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1.
An evaluation of morphometric parameters of two drainage networks derived from different sources was done to determine the influence of sub-basins to flooding on the main channel in the Havran River basin (Balıkesir-Turkey). Drainage networks for the sub-basins were derived from both topographic maps scaled 1:25.000 and a 10-m resolution digital elevation model (DEM) using geographic information systems (GIS). Blue lines, representing fluvial channels on the topographic maps were accepted as a drainage network, which does not depict all exterior links in the basin. The second drainage network was extracted from the DEM using minimum accumulation area threshold to include all exterior links. Morphometric parameters were applied to the two types of drainage networks at sub-basin levels. These parameters were used to assess the influence of the sub-basins on the main channel with respect to flooding. The results show that the drainage network of sub-basin 4—where a dam was constructed on its outlet to mitigate potential floods—has a lower influence morphometrically to produce probable floods on the main channel than that of sub-basins 1, 3, and 5. The construction of the dam will help reduce flooding on the main channel from sub-basin 4 but it will not prevent potential flooding from sub-basin 1, 3 and 5, which join the main channel downstream of sub-basin 4. Therefore, flood mitigation efforts should be considered in order to protect the settlement and agricultural lands on the floodplain downstream of the dam. In order to increase our understanding of flood hazards, and to determine appropriate mitigation solutions, drainage morphometry research should be included as an essential component to hydrologic studies.  相似文献   

2.
Labrador has been covered by reconnaissance-scale geochemical surveys under the National Geochemical Reconnaissance program. Lake sediment and water were the chief sample media, but stream sediment and water were employed in the mountainous terrain of northern Labrador. The main objective of these surveys was mineral resource assessment, but the data are also relevant to geological and environmental studies, and would be most useful to the non-specialist if the data from the two drainage types could be combined to produce unified element distribution maps for the whole region.A comparison of stream and lake data for a 5,700-km2 area where both drainage types were sampled suggests that only the pH of the lake and stream waters are directly comparable, showing a common range and similar spatial distribution. Comparing the two types of sediment, most elements show obvious differences in either median content or range or both, indicating that stream and lake sediment are geochemically distinct media, and their element contents cannot be compared directly. The distribution patterns of Cu, Ni and U reflect similar bedrock features in both sediment types. In contrast, Co, F, Fe, Hg, Mn, V and Zn show little or no spatial correlation between stream and lake sediment, but are strongly intercorrelated in the stream sediment data set.The sediments collected from lakes in Labrador represent disturbed column, about 40 cm in length, of organic debris that accumulated over the past several hundred years. Metal accumulation in the sediment is largely through fixation from inflowing surface and groundwater by microorganisms, coprecipitation with hydrous Fe and Mn oxides, sorption by clay minerals and chemical and biochemical processes at and just below the sediment/water interface. The stream sediments in this study were collected from active sediment, and represent principally the mechanical-weathering products of bedrock, with variable amounts of organic matter and hydrous Fe and Mn oxides. Considering the difference in the two sediment types, it is probably not surprising that there is rather limited spatial correlation between the geochemistry of the two sediment types indicating that to a large extent each medium reflects a different facet of the bedrock geochemistry. Only for a few elements should the data sets be merged. The degree of spatial correlation for U, Ni and Cu increases as the data are generalized by averaging into larger blocks, suggesting that the combined data sets will be more successful in defining broad crustal geochemical features rather than local details. The implication of this study for international geochemical mapping is that geochemical patterns for many elements are strongly dependent on the sample medium chosen. Therefore, when it is necessary to change sample media in passing from one terrain type to another, a comparative study must be carried out to determine how the geochemistry of the different sample media compare spatially.  相似文献   

