Relief-rejuvenation and topographic length scales in a fluvial drainage basin, Napf area, Central Switzerland   总被引：2，自引：2，他引：2  

Fritz Schlunegger  Heinz Schneider 《Geomorphology》2005,69(1-4):102-117

This paper explores how, and to what extent, a phase of relief-rejuvenation modifies the mode of surface erosion in an approximately 63 km² drainage basin located at the northern border of the Swiss Alps (Luzern area). In the study area, the retreat of the Alpine glaciers at the end of the Last Glacial Maximum (LGM) caused base level to lower by approximately 80 m. The fluvial system adapted to the lowered base level by headward erosion. This is indicated by knickzones in the longitudinal stream profiles and by the continuous upstream narrowing of the width of the valley floor towards these knickzones. In the headwaters above these knickzones, processes are still to a significant extent controlled by the higher base level of the LGM. There, frequent exposure of bedrock in channels and especially on hillslopes implies that sediment flux is to a large extent limited by weathering rates. In the knickzones, however, exposure of bedrock in channels implies that sediment flux is supply-limited, and that erosion rates are controlled by stream power.The morphometric analysis reveals the existence of length scales in the topography that result from distinct geomorphic processes. Along the tributaries where the upstream sizes of the drainage basins exceed 100,000–200,000 m², the mode of sediment transport and erosion changes from predominantly hillslope processes (i.e., landsliding, creep of regolith, rock avalanches and to some extent debris flows) to processes in channels (fluvial processes and debris flows). This length scale reflects the minimum size of the contributing area for channelized processes to take over in the geomorphic development (i.e., threshold size of drainage basin). This threshold size depends on the ratio between production rates of sediment on hillslopes, and export rates of sediment by processes in channels. Consequently, in the headwaters, erosion rates and sediment flux, and hence landscape evolution rates, are to a large extent limited by weathering processes. In contrast, in the lower portion of the drainage basin that adjusts to the lowered base-level, rates of channelized erosion and relief formation are controlled mainly by stream power. Hence, this paper shows that base-level lowering, headward erosion and establishment of knickzones separate drainage basins in two segments with different controls on rates of surface erosion, sediment flux and relief formation.  相似文献   

Impact of human activity on the estuary of the Qiantang River and the reclamation of tidal flats and river regulation     

Li-Hua Feng  Yi-Xin Bao 《Environmental Geology》2005,49(1):76-81

The swing of the main channel of the Qiantang River is controlled by the high-water and low-water changes in the river, as well as the impact of large-scale reclamation of tidal flats. Its evolution in modern times is the result of the combined functions of natural and man-made factors. This paper analyzes the cause of the formation of the South Channel and Xisan Tidal Furrow and proposes the regulation principle of “To regulate the river and reclaim tidal flats by taking the advantage of local topography”. It is suggested to cut off the South Channel and Xisan Tidal Furrow completely to restrict the swing of the main channel and to increase the reclamation area of the tidal flat at the same time.  相似文献   

Deep Mechanical Background for the Cenozoic Volcanism in the Tibetan Plateau   总被引：1，自引：0，他引：1  

Xiong Xiong* Institute of Geodesy  Geophysics  Chinese Academy of Sciences  Wuhan  China  Institute of Geology  Geophysics  Chinese Academy of Sciences  Beijing  China Wang Jiye Institute of Geodesy  Geophysics  Chinese Academy of Sciences  Wuhan  China  Graduate School  Chinese Academy of Sciences  Beijing  China Teng Jiwen Institute of Geology  Geophysics  Chinese Academy of Sciences  Beijing  China 《中国地质大学学报(英文版)》2005,16(4)

INTRODUCTION Volcanoesaremostlyobservedinoceanicridges,hotspotsandcontinentalriftzones(Hongetal.,2003),andarerarelyobservedincontinentalinteri ors.However,sincethevolcanoeswithintheconti nentinteriorscannotbeattributedtotheplate/block marginprocess,theydr…  相似文献   

北方农牧交错带退耕还林还草经济政策优化调控   总被引：4，自引：0，他引：4  

甘超华  马礼  南秋菊 《地域研究与开发》2005,24(4):66-69

退耕还林还草作为北方农牧交错带生态重建的切入点,在实际操作中也不可避免地存在一定的问题。文章对当前退耕还林还草的社会经济特征、政策安排、协调机制及制约因素进行了分析。提出:区域粮食适度自给定位、部门协调机制创新、产权制度改革、区域产业专业化分工、适度生态移民及建立生态补偿机制为进行政策优化调控的主要途径。  相似文献   

