Remote sensing and geomorphometry for studying relief production in high mountains

Mountain topography is the result of highly scale-dependent interactions involving climatic, tectonic, and surface processes. No complete understanding of the geodynamics of mountain building and topographic evolution yet exists, although numerous conceptual and physical models indicate that surficial erosion plays a significant role. Mapping and assessing landforms and erosion in mountain environments is essential in order to understand landscape denudation and complex feedback mechanisms. This requires the development and evaluation of new approaches in remote sensing and geomorphometry. The research herein evaluates the problem of topographic normalization of satellite imagery and demonstrates the use of terrain analysis using a digital elevation model (DEM) to evaluate the relief structure of the landscape in the western Himalaya. We specifically evaluated the Cosine-correction and Minnaert-correction methods to reduce spectral variation in imagery caused by the topography. Semivariogram analyses of the topography were used to examine the relationships between relief and surface processes. Remote-sensing results indicate that the Minnaert-correction method can be used to reduce the “topographic effect” in satellite imagery for mapping, although extreme radiance values are the result of not accounting for the diffuse-skylight and adjacent-terrain irradiance. Geomorphometry results indicate that river incision and glaciation can generate extreme relief, although the greatest mesoscale relief is produced by glaciation at high altitudes. At intermediate altitudes, warm-based glaciation was found to decrease relief. Our results indicate that glaciation can have a differential influence on the relief structure of the landscape. Collectively, our results indicate that scale-dependent analysis of the topography is required to address radiation transfer issues and the polygenetic nature of landscape denudation and relief production.     

Hillslope-to-valley transition morphology: New opportunities from high resolution DTMs

The search for the optimal spatial scale for observing landforms to understand physical processes is a fundamental issue in geomorphology. Topographic attributes derived from Digital Terrain Models (DTMs) such as slope, curvature and drainage area provide a basis for topographic analyses. The slope–area relationship has been used to distinguish diffusive (hillslope) from linear (valley) processes, and to infer dominant sediment transport processes. In addition, curvature is also useful in distinguishing the dominant landform process. Recent topographic survey techniques such as LiDAR have permitted detailed topographic analysis by providing high-quality DTMs. This study uses LiDAR-derived DTMs with a spatial scale between 1 and 30 m in order to find the optimal scale for observation of dominant landform processes in a headwater basin in the eastern Italian Alps where shallow landsliding and debris flows are dominant. The analysis considered the scaling regimes of local slope versus drainage area, the spatial distribution of curvature, and field observations of channel head locations. The results indicate that: i) hillslope-to-valley transitions in slope–area diagrams become clearer as the DTM grid size decreases due to the better representation of hillslope morphology, and the topographic signature of valley incision by debris flows and landslides is also best displayed with finer DTMs; ii) regarding the channel head distribution in the slope–area diagrams, the scaling regimes of local slope versus drainage area obtained with grid sizes of 1, 3, and 5 m are more consistent with field data; and iii) the use of thresholds of standard deviation of curvature, particularly at the finest grid size, were proven as a useful and objective methodology for recognizing hollows and related channel heads.     

Insights into the “tectonic topography” of the present-day landscape of the central Iberian Peninsula (Spain)

The landscape of today's central Iberian Peninsula has been shaped by ongoing tectonic activity since the Tertiary. This landscape comprises a mountain ridge trending E–W to NW–SE, the Central System, separating two regions of smooth topography: the basins of the rivers Duero and Tajo. In this study, we explore interrelationships between topography and tectonics in the central Iberian Peninsula. Regional landscape features were analysed using a digital elevation model (DEM). Slope gradients and slope orientations derived from the DEM were combined to describe topographic surface roughness. Topography trend-surfaces inferred from harmonic analysis were used to define regional topographic features. Low roughness emphasizes the smooth nature of the basins' topography, where surfaces of homogeneous slope gradient and orientation dominate. High roughness was associated with abrupt changes in gradient and slope orientation such as those affecting crests, valley bottoms and scarp edges present in the mountain chain and in some deep incised valleys in the basins. One of the applications of roughness mapping was its capacity to isolate incised valley segments. The area distribution of incised rivers shows their prevalence in the east. On a regional scale, the topographic surface can be described as a train of NE–SW undulations or waves of 20 km wavelength. These undulations undergo changes in direction and interruptions limited by N–S-trending breaks. E–W and NE–SW troughs and ridges clearly mark structural uplifts and depressions within the Central System. These structures are transverse to the compressive NW–SE stress field that controlled the deformation of the central Iberian Peninsula from the Neogene to the present. They represent the upper crustal folding that accommodates Alpine shortening. N–S breaks coincide with Late Miocene faults that control the basins' sedimentation. Further, associated palaeoseismic structures suggest the recent tectonic activity of N–S faults in the eastern part of the Tajo Basin. Apatite fission track analysis data for this area suggest the occurrence of a significant uplift episode from 7 to 10 Ma which induced the river incisions appearing in the roughness map. N–S and NE–SW faults could be seismogenic sources for the current moderate to low seismic activity of the east Tajo Basin and southeast Central System. Although N–S fault activity has already been established, we propose its significant contribution to shaping the landscape.     

