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1.
The dynamic response of lakes in the Tuohepingco Basin of the Tibetan Plateau to climate change 总被引：1，自引：0，他引：1
The Tibetan Plateau (TP) is particularly sensitive to the influences of climate change. As indicators of climate change, lakes on the TP play a key role in the Earth’s climatic system. Lake Yazi (LY), Lake Tuohepingco (LT) and Lake Changtiao (LC) in the Tuohepingco Basin are three inland lakes on the plateau. The extents of LY, LT and LC were obtained using object-based image analysis for remote sensing and 22 images from Landsat satellites (from September to December between 1972 and 2015). Inter-annual changes in the extent of LY, LT and LC were then analyzed. The results show that the total area of the three lakes underwent a change from shrinkage to expansion between 1972 and 2015. In general, there was a trend toward shrinkage during 1972–1999, distinct expansion during 2000–2007 and slight expansion during 2008–2015. Moreover, we found that 14 other lakes have also expanded dramatically since 2000. Lakes at 30°N and 35°N (LY, LT and LC are also located in this region) exhibited the same dramatic period of expansion between 2000 and 2005. In other words, 2000 appears to be a critical transition point for changes in lake size on the TP. Lakes at the same latitudes in the Tibetan Plateau interior may have a similar period of dramatic expansion after 2000. The warming-triggered deglaciation or permafrost degradation, increased precipitation and decreased evapotranspiration may be the influencing factors of lake expansion in the Tuohepingco Basin. Temperature showed relatively higher correlation with lake extent, while precipitation and evaporation were slightly correlated with lake area. Given the importance of wetlands to human society, these are no trivial issues, and we now need accelerated research based on long-term and continuous remote sensing. 相似文献
2.
J. L. Wooden G. K. Czamanske M. L. Zientek 《Contributions to Mineralogy and Petrology》1991,107(1):80-93
Analyses of the Pb isotopic compositions of plagioclase from 23 samples covering the stratigraphic thickness of the Stillwater
Complex indicate a narrow range of apparent initial isotopic compositions (206Pb/ 204Pb=13.95; 207Pb/204Pb=14.95–15.01; 208Pb/204Pb=33.6). The uniformity of our data is in contrast to, but not necessarily contradictory to, other recent investigations
which give indications that the complex formed by repeated injection of magmas with at least two distinct compositions that
were presumably derived from different source regions. Samples from the Basal series of the complex have consistently higher
207Pb/204Pb ratios, suggesting either minor contamination from adjacent country rocks or a slight distinction between parental magmas.
Apparent initial Pb isotopic compositions of the complex are very radiogenic compared to Late Archean model-mantle values,
but are nearly identical to initial Pb isotopic compositions found for the the adjacent, slightly older (2.73–2.79 Ga), Late
Archean crustal suite in the Beartooth Mountains. Contamination of magmas parental to the Stillwater Complex by the Late Archean
crustal suite is rejected for two reasons: (1) Th and U concentrations in Stillwater rocks and plagioclase are very low (about
0.08 and 0.02 ppm respectively), yet Th/U ratios are uniform at about 4, in contrast to the highly variable (2–26) but often
high Th/U ratios found for the Late Archean crustal complex; (2) it seems improbable that any contamination process would
have adjusted the isotopic compositions of the diverse magmas entering the Stillwater chamber to near-identical values. The
preferred hypothesis to explain the Pb isotopic data for the Stillwater Complex and the associated Late Archean crustal suite
involves a major Late Archean crust-forming event that resulted in a compositionally complex crust/mantle system with relatively
homogeneous and unusual Pb isotopic compositions. The parental magmas of the Stillwater Complex were generated at different
levels within this crust/mantle system, before isotopic contrasts could develop by radioactive decay within compositionally
discrete reservoirs. This situation limits the utility of all isotopic tracer systems in discriminating among the various
mantle and crustal reservoirs that may have affected the final isotopic character of the Stillwater magmas. The late Archean
crustal complex and the Stillwater Complex melts were ultimately derived from the same distinct mantle without obvious direct
interaction with the Middle to Early Archean crust present in the region. 相似文献
3.