3.
Research undertaken by IGCP 259 (International Geochemical Mapping) indicates that wide-spaced sampling is a fundamental concept of international geochemical mapping as it appears to provide the only practical way to obtain a relative rapid (10–20 years) overview of global geochemistry. The main aim of this study is to test the suitability of floodplain sediment as a global sampling medium.Thirteen floodplain sediment samples and 13 stream sediment samples were taken at the exit of 13 super large drainage basins (SCB). The areal extents of these basins are in the order of 1000–10,000 km2. Within each SCB, 3–11 stream sediment samples each representing a component catchment basin (CCB) of 100–1000 km2 were also taken. Fourty nine elements were analyzed. The results were compared with the average values derived from calculating thousands of stream sediment data available from China's National Geochemical Mapping (RGNG) program. Strong similiarities were demonstrated in distribution and trends among the three levels of data. The set of floodplain sediment data shows great coincidence with RGNR stream sediment data.  相似文献   

4.
Much of the topography of Korea is ancient, but many Quaternary fault outcrops and marine terraces have been observed in the southeastern part of the Korean Peninsula. Sufficient evidence of a Quaternary fault is lacking in the geological features of the commercially developed Jukbyun and Uljin areas. Knickpoints that develop on streams can be formed by tectonic activity such as faulting and folding, or can result simply from the differential erosion rates of bedrock. In this study, we investigated the relationship between stream steepness and faults in the Jukbyun area. Stream profile analyses of the Bugu and Namdae basins were performed using a digital elevation model to estimate Quaternary tectonic movements. Stream parameters obtained from analysis of the longitudinal stream profiles of the Bugu and Namdae drainage basins in the northeastern part of the Korean Peninsula indicated neotectonic movement. Thirty of the thirty-nine knickpoints that developed in the downstream areas of the fluvial channels corresponded to fault zones. It is thought that fault activity results in knickpoints in river systems. The normalized relative slope (Ksn) value (54.9) of the BS1 stream in the Bugu drainage basin was higher than that (28.8–36.3) of the other streams in both basins, despite a similar lithology to NS1 and NS2. Therefore, we concluded that stream steepness might be a result of tectonic forcing rather than a product of rock strength in the study area and that stream parameters could provide indirect evidence of Quaternary tectonics in ancient landforms.  相似文献   

5.
Near the shores of Great Slave Lake, natural sources of heavy metals include gold and base metal deposits, mineralized greenstone belts and sedimentary bedrock, and uraniferous granites. Potential anthropogenic sources of heavy metals (As, Zn, Pb) include large gold and base metal extraction -processing facilities on the shores of the lake. Six sediment cores were collected on a traverse of the lake. Heavy metal concentrations and distributions are related to the regional bedrock geochemistry in the drainage basin. Higher uranium concentrations in the northernmost core are attributed to extensive uraniferous areas north of the lake. More subtle variations of concentration are related to sedimentologic characteristics and processes in the lake. The west basin is a natural sink for most of the heavy metals determined. Two centrally located west basin cores had mean zinc concentrations of 145 ppm, whereas cores closer to the north and south shores had mean concentrations of 80–110 ppm. Mn, Ni, and Pb were enriched in some of the cores from the area of shallower water near the MacKenzie River outlet, rather than in the central west basin. The enrichment is related to Mn-, Ni-, and Pb-rich amorphous coatings on quartz grains. Elevated zinc or lead levels from anthropogenic activities were not detected but elevated levels are suspected for arsenic. In the two cores from the center of the west basin, surface sediment contains up to 12 ppm arsenic, not high in comparison with noncontaminated freshwater lakes elsewhere in Canada but considerably elevated relative to concentrations of 1 ppm found deeper in the same cores. Contribution from the National Water Research Institute, Western and Northern Region. Presented at the 10th International Sedimentological Congress, Jerusalem, Israel, 1978.  相似文献   

6.
The analysis of drainage basin morphotectonic indices is applied in assessment of the influence of tectonic activity on thirteen selected drainage basins of the streams having linear courses and flowing over two very prominent regional structures of northeast India, viz. the Belt of Schuppen and the Dauki fault. Such analysis has been made in order to assess the influence of tectonic activity of these structures on the morphology of the drainage basins of those streams.The different morphotectonic indices considered are: Basin elongation ratio, hypsometric integral, steepness index and profile concavity, drainage basin asymmetry, valley floor width to valley height ratio, longitudinal profiles, stream length gradient index and mountain front sinuosity. Results of the analysis of the morphotectonic indices of the drainage basins infer that morphology of both the streams and drainage basins have been influenced by the regional structures and the present tectonic status of these two structures varies from active to slightly active phase. No significant influence of lithology is seen in the distribution pattern of the anomalous knick points along the longitudinal profiles. The study also reveals that presently the state of tectonic activity is not uniform within the same regional structure and the Belt of Schuppen is relatively more active as compared to the Dauki fault.  相似文献   

