便行政管理。本研究基于河流连续体概念,综合了河流的物理、生物特征,引用北美新近提出的"基于生态功能的河流等级划分",阐明了对浙江省河流进行基于生态功能的等级划分意义。从流域尺度上构建了浙江省河流的等级层次结构,分析了浙江省河流各等级层次的物理特征和生态特征的共性和差异,明确了河流各等级层次的生态保护目标和相关对策。以钱塘江流域为例,对其进行了详细的等级划分,探讨了基于生态功能的河流等级划分在确定浙江省河流生态需水的作用及应用前景,并明确了在人类活动影响下,浙江省水行政管理机构在河流等级层次的相关管理范围和内容。 相似文献
12.
Over the past 10 years many restoration projects have been undertaken in Austria, and river engineering measures such as spur dykes and longitudinal bank protection, which imposed fixed lateral boundaries on rivers, have been removed. The EU-Life Project “Auenverbund Obere Drau” has resulted in extensive restoration on the River Drau, aimed to improve the ecological integrity of the river ecosystem, to arrest riverbed degradation, and to ensure flood protection. An essential part of the restoration design involved the consideration of self-forming river processes, which led to new demands being imposed on river management.This paper illustrates how model complexity is adapted to the solution and evaluation of different aspects of river restoration problems in a specific case. Point-scale monitoring data were up-scaled to the whole investigation area by means of digital elevation models, and a scaling approach to the choice of model complexity was applied. Simple regime analysis methods and 1-D models are applicable to the evaluation of long-term and reach-scale restoration aims, and to the prediction of kilometre-scale processes (e.g. mean river bed aggradation or degradation, flood protection). 2-D models gave good results for the evaluation of hydraulic changes (e.g. transverse flow velocities, shear stresses, discharges at diffluences) for different morphological units at the local scale (100 m–10 m), and imposed an intermediate demand on calibration data and topographic survey. The study shows that complex 3-D numerical models combined with high resolution digital elevation models are necessary for detailed analysis of processes (1 m–0.01 m), but not for the evaluation of the restoration aims on the River Drau. In conclusion, model choice (complexity) will depend on both lower limits (determined by the complexity of processes to be analysed) and upper limits (field data quality and process understanding for numerical models). 相似文献
13.
Molly Pohl 《The Professional geographer》2004,56(3):422-431
Dams are a major source of fragmentation and degradation of rivers. Although substantial research has been conducted on the environmental impacts of large structures in the United States, smaller dams have received less attention. This study evaluated the impact of two dams of moderate size, the Elwha Dams, on the downstream channel system using field data collection at river cross‐sections. The relationship of average boundary shear stress (τo) to critical shear stress (τcr) served as the basis for determining channel bed material mobility under the two‐year and ten‐year flood events. The channel had the greatest channel bed mobility at the natural cross‐section upstream from the dams, low bed mobility between the structures, and an increase in channel bed mobility in the low gradient river segment near the mouth of the river. Low bed mobility tended to be associated with a lack of channel system complexity, including reduction or loss of bars and low alluvial terraces and their associated young riparian communities. Although these run‐of‐the‐river dams do not modify streamflow greatly, the loss of sediment from the channel system has had a substantial impact on bed mobility and geomorphic and biotic complexity of the Elwha River. 相似文献
14.
Distance is a key variable in explicating environmental, social, and economic conditions and in defining spatial and temporal patterns. Prior research has primarily focused on using simple to complex algorithms for calculating distances along road networks. In contrast, few algorithms are available for distance calculations along fluvial networks which are often more erratic, divergent and transient than road networks. Fluvial transportation is relied upon worldwide, particularly in developing regions, where communities use river networks for transportation, access to natural resources and for trade and commerce. This paper presents a methodology developed for mapping complex fluvial networks for travel distance analysis. The methodology was applied in four major river basins in western Amazonia over some 35,000 km of river length and incorporating 919 communities as origins/destinations. A cost path and network analysis methodology was created using vector and raster datasets in a Geographic Information System (GIS) to assess interactions among communities and the distances traveled by river to reach district capitals, major urban centres and marketplaces. An accuracy assessment using distance values calculated from a previous study using a different methodological approach in the region as well as Google Earth Pro, found a high degree of concordance for distance calculations. Our methodology creates a very flexible approach for complex river systems that can be use to calculate river distances in an adaptive and efficient manner and that can be use in other regions of the world where rural communities must rely on rivers for transportation. 相似文献
15.
Human modifications to the sediment regime of the Lower Mississippi River flood plain 总被引：3，自引：0，他引：3
The Mississippi River is one of the most regulated rivers in the world. Human modifications constructed mainly after 1920 include dams and reservoirs, artificial levees, dikes, concrete revetments and a series of channel cutoffs. This paper examines some of the effects of these modifications on the channel and sediment budget of the river. In particular, the changes to the thalweg profile and the size of channel bars are examined in detail. It is concluded, that prior to the 1930s, when major modifications were introduced, the Lower Mississippi River was an aggrading meandering river. The role of the flood plain has also changed. Prior to modifications, the flood plain was the major sediment source as the result of bank caving. Today the flood plain provides only a minor amount of sediment. It can be shown that major degradation to the channel including the growth of channel bars has occurred as a result of these engineered modifications. The data also indicates that the different geomorphic regions respond to modifications in different ways. 相似文献
16.