In order to determine how As speciation in lacustrine sediment changes as a function of local conditions, sediment cores were taken from three lakes with differing hydrologic regimes and subjected to extensive chemical and TEM analysis. The lakes (Killarney, Thompson and Swan Lakes) are located within the Coeur d’ Alene River system (northern Idaho, USA), which has been contaminated with trace metals and As, from over 100 a of sulfide mining. Previous analyses of these lakebed sediments have shown an extensive amount of contaminant metals and As associated with sub-μm grains, making them extremely difficult to analyze using standard methods (scanning electron microscopy, X-ray diffraction). Transmission electron microscopy offers great advantages in spatial resolution and can be invaluable in determining As speciation when combined with other techniques. Data indicate that because of differences in local redox conditions, As speciation and stability is dramatically different in these lakes. Killarney and Thompson Lakes experience seasonal water-level fluctuations due to drawdown on a downstream dam, causing changes in O2 content in sediment exposed during drawdown. Swan Lake has relatively constant water levels as its only inlet is dammed. Consequently, Killarney and Thompson Lakes show an increase in labile As-bearing phases with depth, while Swan Lake data indicate stable As hosts throughout the sediment profile. Based on these observations it can be stated that As in lakebed sediments is much less mobile, and therefore less bioavailable, when water is kept at a constant level. 相似文献
4.
The vertebrate fauna of the type Cromerian 总被引：1，自引：0，他引：1
The rich vertebrate fauna of the Upper Freshwater Bed of West Runton, Norfolk. England (pollen assemblage zone Cr II) is described and related to vegetational conditions based on analysis of fossil pollen and macroscopic plant remains. The general picture is of fishes associated with aquatic plants in a slow-flowing river, amphibious vertebrates living in water-side habitats, and a terrestrial fauna generally reflecting a regional mixed oak forest cover, but also including grassland voles associated with local herb-dominated communities on the river floodplain. The fossil assemblage appears to have accumulated largely from the remains of terrestrial vertebrates washed in as the river eroded its banks. A reassessment of the fossil voles indicates that only five species occur, although ten were previously described. The sparse fauna from the overlying marine gravel (zone Cr III) is also discussed. On the basis of vertebrate faunas and other evidence, sites in England and Germany arc correlated with the type Cromerian. 相似文献
5.
Lakes of the Klutlan moraines originate by down-melting of stagnant ice under a mantle of rock debris and vegetation ranging from scattered herbs and shrubs on the younger moraines to multiple-generation closed spruce forest on the oldest moraines, which are 600–1200 yr old. Lakes on the youngest moraines are temporary, turbid with glacial silt, and marked by unstable ice-cored slopes. On older moraines most lakes have clear water and stable slopes. On the oldest moraines many lakes have brown water caused by dissolved humic materials derived from the thick forest floor, but even here some slopes are unstable because of continued melting of buried ice. Morainic lakes contain bicarbonate waters of moderate alkalinity and conductivity and low levels of nutrients. The highly diverse phytoplankton is dominated by chrysophytes and cryptomonads, with few diatoms. Extremely low values for phytoplankton biomass place most of the lakes in an “ultraoligotrophic” category. Zooplankton is dominated by copepods, which were found even in ice ponds only a few years old, and by the cladoceran Daphnia pulex. Surface-sediment samples contained a total of 16 species of chydorid Cladocera. Of these, Alonella excisa and Alona barbulata are apparently the pioneer species in the youngest lakes. Chydorus sphaericus only appears in lakes of the oldest moraines. A successional pattern is not conspicuous, however, partly because some of the lakes on the older moraines originated by recent collapse over buried ice. Lakes on the upland outside the dead-ice moraines yielded 39 species in the zooplankton. The distinctive assemblage on upland lakes may relate more to different water chemistry than to age. 相似文献
6.
7.
北五湖系鲁中黄河以南自北向南分布的一系列湖泊,分别为安山湖、马踏湖、南旺湖、蜀山湖和马场湖。近100年来,北五湖陆续淤废,目前,除安山湖、马场湖尚有残存外,其他诸湖悉数消亡。作者收集了北五湖地区1907年测绘地图、成像于1954年的航片、成像于1975年的MSS遥感影像、分别成像于1987年、2001年的TM遥感影像以及成像于2005年的SPOT5 XS遥感影像等6个时段的数据。对其进行了几何纠正与配准、图像镶嵌等预处理,通过目视解译和遥感分类等手段提取了该地区6个时期的北五湖空间分布数据。根据北五湖6个不同时段的空间展布特征,分析了近100年以来北五湖消亡的幅度、速度和时间等特征,并在此基础上初步探讨了北五湖消亡的原因。 相似文献
8.