7.
Lake sediment composition as an indicator of mineralization within the catchment area has found widespread application in recent years, particularly in Canada. Results have indicated, however, the existence of varying relationships between lake sediment composition and mineralization resulting from local features of the limnological environment. Accordingly it was considered appropriate to examine the nature of metal transport in the lake and stream environment, the partitioning of metal between the stream waters and stream sediments and between lake waters and lake sediments to obtain some understanding of the factors that affect the lake sediment-mineralization relationship. This investigation was carried out over an area containing Pb-Zn occurrences of supposed “Mississippi-Valley type” in Grenville and Paleozoic bedrock in southeastern Ontario.The headwater drainage systems comprise active streams, swamps, beaver ponds and small lake-bog systems giving way downstream to open lakes. The beaver swamps and seasonal swamps act as drainage sinks for metals, restricting the extent of geochemical dispersion in drainage systems adjacent to mineralization. Selective extraction analysis of bog, stream and lake sediments indicates that metals are preferentially concentrated with amorphous iron oxides, which readily adsorb and complex lead and zinc and are stable in the alkaline environment common in swamps adjacent to carbonate-hosted lead-zinc mineralization. The accumulation of lead and zinc with amorphous iron oxides combined with the adsorbing and chelating action of organic matter on lead and zinc makes organic-rich sediments from these small swampy areas an excellent sample medium for reflecting local mineralization. Down drainage anomalies of these elements can be accentuated by selective analysis for the amorphous iron oxide-held metal, involving selective extraction techniques.In contrast, within larger lake systems, the analysis of water samples indicates that geochemical dispersion in surface waters in the high pH environment (pH = 8.0) associated with the carbonate-hosted lead-zinc deposits is extremely restricted. In this environment, anomalous metal contents in lake water were not evident in lakes adjacent to mineralization, while anomalous lake sediment compositions exist only in lakes immediately adjacent to Pb-Zn mineralization and do not extend down the drainage system. The restricted dispersion necessitates basing geochemical reconnaissance surveys on collection and analysis of samples from the headwater organic-rich swamps at a higher sample density and resulting higher cost than in areas where a lower sample density is acceptable due to a wider dispersion.  相似文献   

8.
GIS and Remote Sensing have proved to be an indispensible tool in morphometric analysis. The identification of morphometric properties based on a geographic information system (GIS) was carried out in two watersheds in the Thrissur district of Kerala, India. These watersheds are parts of Western Ghats, which is an ecologically sensitive area. Quantitative geomorphometric analysis was carried out for the Chimmini and Mupily watersheds independently by estimating their (a) linear aspects like stream number, stream order, stream length, mean stream length, stream length ratio, bifurcation ratio, length of overland flow, drainage pattern (b) aerial aspects like circulatory ratio, elongation ratio, drainage density and (c) relief aspects like basin relief, relief ratio, relative relief and ruggedness number. The drainage areas of Chimmini and Mupily watersheds are 140 and 122 km2 respectively and show patterns of dendritic to sub-dendritic drainage. The Chimmini watershed was classified as a sixth order drainage basin, whereas Mupily watershed was classified as a fifth order basin. The stream order of the basin was predominantly controlled by physiographic and structural conditions. The increase in the stream length ratio from lower to higher order suggests that the study area has reached a mature geomorphic stage. The development of stream segments is affected by rainfall and local lithology of the watersheds. The slope of both watersheds varied from 0° to 50° and 0° to 42° respectively and the slope variation is chiefly controlled by the local geology and erosion cycles. Moreover, these studies are useful for planning rain water harvesting and watershed management.  相似文献   