荒漠绿洲植被变化与景观格局耦合关系的研究--以新疆三工河流域为例

以三工河流域为例,讨论了荒漠绿洲植被变化与景观格局的耦合关系.结果表明:(1)荒漠绿洲景观格局最明显的特征是植被变化,其通过地貌、水资源利用、河流廊道或排碱渠等景观要素的变化对植被产生影响;随着景观格局的变化,流域内的植丛高度、盖度及生物量均表现出较大变异性.(2)以河流廊道为核心,通常荒漠绿洲景观格局呈带状分布且由内向外,随水热条件的改变,植被类型由乔灌木林依次向灌丛草甸、盐化草甸、荒漠化草甸和荒漠层次结构过渡.(3)人类活动影响强烈的景观要素,其缀块多样性、破碎度、分离度和缀块密度等指数的值较高,而人类活动影响较小的,其优势度和均匀度较高.(4)各景观要素的稳定性不同,通常自然缀块的稳定性高于人工缀块,其排序为:荒漠>砾石地、沙地>城镇工矿用地>水体>灌溉水田>牧草地>菜地>林地>农村居民点>盐碱地>荒草地>水浇地>果园.     

The perfect landscape

The “perfect storm” metaphor describes the improbable coincidence of several different forces or factors to produce an unusual outcome. The perfect landscape is conceptualized as a result of the combined, interacting effects of multiple environmental controls and forcings to produce an outcome that is highly improbable, in the sense of the likelihood of duplication at any other place or time. Geomorphic systems have multiple environmental controls and forcings, and degrees of freedom in responding to them. This allows for many possible landscapes and system states. Further, some controls and forcings are causally contingent. These contingencies are specific to time and place. Dynamical instability in many geomorphic systems creates and enhances some of this contingency by causing the effects of minor initial variations and small disturbances to persist, and grow disproportionately large, over time. The joint probability of any particular set of global controls is low, as the individual probabilities are < 1, and the probability of any set of local, contingent controls is even lower. Thus, the probability of existence of any landscape or earth surface system state at a particular place and time is negligibly small: all landscapes are perfect. Recognition of the perfection of landscapes leads away from a worldview holding that landforms and landscapes are the inevitable outcomes of deterministic laws, such that only one outcome is possible for a given set of laws and initial conditions. A perfect landscape perspective leads toward a worldview that landforms and landscapes are circumstantial, contingent results of deterministic laws operating in a specific environmental context, such that multiple outcomes are possible.     

Crustal uplift in southern England: evidence from the river terrace records

Much of the past work on the Quaternary rivers of northwest Europe has been concerned with river terraces, which characterise almost every valley. While these terraces are undoubtedly striking features of the landscape, the incision achieved by Quaternary rivers is equally significant in terms of river behaviour, and for an understanding of the factors affecting landform development during the Quaternary. This paper examines the incision achieved during the Quaternary by the Thames, in both its upper and lower catchments, and by the Hampshire Avon in southern England. Valley incision rates of ca. 0.07–0.10 m ka⁻¹ have been identified, although in the lower catchment of the Thames, these have been enhanced by additional incision in response to glacio-isostasy and valley shortening. A model is proposed in which regional uplift is recognized as the primary cause of incision by these Quaternary rivers. Possible mechanisms for regional uplift are considered.     