Karst rivers’ particularity: an example from Dinaric karst (Croatia/Bosnia and Herzegovina) 总被引：2，自引：2，他引：0
The very complex system of sinking, losing and underground transboundary Karst rivers, lakes and aquifers in the central part of the deep and bare Dinaric karst in Croatia and Bosnia and Herzegovina is analysed. The groundwater and surface water are hydraulically connected through numerous karst forms which facilitate the exchange of water between the surface and subsurface. A complex underground conduit system is an inherent characteristic karst system analysed. Groundwater and surface water exchange with both adjacent and distant aquifers through underground routes or inflows from surface streams and artificial reservoirs. Because of a complex surface and underground karst features, which strongly influenced its hydrological and hydrogeological regime, the main open stream flow, with a longitude of about 106 km, undergoes eight name changes. In this paper, it is noted as “the eight-name river”. In fact, it represents one river with losing, sinking and underground stream sections. Different surface and underground karst forms play crucial roles in the way the water flowing over the surface and on the underground sections of its catchment. The analysed area is full of varied and often spectacular surface landforms, including for example the Blue and Red Lakes and the Kravice Waterfall. The analyses made in the paper show the existence of a decreasing trend of mean annual discharges on the eight-name river, which can cause numerous problems in the regional water resource management of this transboundary river and catchment. 相似文献
9.
A review of some unique aspects of the limnology of shallow Southern African man-made lakes 总被引：1，自引：0，他引：1
J. A. Thornton Dr. 《GeoJournal》1987,14(3):339-352
Large numbers of man-made lakes occur throughout S Africa. Although primarily designed for specific water supply purposes, most of these lakes fulfill a multipurpose role. The need for such large numbers of reservoirs is related to vagaries in rainfall and runoff; these uncertainties have led to the development of a philosophy of conserving water quantity, which has created some unusual limnological conditions. Examination of the hydrography and limnology of 64 S African impoundments reveals that both basin morphometry and water retention times are affected by natural (drought/flood) and artificial (drawdown) factors which, in turn, affect the hydrodynamics of the lakes, whilst the introduction of treated effluents as a water conservation measure gives rise to a large percentage of enriched lakes having extremely high productivity. 相似文献
10.
A number of problems associated with detailed investigation of subsurface geological structure below the bottoms of freshwater reservoirs (rivers, ponds, and lakes) arise during the study of the upper portions of geological sections. The present study is aimed at marginal estimation of the penetrating depth of ground radar investigations in freshwater reservoirs (based on bottom reflections), taking into account the complex nature of conductivity and its dependence upon the frequency under ideal conditions with no noise. The same method can be applied for estimating any given dynamic recording range. 相似文献
11.
滑坡坝及形成的堰塞湖在世界各国特别是山区一带广泛分布。它能够形成天然水库 ,在发生溃坝和洪水漫坝的情况下 ,造成的灾害损失很大。历史上产生过许多天然滑坡坝 ,有些很快发生溃坝 ,有些存在时间很长。存在时间较长的滑坡坝 ,它周围风景秀丽 ,成为旅游热点地区 ,同时也可开发其水资源。滑坡坝的形成有滑坡堵江、崩塌堵江和泥石流堵江 3种形式 ,它的形成机制对于滑坡坝的存在时间起了重要的作用。在开发和利用滑坡坝时 ,它的成因机制需要分析评价。本文对某滑坡坝的形成机制进行了研究 ,在研究某滑坡坝时发现 ,它由左右两岸的滑坡共同作用形成 ,这种形成模式在国内外报道很少 ,对类似工程也有一定的借鉴意义 相似文献
12.