9.
Based on data from two runoff plots and ten stations in hilly loess region Dalihe drainage basin ranging in area from 0.0006 to 3983 km2 on the Loess Plateau, the relationship between mean annual specific sediment yield (Y s) and drainage area (A) is studied, which is different from those for many other drainage areas of the world, neither at the scale of whole basin nor at local scale on the Loess Plateau. With increasing drainage area, the mean annual specific sediment yield experiences two peak values: the first peak value appears at 0.00408 km2 in area corresponding to the whole slope surface, and the second peak value appears at 96.1 km2 in area. The non-linear variation in the Y s–A can be explained as follows: the first peak value can be explained by the abrupt increase in slope gradient and flow shear stress resulting in highly increased sediment concentration and specific sediment yield. And the second peak value can be explained by the combined influence of flow shear stress and drainage density, represented by dimensionless variable Ω.  相似文献   

10.
Sixteen hundred stream sediments (<150 μm fraction) collected during regional geochemical surveys in central and SW Nigeria have high median and maximum concentrations of Zr that exceed corresponding Zr concentrations found in stream sediments collected from elsewhere in the World with similar bedrock geology. X-ray diffraction studies on a sub-set of the analysed stream sediments showed that Zr is predominantly found in detrital zircon grains. However, the main proximal source rocks (Pan-African ‘Older Granites’ of Nigeria and their Proterozoic migmatitic gneiss country rocks) are not enriched in zircon (or Zr). Nevertheless, U–Pb LA-ICP-MS dating with cathodoluminescence imaging on detrital zircons, both from stream sediment samples and underlying Pan-African ‘Older Granites’ confirms a local bedrock source for the stream sediment zircons. A combination of tropical/chemical weathering and continuous physical weathering, both by ‘wet season’ flash flooding and ‘dry season’ unidirectional winds are interpreted to have effectively broken down bedrock silicate minerals and removed much of the resultant clay phases, thereby increasing the Zr contents in stream sediments. The strong correlation between winnowing index (Th/Al) and Zr concentration across the study area support this interpretation. Therefore, ‘anomalous’ high values of Zr, as well as other elements concentrated in resistant ‘heavy’ minerals in Nigeria’s streams may not reflect proximal bedrock concentrations of these elements. This conclusion has important implications for using stream sediment chemistry as an exploration tool in Nigeria for primary metal deposits associated with heavy minerals.  相似文献   

11.
It has been proposed that erosional unloading by focused incision can be accommodated by flexural rebound, which in turn, potentially exerts a positive feedback on erosion. This paper presents morphometric data from the La Paz drainage basin in the Bolivian Andes to illustrate the effects of such a feedback mechanism between erosion and crustal bending. The Rio La Paz is a prominent system with the headwaters on the Altiplano and a trunk stream segment that cuts across the Cordillera Real thereby connecting large portions of the Altiplano with the Amazon Basin. The flexural feedback model explains why all drainages beyond the watershed disperse their waters to the Altiplano. It also provides an explanation for the presence of the highest peaks just next to the location where the La Paz River cuts into the bedrock across the Cordillera Real.  相似文献   

