摘要:

ABSTRACTThrough an analysis of both the historical and the present-day processes, an effort has been made to deal with the long-term environmental effects induced by man's regulation of the Yellow River of China, which was started more than 2,000 years ago. The artificial levee construction on the Yellow River was proposed as a prerequisite for regional development of the Lower Yellow River alluvial plain. However, the resulting formation of a“hanging river”led to frequent occurrences of course changes, which closely control the micro-landform formation, the soil formation and distribution, the surface water and groundwater features, and so on. Particularly, a descriptive model is proposed to explain the formation process of micro-landforms controlled by the artificial levee construction and the course change of the Yellow River. In history, the Yellow River regulation strategies of“narrowing water flow by diking to scour sediment”and“storing the Huaihe River's clear water to scour the Yellow River's sediment’were proposed and put into action, giving rise to far-reaching environmental effects. The practice of the former greatly increased the delta's extending rate, and the practice of the latter resulted in a sharp expansion of the Lake Hongzhehu, one of the earliest large artificial lakes in the world, and consequently increased the frequency of flood and water-logging hazards in the middle Huaihe River basin.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880385

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACTIt is readily apparent from both map and field observation that the patterns of channels and ridges in fluvially eroded terrain are closely intertwined. The purpose of this paper is the formulation of some of the principles governing their relationship. Specifically, the paper models the sequence and length distribution of ridge paths on the basis of the sequence and length distribution of channel paths within a given drainage basin.The modeling approach first codifies the channel and ridge networks within drainage basins in graph theoretical terms; it then takes advantage of the fact that ridge lines tend to run quasi-parallel to neighboring channels thus permitting the estimation of the former from the latter. Data were obtained from two drainage basins of the (environmentally homogeneous) Cumberland Plateau of Kentucky and the (environmentally complex) San Gabriel Mountains of southern California.Comparison of the observed length values of ridge paths with their values as derived from channel path data demonstrate the intimate relation between channels and ridges, thereby providing support for the model as a formal description of the ridge-channel interdependence.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880386

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACTAlluvial fans have developed under semiarid conditions in the continuous permafrost zone along the Aklavik Range of northwestern Canada. The fans are formed by a series of steep, narrow, coalescing aprons debouching from nivation-modified hollows in shale and siltstone.The morphology and development of one of these fans, the Mount Goodenough North fan, has been studied in detail. Channel development is largely confined to the fan head area, and the lower parts of the fan are marked by gelifluction terracettes and earth hummocks. The non-channelized parts of the fan are dominantly composed of silty diamicton units.The alluvial fan was formed by gelifluction and snowmelt-induced debris flow, with fluvial processes playing a minor role. These processes are a consequence of the semi-arid conditions marked by summer water influx, generated through snowmelt and degradation of segregated ice developed preferentially in the silty diamictons. The dominant controls on the mode of fan development and sedimentation are the composition and texture of the source material, and the presence of snowfall accumulations and segregated ice. The aridity of the climate limits the amount of fluvial activity, but in itself does not represent the controlling factor in fan morphology and sedimentation.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880387

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACTEskers sometimes become much wider and higher for short sections. As a special form element such sections are calledesker enlargementsin this paper. Comparative studies of these relatively uncommon phenomena have been carried out in inland areas of northern Sweden above the highest shoreline of the Baltic.The esker enlargements show some common distinctive features such as dead ice kettles, often a rather flat topped surface with shallow erosion channels and superimposed nets of low eskers (esker ribs) descending diagonally the side of the main esker in the flow direction, and sometimes a broken topography with high mounds. All these features indicate fractured ice during at least a part of their formation, during the last deglaciation period. The continuation of the esker upstream and downstream of the esker enlargement is often a low, narrow and winding ridge, sometimes in a depression, often a lake (esker in the lake).The formation of esker enlargements seems to be caused by changed hydrologic conditions, partly due to the surrounding topography e.g. in the form of a widening, sometimes curving valley. In only one locality out of three seismically investigated ones, is there a connection between the esker enlargement and the underlying bedrock topography, in the form of a rising bedrock surface in the flow direction. However, changed hydrologic conditions can also be caused by previously deposited esker material or till, forcing the water flow to rise with a diminished speed of water flow and transport capacity as a result. Under these conditions the esker enlargements are formed in at least two steps: (1) subglacial deposits of esker or till material, (2) transformation of the upper parts of these deposits by subglacial and subaerial glaciofluvial processes. Apart from the positions of the sites in a rapidly melting land ice with ice-dammed lakes, the large amount of material necessary for the formation of esker enlargements is sometimes due to the confluence of drainage systems shown in the form of connecting eskers.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880388

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACTRecent fluctuations of the San Rafael Glacier contrast in timing, direction and intensity with regional trends of glacier behaviour. In this paper, a link is identified between the oscillation history, the topographic situation and variations in winter precipitation. The San Rafael Glacier is the lowest latitude tidewater glacier in the world with unusually high annual mean velocities. Since the late nineteenth century, it has retreated and advanced rapidly over a total distance of 14 km and is now 60 km2smaller than it was 100 years ago. Retreat at up to 300 m a-1during the 1980s halted in 1990. Since then, a slight readvance has occurred at a time of accelerated regional retreat. Calving glaciers are known to respond indirectly to climate change, but whereas some may oscillate in non-climatic, cyclical ways over decadal and century timescales, the San Rafael Glacier seems to respond rapidly to changes in precipitation. The influence of calving dynamics has, at different times, both damped and amplified the response to climate change, but the topographic geometry does not permit large-scale, non-climatic fluctuations. The Holocene record of glacier behaviour is therefore probably free of calving anomalies.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880389

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACTCase studies on desertification in northern Burkina Faso, in the Western Sahel, using satellite-aided ground navigation technology, have shown that a noticeable environmental degradation took place from the late 1960s to 1990. Analyses of aerial photographs and satellite images indicate that the most severe land degradation occurred during the first of a series of droughts, which started in the late 1960s, when large areas with bare ground developed. Despite increased rainfall since 1985, the areas with bare ground have not recovered. The main cause is to be found in a combination of human impact and of repeated droughts.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880390

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor

摘要:

ABSTRACTDaily rainfall data have been analysed for 11 stations in the Wet Zone of Sri Lanka for the Southwest Monsoon season (May to September, 1971–1985) to yield the spatial organization of daily rainfall, by calculating the product-moment correlation coefficients. The patterns of inter-station daily rainfall correlation have been discussed in relation to high correlation coefficients (r=0.5-0.6) at a distance of 0–40 km. The daily rainfall correlation and the distance-decay function were distinct up to a distance of about 80 km. Beyond this distance, similar coefficients of about 0.2 appeared at most stations. The daily rainfall linkage patterns have shown the degree of association for daily rainfall between the stations. These data reflected the different characteristics of spatial organization during the Southwest Monsoon season. Simple linkage analysis yielded three distinct daily rainfall affinity areas. Apparently, the Central Highlands act as a climate barrier and provide a broad-scale effect on the daily rainfall organization in different spatial patterns over the Wet Zone, which covers the southwest sector of Sri Lanka.

ISSN:0435-3676    

DOI:10.1080/04353676.1993.11880391

出版商:Taylor&Francis    

年代:1993

数据来源: Taylor