A paleolimnological study was undertaken to investigate changes in three Minnesota lakes over the last 100 years and to demonstrate the stratigraphic effects of cultural eutrophication in two of them. The study combined the analysis of the lake sediment from short cores with stratigraphic analyses of pollen, plant macrofossils, mollusks, diatoms and certain other algae, chydorid Cladocera, and Daphnia ephippia.The rise of Ambrosia type pollen (ragweed) marks the onset of interference with the landscape by European man, which can be closely dated. Calculations of sedimentation rates from this base gave reasonable correlations of other stratigraphic events with historical events.Elk Lake is considered “unpolluted” today and was chosen as a control. Man's effects are limited to logging some of the surrounding forest and to the construction of a dam. Small changes in the lake's fauna and flora are demonstrated, showing the sensitivity of the lake to changes in its catchment area.Lake Sallie and St. Clair Lake, in the same watershed as the city of Detroit Lakes, have been affected not only by logging but also by addition of nutrients from agricultural runoff and sewage effluent. Considerable responses by the lake organisms are apparent. In Lake Sallie the changes were gradual, but in St. Clair Lake they were very abrupt because the lake was partially drained at the same time and the water volume was thereby reduced.The merits of such an integrated study, the types of information gained from the analyses of the various fossils, and the wider application of the results are discussed. 相似文献
13.
《Applied Geochemistry》1998,13(1):59-75
Sediment cores were collected from depositional basins in lakes Michigan, Ontario and Superior to assess spatial and temporal variations in Cu accumulation rates and inventories. Sediment chronologies were determined via 210Pb dating, and 210Pb inventories were used to correct for the effects of sediment focusing. Among the lakes studied, Cu loading histories generally decrease to the present, signifying a regional reduction in the anthropogenic release of Cu to the environment. Focusing-corrected Cu accumulation rates in surficial sediments vary significantly within lakes Michigan and Ontario, suggesting that recent inputs of Cu to these 2 lakes may have been dominated by localized sources. Variations in Cu accumulation rates within Lake Superior are interpreted to be insignificant, suggesting either that Lake Superior is well-mixed with respect to copper inputs or that the lake is dominated by a single source for Cu. Recent measurements of atmospheric fluxes of Cu within the Great Lakes region generally indicate that the atmosphere accounts for less than 50% of the Cu accumulating in surficial sediments. Historical Cu accumulation rates from Lake Superior are poorly correlated with Cu production associated with local smelters, but favorably agree with national production rates. Trends in historical Cu accumulation rates within all 3 lakes may indicate that recent declines in Cu loading can be attributed to the enactment of stricter environmental regulations as well as changes in the use of Cu. Focusing-corrected Cu inventories in lakes Michigan, Ontario, and Superior are generally invariant within a given lake, indicating that, historically, these lakes have been dominated either by a single source or were well-mixed with respect to Cu inputs. The disparity between the results from recent accumulation rates and historical inventories may reflect differences in how the Great Lakes respond to contaminant loading on short (e.g. years) versus long (e.g. decades) timescales. 相似文献
14.
Sources,Distribution and Behavior of Major and Trace Elements in a Complex Karst Lake System 总被引：1，自引：0，他引：1
Jelena Dautović Željka Fiket Jadranka Barešić Marijan Ahel Nevenka Mikac 《Aquatic Geochemistry》2014,20(1):19-38
Sources and distribution of major and trace elements were investigated in the Plitvice Lakes, a pristine cascade hydrological system of sixteen karst lakes situated in a sparsely populated area of the central Croatia. Water and surface sediment samples from 17 locations, including springs, tributaries and lakes, were analyzed for the content of 22 elements by high-resolution inductively coupled mass spectrometry. Principal component analysis of the collected data set showed that different springs and tributaries displayed distinct multielemental compositions, reflecting primarily the differences in their corresponding geological backgrounds. It was shown that the springs situated in the Upper and Middle Jurassic dolomite bedrock represented the main source of several trace elements, including some toxic metals (Cd, Zn, Ni and Tl), to the Plitvice Lakes system. The concentrations of most of the trace elements (Mn, Fe, Al, Cd, Zn, Cu, Ni, Pb, Co, Cr and Tl) showed decreasing spatial trends in the downstream direction, from sources to the lakes. Such a distribution was interpreted to be a consequence of an efficient removal of the dissolved elements in the lentic parts of the system, mainly by co-precipitation with authigenic calcite and Mn oxides. Nevertheless, most of the elements in the lake sediments were highly correlated with Al, which indicated their prevalent association with terrigenic material. It was shown that the multicascade system of the Plitvice Lakes had an enhanced autopurification efficiency regarding the elimination of most of the trace metals from the aqueous phase. 相似文献
15.