12.
Priebe  E. H.  Hamilton  S. M.  Lemieux  A.  Rowan  D. J.  Clark  I. D. 《Hydrogeology Journal》2023,31(3):641-659

Modern, ambient tritium concentrations in precipitation are lower and more temporally consistent now that they have recovered from the historic thermonuclear bomb peak of the mid-1960s. With the bomb peak no longer the overriding influence on atmospheric tritium concentrations, anthropogenic point sources, such as nuclear-generating stations (NGS), have the largest influence, though the extent and temporal variability of this influence remains uncharacterized. The lack of precipitation monitoring locations means that spatial trends in tritium concentrations in precipitation are unknown. To address this data gap, tritium concentrations in shallow modern groundwater are interpolated throughout southern Ontario (Canada), at the center of the Great Lakes Basin, and the interpolation is tested as a precipitation proxy with a statistical comparison that shows good agreement between the shallow groundwater and precipitation datasets. The shallow groundwater tritium interpolation is used to delineate the extent of NGS influence as representing 66% of the study area. Recharge timings in the subcropping bedrock aquifers of the study area are interpreted qualitatively in areas outside of NGS influence to be primarily a mix of pre-bomb and modern recharge, with no indication of peak recharge levels remaining. The influence of drift thickness on the proportion of tritium-dead versus tritium-live samples is observed spatially and confirmed by comparing data distributions. The oldest waters (pre-1953) tend to occur in subcropping bedrock aquifers underlying the thickest sediment packages.

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13.
通过数字高程模型(DEM)的空间分析技术,系统提取研究区洮河水系流域盆地典型的河流地貌参数,并利用活动造山带地区发育的基岩河道纵向剖面形态等典型参数,分析洮河水系流域盆地地貌发育不对称性特征及洮河在岷县流向的急剧转变成因。研究表明洮河上游南岸水系较北岸水系形状指数高、流域面积大、水系发育更成熟,下游东岸河流河长较短、流域面积小、形状指数低于西岸,西岸水系更发育,且上游流域要比下游河道平缓,水系的相对落差更低,发育更成熟,表明上游河道形成时间早于下游河道。临潭—宕昌断裂带的逆冲隆升作用是造成洮河上、下游水系形态差异的主要原因。岷县东侧山脉的快速隆起致使古洮河被截断,之后被东北侧河流溯源侵蚀,切穿西秦岭北缘构造带,进行河流袭夺,从而形成了现今的洮河。最后探讨了对青藏高原东北端晚新生代以来的构造响应。  相似文献   

14.
Neotectonic activity in Mahe (Mayyazhi) river basin and its implication in landform development and stream characteristics were investigated with the help of the digital elevation model-derived geomorphic indices. The analysis is based on the commonly used geomorphic indices such as hypsometric curve and integral, drainage basin asymmetry, transverse topographic symmetric factor (T), stream length (SL) gradient index and longitudinal profiles of the stream. Hypsometric integrals indicate that the basin has reached the base level and lesser amount of material remains for erosion and transportation. The basin is asymmetric and has a south-south-east directional tilt, but with a probable spin caused due to the presence of major strike slip fracture, the mouth portion of the Mahe river has been tilted towards north-north-west. The SL index with sudden and non-uniform variations supports the finding and indicates the relative tectonic activity and its influence over the river networks in the area. Longitudinal profile of the stream also reveals gradational changes in the profile with the presence of knick points. Cross comparison of the analysed geomorphic indices points towards neotectonic activity in the basin, which modified the river basin to the present morphology and is reflected in the characteristics of the river and the basin as a whole.  相似文献   

15.
The Sample Catchment Basin Approach (SCBA) is one of the techniques widely employed in the analysis of stream sediment geochemical data and delineation of anomalous catchments. However, this method fails to take into account the real catchment basin boundaries of each sample by incorporating only the incremental area between two adjacent samples. In other words, the SCBA incorrectly assumes that the geochemistry of every sample catchment within a drainage is independent from upstream sample catchment(s) feeding into this drainage. The chemical composition of sediment at the basin outlet originates from the whole basin upstream and not the incremental area as postulated in the SCBA. Consequently, the calculated background values for various lithologies and the background value for the element of interest at the basin outlet might be far from reality.This study used a True Sample Catchment Basin Approach (TSCBA), that reflects the true catchment boundary of every stream sediment sample, and in which all calculations are carried out on the premise that this boundary and the true area affect the composition of each sample. The results obtained from the application of both the SCBA and TSCBA to a gold endowed study area in western Iran clearly illustrated the superiority of the TSCBA over the SCBA. In addition, this study demonstrated the advantage of using the modified dilution correction equation of Mokhtari and Garousi Nezhad (2015), as compared to the existing Hawkes’s equation commonly used for dilution correction of residual values.  相似文献   