Space–time variability of denudation rates at the catchment and hillslope scales on the Tyrrhenian side of Central Italy

Studies on denudation rates can provide insight into the influence of climate change, tectonics, and human activities on landscape evolution. Research performed in Central Italy has shown considerable spatial variability of denudation rates in the major river basins. These studies have focused mainly on the Tyrrhenian side of the Italian peninsula, where Plio-Pleistocene marine deposits filling NW–SE elongated sedimentary basins have been uplifted during the Quaternary up to several hundreds of meters above present sea level. Small sub-catchments developed on clays are affected by sharp- and/or rounded-edged badlands (i.e. calanchi and biancane), representing denudation “hot spots” in the present-day morphoclimatic framework.In this paper, we analyze the relationships between indirectly estimated denudation rates at the catchment scale and field monitoring data at the hillslope scale. We attempt to better understand and quantify all hillslope processes that contribute to seasonal variability of denudation, to help with predicting the net input from “hot spots” to the overall estimated sediment yield at the basin outlets. At the hillslope scale, we discuss, in particular, the variability of denudation rates at calanchi and biancane badlands as a function of their different morphoevolution.     

Morphodynamic river processes and techniques for assessment of channel evolution in Alpine gravel bed rivers

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).     

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.     

A model for the active origin and development of source-bordering dunefields on a semiarid fluvial plain: A case study from the Xiliaohe Plain, Northeast China

Source-bordering dunefields have been reported in some drylands of the planet, but scarcely in China where there are extensive drylands. This article reports them in China for the first time, and presents a model for their active origin and development on a semiarid fluvial plain by means of satellite image analyses and field investigations. Local- and regional-scale examples are chosen to analyze the spatial patterns of dunefields, as well as the relationships with the fluvial systems in the central part of Naiman Banner where the Jiaolai River runs, and the lower Laoha River, and the middle and lower Ulijimulun River (principal tributaries of the Xiliaohe River). The active origin and development of source-bordering dunefields can be divided into four stages in terms of the spatial patterns of dunefields and channel dynamics: Stage I — individual dunes on the downwind margins of river valleys where running water constantly erodes the steep slopes of valley and where the downwind slopes orient to local dominant winds; Stage II — individual local-scale dunefields formed by deflation of the steep valley slopes and extending antecedent dunes downwind, together with the downstream displacement of meanders; Stage III — individual large-scale dunefield belts along the downwind margins of river valleys formed through frequent lateral migrations of channel; Stage IV — regional-scale dunefields formed mainly by river diversions due to climatic changes or tectonic movements. On the one hand, it is the running water's lateral migration, especially meandering, that prepares suitable places for aeolian systems in terms of both wind flow fields and sand sources, and subsequently it can further cause separate local-scale source-bordering dunefields to link together as a regional-scale dunefield belt given sufficient time. On the other hand, diversions of the river are bound to occur following changing hydrologic regimes resulting from tectonic movements or significant climate change (at regional and millennium scales). As a result, when some dunefield belts as well as the adjacent channels are abandoned, new channels work elsewhere in the same way to actively form new source-bordering dunefields and even dunefield belts at a regional scale.     

Perfection and complexity in the lower Brazos River

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.     

Constructal view of scaling laws of river basins

River basins are examples of naturally organized flow architectures whose scaling properties have been noticed long ago. Based on data of geometric characteristics, Horton [Horton, R.E., 1932. Drainage basin characteristics. EOS Trans. AGU 13, 350–361.], Hack [Hack, J.T., 1957. Studies of longitudinal profiles in Virginia and Maryland. USGS Professional Papers 294-B, Washington DC, pp. 46–97.], and Melton [Melton, M.A, 1958. Correlation structure of morphometric properties of drainage systems and their controlling agents. J. of Geology 66, 35–56.] proposed scaling laws that are considered to describe rather accurately the actual river basins. What we show here is that these scaling laws can be anticipated based on Constructal Theory, which views the pathways by which drainage networks develop in a basin not as the result of chance but as flow architectures that originate naturally as the result of minimization of the overall resistance to flow (Constructal Law).     