Metrics to assess how longitudinal channel network connectivity and in‐stream Atlantic salmon habitats are impacted by hydropower regulation          下载免费PDF全文

Willem B. Buddendorf  Iain A. Malcolm  Josie Geris  Mark E. Wilkinson  Chris Soulsby 《水文研究》2017,31(12):2132-2142

Habitat fragmentation in channel networks and riverine ecosystems is increasing globally due to the construction of barriers and river regulation. The resulting divergence from the natural state poses a threat to ecosystem integrity. Consequently, a trade‐off is required between the conservation of biodiversity in channel networks and socio‐economic factors including power generation, potable water supplies, fisheries, and tourism. Many of Scotland's rivers are regulated for hydropower generation but also support populations of Atlantic salmon (Salmo salar L.) that have high economic and conservation value. This paper investigates the use of connectivity metrics and weightings to assess the impact of river barriers (impoundments) associated with hydropower regulation on natural longitudinal channel connectivity for Atlantic salmon. We applied 2 different weighting approaches in the connectivity models that accounted for spatial variability in habitat quality for spawning and fry production and contrasted these models with a more traditional approach using wetted area. Assessments of habitat loss using the habitat quality weighted models contrasted with those using the less biologically relevant wetted area. This highlights the importance of including relevant ecological and hydrogeomorphic information in assessing regulation impacts on natural channel connectivity. Specifically, we highlight scenarios where losing a smaller area of productive habitat can have a larger impact on Atlantic salmon than losing a greater area of less suitable habitat. It is recommended that future channel connectivity assessments should attempt to include biologically relevant weightings, rather than relying on simpler metrics like wetted area which can produce misleading assessments of barrier impacts.  相似文献   

基于垂直剖面函数的天顶对流层延迟插值算法     

徐康  邓兴升 《大地测量与地球动力学》2020,40(1):66-70

通过分析由ERA-Interim气象再分析资料积分方法得到的天顶对流层总延迟随高程变化的规律,提出一种基于垂直剖面函数的天顶对流层延迟(ZTD)插值算法。该算法以ZTD的垂直分布规律为基础,通过垂直剖面函数实现ZTD在高程方向上的精准投影延拓,可以避免因高差较大造成的空间内插结构畸形。采用IGS站提供的高精度对流层产品进行实验验证表明,该算法相对于传统算法能够有效提高ZTD改正值的精度,尤其在高差超过1 km的情况下,相对于反距离加权法精度提升了96%,相对于空间回归法精度提升了79%。  相似文献   

Mid-late Holocene anthropogenic and natural variations in geochemistry and sedimentation in the Yazoo Basin: Clark Lake,Sharkey County,Mississippi     

Stan Galicki  Maria Bujenovic  Theresa Woehnker  Jeb Galtney  Trevor Galicki 《自然地理学》2019,40(3):209-226

A 4000-yr history at Clark Lake reveals natural variations in sediment geochemistry and nutrient levels and anthropogenic influence on twentieth century sedimentation rates. Sediment texture and ridge and swale topography indicate that the channel system creating Clark Lake is now occupied by the Yazoo River. Between 1.24 and 0.60 m (2522–865 cal yr BP), total carbon percentages and the C/N ratio average 42% and 17, respectively. The base of the 1650-yr interval and onset of high organic activity in the lake coincides with the abandonment of the Yazoo Meander Belt. The top of the interval is marked by a drop in C/N ratio to 11 and a geochemical transition zone. No change in the rate of sediment accumulation or clay mineralogy was observed despite increased cumulative percentages of Al, Fe, K, Mg, Mn, Ca, and Na. The decrease in organic activity at 865 cal yr BP is attributed to unfavorable growth conditions related to the entrenchment of the Yazoo River and changes in the hydroperiod of the area. Significant floodplain alteration was completed in 1978, but the only major effect was increased sedimentation in the late twentieth century due to the completion of the Whittington Auxiliary Channel.  相似文献   

Deducing the spatial variability of exchange within a longitudinal channel water balance          下载免费PDF全文