16.
The recent development of digital representation has stimulated the development of automatic extraction of topographic and hydrologic information from digital elevation model input, using geographic information system (GIS) and hydrologic models that integrate multiple databases within a minimal time. The objective of this investigation is to compare the drainage extracted from Shuttle Radar Topography Mission (SRTM) data with the drainage digitized from topographic data (1:50,000) and also to draw attention to the functions of an add-on tool in ArcGIS 9.2 (Arc Hydro v.2) of Kuttiyadi River basin. The analysis reveals that the watershed extracted from the SRTM digital elevation model (DEM) (90 m resolution) is having an area of 668 km2 and that from toposheet is 676 km2. The river mouth in the drainage network from the SRTM DEM is found to be shifted to the northern side from where it actually exists. The drainage network from SRTM DEM at stream threshold 15 (0.0002 % of maximum flow accumulation) is delivering best results than the other threshold value in comparison with the drainage pattern derived from toposheets. The study reveals the importance, reliability, and quaintness of drainage network and watershed derived from the SRTM using the Arc Hydro Tool, an extension for Environmental Systems Research Institute ArcGIS. The advantage of the Arc Hydro Tool is that it would help a novice with little GIS knowledge to run the model to obtain watershed and drainage network.  相似文献   

17.
Suspended sediment concentration is a major variable influencing soil erosion and loss, study on which at different spatial scales is of great meaning to understand soil erosion mechanism and sediment transport process. Based on data from 4 sloping surfaces and 7 basins ranging from 0.0003 to 187 km2 in area, the suspended sediment concentration in flood season (SSC) with drainage area is studied. With increasing drainage area on the slope surfaces, the mean suspended sediment concentration in flood season (MSSC) enhances continuously until a peak value of 685 kg m−3 occurs at the whole slope surface No. 7 runoff plot resulting from harder and harder erosion forms downslope. Entering basin systems, the diluted action of subsurface water on the toeslope on MSSC and small water flow power Ω make a minimum MSSC value of 568 kg m−3 occur in the first-order basin system Tuanshangou basin at an area of 0.18 km2, and then from Tuanshangou basin to larger basins, the positive feedback function among drainage density, water flow energy, and hyperconcentrated flow as well as its reduction of settling velocity of coarser particles generates continuously increasing MSSC with drainage area.  相似文献   

18.
Paleofloristic data imply that paleoclimate changed in the Swiss Alps at the Oligocene/Miocene boundary from humid and hot conditions toward a climate with high temperature and low humidity. The aridization is associated with a change in depositional pattern from alluvial fans to lakes and floodplains, suggesting decreasing sediment discharge. A further 25-40% decrease of sediment discharge occurred at ca. 20 Ma when the orogenic core of the Alps became exposed to the surface. We applied a surface processes model to explore potential controls on the pattern of sediment discharge and on the evolution of the Alpine drainage basin. The model is based on the presumption that the rates of fluvial incision into bedrock are proportional to shear-stress exerted by the flowing water. The model results imply that the paleoclimate change resulted in an instantaneous decrease of sediment discharge and a vertical topographic growth until steady-state conditions between erosional and crustal mass flux are established. However, exposure of the crystalline core of the Alps at ca. 20 Ma is likely to have resulted in the 25-40% decrease of sediment discharge and the reorganization of the drainage pattern from an orogen-normal to an orogen-parallel orientation of dispersion.  相似文献   