Runoff Scaling in Large Rivers of the World

Runoff and precipitation scaling with respect to drainage area is analyzed for large river basins of the world, those with mean annual runoff in excess of 10 k³/yr. The usefulness of the specific runoff (runoff per unit drainage area, m/yr) to categorize runoff scaling laws across the complete spectrum of climatic and hydrologic conditions is evaluated. It is found that (1) runoff scales with drainage are in those river basins with specific runoff in excess of 0.15 m/yr (r² = 0.88); (2) runoff scaling with drainage area shows remarkably high statistical correlation (r²= 0.97) in river basins with specific runoff equal to or larger than 1.0 m/yr; (3) runoff does not Inc.rease with Inc.reasing drainage area in river basins with specific runoff below 0.15 m/yr, where no discernible statistical association was found between runoff and drainage area; and (4) precipitation depth (m/yr) is inversely proportional to drainage area raised to a fractional exponent in river basins with specific runoff in excess of 0.15 m/yr.     

Complexity in a cellular model of river avulsion

We propose a new model of river avulsion that emphasizes simplicity, self-organization, and unprogrammed behavior rather than detailed simulation. The model runs on a fixed cellular grid and tracks two elevations in each cell, a high elevation representing the channel (levee) top and a low one representing the channel bottom. The channel aggrades in place until a superelevation threshold for avulsion is met. After an avulsion is triggered a new flow path is selected by steepest descent based on the low values of elevation. Flow path depends sensitively on floodplain topography, particularly the presence of former abandoned channels. Several behavioral characteristics emerge consistently from this simple model: (1) a tendency of the active flow to switch among a small number of channel paths, which we term the active channel set, over extended periods, leading to clustering and formation of multistory sand bodies in the resulting deposits; (2) a tendency for avulsed channels to return to their previous paths, so that new channel length tends to be generated in relatively short segments; and (3) avulsion-related sediment storage and release, leading to pulsed sediment output even for constant input. Each of these behaviors is consistent with observations from depositional river systems. A single-valued threshold produces a wide variety of avulsion sizes and styles. Larger “nodal” avulsions are rarer because pre-existing floodplain topography acts to steer flow back to the active channel. Channel stacking pattern is very sensitive to floodplain deposition. This work highlights the need to develop models of floodplain evolution at large time and space scales to complement the improving models of river channel evolution.     

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.     

Landscape disequilibrium on 1000–10,000 year scales Marsyandi River, Nepal, central Himalaya

In an actively deforming orogen, maintenance of a topographic steady state requires that hillslope erosion, river incision, and rock uplift rates are balanced over timescales of 10⁵–10⁷ years. Over shorter times, <10⁵ years, hillslope erosion and bedrock river incision rates fluctuate with changes in climate. On 10⁴-year timescales, the Marsyandi River in the central Nepal Himalaya has oscillated between bedrock incision and valley alluviation in response to changes in monsoon intensity and sediment flux. Stratigraphy and ¹⁴C ages of fill terrace deposits reveal a major alluviation, coincident with a monsoonal maximum, ca. 50–35 ky BP. Cosmogenic ¹⁰Be and ²⁶Al exposure ages define an alluviation and reincision event ca. 9–6 ky BP, also at a time of strong South Asian monsoons. The terrace deposits that line the Lesser Himalayan channel are largely composed of debris flows which originate in the Greater Himalayan rocks up to 40 km away. The terrace sequences contain many cubic kilometers of sediment, but probably represent only 2–8% of the sediments which flushed through the Marsyandi during the accumulation period. At 10⁴-year timescales, maximum bedrock incision rates are 7 mm/year in the Greater Himalaya and 1.5 mm/year in the Lesser Himalayan Mahabarat Range. We propose a model in which river channel erosion is temporally out-of-phase with hillslope erosion. Increased monsoonal precipitation causes an increase in hillslope-derived sediment that overwhelms the transport capacity of the river. The resulting aggradation protects the bedrock channel from erosion, allowing the river gradient to steepen as rock uplift continues. When the alluvium is later removed and the bedrock channel re-exposed, bedrock incision rates probably accelerate beyond the long-term mean as the river gradient adjusts downward toward a more “equilibrium” profile. Efforts to document dynamic equilibrium in active orogens require quantification of rates over time intervals significantly exceeding the scale of these millennial fluctuations in rate.     