全新世以来山东半岛东北部海面变化的河流地貌响应 总被引：6，自引：1，他引：5
对全新世以来山东半岛东北部沿岸的相对海面（基准面）变化,不同河流有截然不同的地貌响应方式。大沽夹河型河流的下游河谷中有分别与相对海面升降对应的溯源堆积期和溯源侵蚀期,但不同规模的河流仍有差异。黄水河型河流河谷未因基准面升降而发生侵蚀或堆积,河谷仅起了输沙通道的作用。此外,大沽夹河型河流的中下游河型在相对海面上升或下降时,还可能发生相应地网状化或曲流化。 相似文献
17.
A large number of rivers in Tuscany have channel planforms, which are neither straight nor what is usually understood as meandering. In the typical case, they consist of an almost straight, slightly incised main channel fringed with large lateral bars and lunate-shaped embayments eroded into the former flood plain. In the past, these rivers have not been recognised as an individual category and have often been considered to be either braided or meandering. It is suggested here that this type of river planform be termed pseudomeandering.A typical pseudomeandering river (the Cecina River) is described and analysed to investigate the main factors responsible for producing this channel pattern. A study reach (100×300 m) was surveyed in detail and related to data on discharge, channel changes after floods and grain-size distribution of bed sediments. During 18 months of topographic monitoring, the inner lateral bar in the study reach expanded and migrated towards the concave outer bank which, concurrently, retreated by as much as 25 m. A sediment balance was constructed to analyse bar growth and bank retreat in relation to sediment supply and channel morphology. The conditions necessary to maintain the pseudomeandering morphology of these rivers by preventing them from developing a meandering planform, are discussed and interpreted as a combination of a few main factors such as the flashy character of floods, sediment supply (influenced by both natural processes and human impact), the morphological effects of discharges with contrasting return intervals and the short duration of flood events. Finally, the channel response to floods with variable sediment transport capacity (represented by bed shear stress) is analysed using a simple model. It is demonstrated that bend migration is associated with moderate floods while major floods are responsible for the development of chute channels, which act to suppress bend growth and maintain the low sinuosity configuration of the river. 相似文献
18.
Sanjeev Joshi 《自然地理学》2018,39(2):140-165
Changes in channel morphology provide relevant insights into sediment transport and deposition in alluvial river systems. This study assessed three to four decades of morphological changes at seven locations along a 327-km reach of the Lower Mississippi River (LMR) to better understand channel adjustment processes of this large alluvial river. The assessment included analysis of three cross-sectional areas at each location during the period 1992–2013, as well as analysis of the changes in river stage and maximum surface slopes under four flow conditions over the last three to four decades . We found that the first 20–25 km LMR reach below its diversion to the Atchafalaya River and the reach from 80 to 140 km experienced significant riverbed aggradation, while the reach in between (i.e. from 20 to 80 km) experienced riverbed degradation. The lower 187-km reach (i.e. from 140 to 327 km) showed negligible sediment trapping. These findings may have relevant implications for management of river sediment diversions along the LMR and other large alluvial rivers in the world. 相似文献
19.
An assessment of socioeconomic-environmental vulnerability of the ten major river basins in the Monsoon Asia-Pacific region was performed. The rivers include Indus, Ganges-Brahmaputra-Meghna (GBM), Irrawaddy, Salween, Chao Phraya, Mekong, Red River, Pearl River, Yangtze and Yellow River. The study looked at a combination of various issues, usually investigated in isolation from each other, on a sub-continental scale by using the river basins as the geographical entities for the study. The analysis was conducted by using various geospatial databases including data on nature and environment, demography, governance, water scarcity as well as social and economic development. A vulnerability profile for each of the river basins was produced by using two different approaches. We found that the South Asian basins (Indus and GBM) appeared essentially more vulnerable than the other investigated basins. The other basins are roughly at the same vulnerability level, but their vulnerability profiles differ substantially. The Chinese basins and the Red River are particularly vulnerable to environmental factors. The weak points of Irrawaddy and Salween are in the hazards and economic development, whereas the former is a challenge to Mekong as well. Only Indus and Yellow River suffer from considerable water scarcity. The other basins except GBM are relatively well-off with the social matters. A transparent and pragmatic river basin vulnerability assessment approach was developed, which can be used in any large river basin of the world, producing intercomparable results. The approach allows further decoupling of the used indices in the case of more specific and targeted analyses without losing the comparability of the overall results. 相似文献
20.
朱圣钟 《云南地理环境研究》2006,18(5):8-13
历史时期四川凉山地区河流变迁研究在凉山地区研究中尚属薄弱环节，该文对历史时期凉山地区主要河流的水文状况进行了复原性探讨，并对各河流的水文状况的变化进行了研究．认为历史时期凉山地区河流河道变化小．各主要河流流域内的植被日趋萎缩，总体水量呈下降趋势，河流的泥沙含量呈上升趋势，这些变化主要是历史时期凉山地区经济开发力度加大破坏了局部地区的生态环境造成的。 相似文献