19.
Molybdenum mining in the Knabena—Kvina drainage basin (1918–1973) left more than eight million tons of tailings in two small lakes in the headwater area of the Knabena river. The piles, that reach above the water surface, were freely eroded until a dam was built to reduce the dispersion in 1976. Sampling of tailings and fluvial sediments took place almost 20 years later. Sampling media were natural sediment sources, 1-cm-thick slices of overbank sediments of various depths, material from the tailings pond, sandbars, stream sediments, fjord sediments, and integrated samples of floodplain surfaces (0–25 cm). In total 734 samples were collected. Chemical analysis (ICP-AES after aqua regia or HNO3 extraction) showed that overbank sediments at a certain depth represent the pre-industrial trace metal concentrations within the drainage basin. The tailings and recent fluvial sediments were enriched in approximately the same element suite. The highest enrichment factors were obtained for Cu (8–53) and Mo (22–57). Fluvial processes in the tailings pond have probably selectively eroded fine-grained, low-density particles. Thus, coarse chalcopyrite may have been left behind, while molybdate associated with fine-grained particles may have been selectively entrained causing dilution of Cu and enrichment of Mo in the downstream fluvial sediments. In the sandbars, the highest Cu and Mo concentrations were found in fine-grained sediments downstream of a low-gradient reach that act as a bedload trap. On the floodplains, it is seen that the first areas to be inundated in a flood situation (proximal to the river and in depressions) have the highest metal concentrations. For regional geochemical mapping it is suggested that overbank sediment profiles along river reaches with a laterally stable or slowly migrating channel, should be sampled. In such floodplains, pre-industrial overbank sediments are usually preserved at depth. In case of laterally unstable reaches upstream of the sampling point, polluted and unpolluted sediments may be interlayered or mixed. Therefore, samples should be collected from various depths or sedimentary units in such profiles. A similar sampling strategy should probably be adopted to detect vertical migration of elements especially in areas with acid rain and low bedrock buffer capacity. To obtain high contrasts between polluted and unpolluted drainage basins, the overbank sediment profiles should be within the proximal part of the floodplain.  相似文献   

20.
《Earth》2007,82(3-4):217-249
Rapidly-flowing ice streams are the arterial drainage routes in continental ice sheets and exert a major influence on ice sheet mass balance. Recent observations have revealed that ice stream flow exhibits considerable variability, with relatively rapid changes taking place in speed and direction. This spatial and temporal variability is intimately linked to the conditions at the base of the ice streams and the distribution of localised patches of basal friction, known as ‘sticky spots’. In this paper, we provide a detailed review of sticky spot observations from both contemporary and palaeo-ice stream beds in order to better understand their nature and influence. Observations and theoretical considerations reveal four primary causes of ‘stickiness’: (i), bedrock bumps; (ii), till-free areas; (iii), areas of ‘strong’ (well drained) till; and (iv), freeze-on of subglacial meltwater. These may act together in one location, or in isolation; and a progressive increase in their distribution could lead to ice stream shut-down. Bedrock bumps are influential under active ice streams, where they provide form drag and can create thinner ice which increases the likelihood of basal freeze-on. Increased bed roughness may prevent the lateral migration of some ice streams but bedrock bumps are unlikely to cause ice stream shut-down because, over long time-scales, ice stream erosion might be expected to reduce their amplitude. The influence of till-free areas beneath an ice stream will depend critically on the amount of water that might be drawn out of the surrounding till to lubricate such areas. They are likely to be most important in ice stream onset zones but their identification has proved difficult beneath active ice streams. If an ice stream operates solely by till deformation, it is conceivable that a progressive increase in the exposure of till-free areas could lead to shut-down through a process of sediment exhaustion. Areas of strong, well drained till have been identified beneath both active and ancient ice streams and are most likely to result from the reorganisation of subglacial meltwater. The collapse of an inefficient ‘cannalised’ system to a more efficient ‘channelised’ system can occur rapidly and this mechanism has been hypothesised as a candidate for ice stream shut-down in both contemporary and palaeo-settings. Basal freeze-on has also been observed and inferred from beneath modern and palaeo-ice streams, and a reduction in basal meltwater supply coupled with ice stream drawdown and the advection of cold ice increases the likelihood of switching off an ice stream. A paucity of data from ice stream sticky spots limits a better understanding of their nature, distribution and evolution beneath ice streams. Future technological advances are likely to improve the resolution of the data collected from the beds of modern ice streams but well-preserved palaeo-ice stream beds also hold potential for investigating their influence on ice stream flow and we present simple landsystems models to aid their identification. Such data will considerably enhance the basal boundary condition in ice stream models which will, ultimately, refine our predictions of the response of contemporary ice sheets to future changes in climate.  相似文献   

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