Influence of melioration in natural ecological processes of a small river valley (Poland)

On the basis of the topographic maps from 1933, 1951 and 1962 (sc 1:10 000, 1:25 000 and 1:100 000, respectively) and the satellite image from 2014 (sc 1:10 000), we determined changes in the channel of the small Brynica river (Southern Poland) as well as in the land-use pattern along this river valley after its melioration. Terrain regulation and drainage brought about an intensification of the overland runoff, and a decrease in irrigation of the territory. Due to soil overdrying, some tracts of the valley are experiencing a moisture deficiency. Peat drying can lead to negative consequences, because shallow peats mostly occur on the study territory. Disappearance of peaty soils acts to decrease the degree of suitability of the riverside territories for economic purposes as well as their capacity for recovery. The schematic map of vegetation distribution along the river valley prior to and after melioration as compiled from results of field observations displays differences between mosaic quasi-natural vegetation and areas of typical agrocenoses. The preservation of the natural values along the Brynica valley is possible through balanced economic management, while the differentiation of topography contributes to creating favorable conditions for the formation of biodiversity.     

Acoustic Doppler current profiler surveys along the Yangtze River

An acoustic Doppler current profiler is used to characterize the river velocity against the morphology of the Yangtze River from Chonqing to the sea. High flow velocities occur in the Three Gorges section and lower velocities in the middle and lower reaches of the river. This is largely due to the change in river pattern from a high gradient deeply-cut valley to a flat fluvial plain. Flow velocities fluctuate in the middle Yangtze due to the presence of meander bends of different length. There are numerous smaller velocity fluctuations in the lower Yangtze channel that reflect multichannel pattern with numerous sand bars and a river morphology affected by bedrock outcrops. Water depths of 40–100 m occur in the Three Gorges valley but decrease to 15–40 m in the middle and lower Yangtze. At the Gezhou Reservoir, 30 km downstream of the Three Gorges damsite velocity drops to low (< 1.0 m s^− 1) 20 km reach. A second low velocity (< 0.5 m s^− 1) zone, about 20 km in length, is located in the lower Yangtze near the coast probably due to the tidal influence. The results from this research will serve as a datum for evaluating changes to the river once the Three Gorges dam is completed in 2009.     

城市河岸土地利用对河流廊道功能影响初探——以上海苏州河为例

城市河岸土地利用往往改变河岸的自然地理条件,对城市河流功能产生影响。通过总结城市河岸廊道功能以及国内外城市河岸规划土地利用的实践,分析了河岸土地利用对城市河流廊道功能的影响,并以上海苏州河为例进行了实证分析。结果表明,苏州河河岸规划土地利用方式提高了河岸开放带宽度,改善了河岸舒适性,控制了河岸建筑高度,有利于保护和开发苏州河河流的景观、遗产和经济廊道的功能。干流河岸在自然廊道功能方面,仍存在一定缺陷,与支流的部分河段相比仍有差距。     

Comments on Jansson, K.N. and Glasser, N.F. (2008) “Modification of peripheral mountain ranges by former ice sheets: The Brecon Beacons, southern UK,” Geomorphology 97, 178–189

Jansson and Glasser (Jansson, K.N., Glasser, N.F., 2008. Modification of peripheral mountain ranges by former ice sheets: the Brecon Beacons, southern UK. Geomorphology 97, 178–189.) have recently provided unconventional interpretations of selected glacial erosional and depositional landforms in the Brecon Beacons, UK, based on remotely sensed imagery. These new interpretations contradict well-established and reliable evidence for the origins and ages of certain glacial landforms of this upland area and elsewhere. They suggest that during a post-Last Glacial Maximum (LGM) ice-sheet event ice flowed up supposed, essentially “fluvial” valleys producing “glacial lineations” and depositing marginal moraines at the valley heads and on cirque floors. We argue that their interpretations of some key landforms are incorrect and that they have ignored much of the previous dating and field geomorphological evidence. Sedimentary and morphological evidence (e.g., lack of erratic content; convex planform with respect to the headwall; relatively large height range of moraines; and close association with headwall extent, height, and steepness) all indicate that higher level cirque-floor and valley-head moraines in the Brecon Beacons (> c. 400 m) were formed by cirque glaciers. Available dating evidence indicates a Younger Dryas age. We demonstrate that the supposed “fluvial” valleys, comprising trough heads with steep headwalls, have more nearly parabolic than V-shaped cross profiles indicating substantial glacial modification. Field evidence shows that proposed key exemplar post-LGM glacial lineations are in fact debris flow deposits. We conclude that whilst the adoption of a macroscale approach can shed new light on large-scale, ice-sheet movements, this approach should not be undertaken without consideration of the associated field evidence.