Noah M. Schmadel  Bethany T. Neilson  Tamao Kasahara 《水文研究》2014,28(7):3088-3103

Developing an appropriate data collection scheme to infer stream–subsurface interactions is not trivial due to the spatial and temporal variability of exchange flowpaths. Within the context of a case study, this paper presents the results from a number of common data collection techniques ranging from point to reach scales used in combination to better understand the spatial complexity of subsurface exchanges, infer the hydrologic conditions where individual influences of hyporheic and groundwater exchange components on stream water can be characterized, and determine where gaps in information arise. We start with a tracer‐based, longitudinal channel water balance to quantify hydrologic gains and losses at a sub‐reach scale nested within two consecutive reaches. Next, we look at groundwater and stream water surface levels, shallow streambed vertical head gradients, streambed and aquifer hydraulic conductivities, water chemistry, and vertical flux rates estimated from streambed temperatures to provide more spatially explicit information. As a result, a clearer spatial understanding of gains and losses was provided, but some limitations in interpreting results were identified even when combining information collected over various scales. Due to spatial variability of exchanges and areas of mixing, each technique frequently captured a combination of groundwater and hyporheic exchange components. Ultimately, this study provides information regarding technique selection, emphasizes that care must be taken when interpreting results, and identifies the need to apply or develop more advanced methods for understanding subsurface exchanges. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Composition and flux of suspended organic matter in the middle and lower reaches of the Changjiang (Yangtze River) — impact of the Three Gorges Dam and the role of tributaries and channel erosion          下载免费PDF全文

Hongyan Bao  Ying Wu  Jing Zhang  Bing Deng  Qing He 《水文研究》2014,28(3):1137-1147

To investigate the sources of particulate organic matter (POM) and the impact of Three Gorges Dam (TGD), two large lakes and erosion processes on determining the composition and flux of POM in low water discharge periods along the middle and lower Changjiang, suspended particulate samples were collected along the middle and lower reaches of the Changjiang (Yangtze River) in January 2008. Organic geochemistry of bulk sediment (particulate organic carbon, organic carbon to nitrogen molar ratio (C/N), stable carbon isotope (δ¹³C) and grain size) and biomarker of bulk sediment (lignin phenols) were measured to trace the sources of POM. The range of C/N ratios (6.4–8.9), δ¹³C (?24.3‰ – ?26.2‰) and lignin phenols concentration Λ8 (0.45 mg/100 mg OC‐2.00 mg/100 mg OC) of POM suggested that POM originated from the mixture of soil, plant tissue and autochthonous organic matter (OM) during the dry season. POM from lakes contained a higher portion of terrestrial OM than the mainstream, which was related to sand mining and hydropower erosion processes. A three end‐member model based on δ¹³C and Λ8 was performed. The results indicated that soil contributed approximately 50% of OM to the POM, which is the dominant OM source in most stations. POM composition was affected by total suspended matter (TSM) and grain size composition, and the direct OM input from two lakes and channel erosion induced OM. The lower TSM concentration in January 2008 was mainly caused by seasonal variations; the impact from the TGD in the dry season was relatively small. A box model indicated that more than 90% of the terrestrial OM transported by the Changjiang in January 2008 was from the middle and lower drainage basins. Channel erosion induced OM, and contributions from Poyang Lake were the major terrestrial OM sources in the dry season. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

2012-2016年调水调沙对黄河口海域大型底栖动物群落结构的影响     

李少文  李凡  宋秀凯  张明亮 《海洋学报(英文版)》2020,39(10):120-128

为了明确黄河调水调沙对河口海域大型底栖动物群落结构的影响，本研究于2012至2016年调水调沙后进行了大型底栖动物样品的采集工作。整个研究时期共鉴定出大型底栖动物7门138种，生物量和丰度2012年显著高于2016年，生物量年间无显著差异；研究时期内优势种以小型多毛类为主，鲜见软体动物、节肢动物和棘皮动物，其中2016年优势种全部为小个体多毛类；MDS显示，随时间的推移距河口不同距离梯度的大型底栖动物逐渐演变为相同的群落结构，且Shannon-Wiener多样性指数和Margalef丰富度指数呈下降趋势；CCA显示研究海域主要底栖动物种类的生态需求为高中值粒径、低溶解氧、pH、黏土和硫化物含量。本研究推测调水调沙工程一定程度上影响了底层生物群落结构，特别是研究海域的A断面。  相似文献