硫酸钠亚型富锂卤水25℃等温蒸发过程的计算机模拟 总被引：5，自引：0，他引：5
我国青藏高原上分布有许多盐湖,其中扎布耶等碳酸盐型盐湖、东台吉乃尔等硫酸镁亚型盐湖是著名的富锂盐湖,已成功开发生产出碳酸锂产品。青藏高原上还有许多硫酸钠亚型盐湖,其卤水锂浓度高,钾、镁、硼等有用成分丰富。针对这些盐湖资源,目前尚未进行开发利用研究。这些卤水的等温蒸发实验研究,是必不可少的关键工作之一。本文使用我们由Pitzer电解质溶液理论建立的Li+,Na+,K+,Mg2+/Cl-,SO24--H2O体系的热力学模型,对25℃下几种硫酸钠亚型富锂卤水等温蒸发过程进行了计算机模拟,预测蒸发过程中盐类矿物的析出顺序、卤水组成变化规律、钠、钾、镁、锂盐的饱和点,并在此基础上进行卤水蒸发过程物料平衡关系的理论计算等。本文对西藏3种硫酸钠亚型卤水和国外2种硫酸钠亚型卤水25℃等温蒸发过程进行了模拟。其析盐顺序的特点是:在第一个矿物石盐饱和析出后,接着析出的含钾矿物是钾芒硝,而不是钾镁矾类矿物。其后钾盐则会以钾岩盐形式析出。锂盐饱和后则主要以复盐Li2SO4.K2SO4形式析出。美国银峰卤水的模拟结果与实验蒸发过程完全一致,并给出其继续蒸发时的析盐状况。另4种硫酸钠亚型卤水的模拟结果不仅可以作为未来实验研究的基本参考和注意点,同样也可作为其盐田设计和工艺安排的基本理论依据。本文研究为该类型卤水资源综合开发利用工艺路线的确立提供理论参考。 相似文献
16.
This study of five small (<3.0 ha) lakes in southwestern Greenland examines the veracity of branched glycerol dialkyl glycerol tetraethers (br GDGTs) as a temperature proxy in lacustrine systems. The proximity (<5 km) of the lakes suggests that their temperature history, and thus their br GDGT records, should be similar. Distributions of br GDGTs in (i) surface sediments from all five lakes, (ii) 14C-dated sediment cores from two lakes (Upper and Lower EVV Lakes) and (iii) soil samples from the area surrounding the lakes were examined. The temporal records of br GDGT-based temperature for the two cores exhibited both similarities and major discrepancies. The differences between the paleotemperature records for the two lakes suggest that br GDGTs are not solely soil-derived, reflecting air temperature, but also indicate an additional br GDGT contribution from another source. Among the broader suite of lake sediments, there was a strong correlation (R2 0.987) between br GDGT-based surface sediment temperatures and measured summer bottom water temperatures for the four lakes with hypoxic/anoxic bottom waters, including Upper EVV Lake. The correlation suggests production of br GDGTs by anaerobic bacteria within the bottom water and/or sediment–water interface, reflecting environmental temperature for the individual lakes and augmenting the uniform, soil-derived signal. Hence, assessment of br GDGTs in Greenland lake sediments provides evidence for their origin from anaerobic autochthonous bacteria and indicates that interpretation of lacustrine br GDGT-based paleotemperature records requires contextual knowledge of individual lake systems and potential source(s) of sedimentary br GDGTs. 相似文献
17.
W. B. Lyons Carmen A. Nezat Larry V. Benson Thomas D. Bullen Elizabeth Y. Graham Jesicca Kidd Kathleen A. Welch James M. Thomas 《Aquatic Geochemistry》2002,8(2):75-95
We have collected and analyzed a series of water samples from three closed-basin lakes (Lakes Bonney, Fryxell, and Hoare) in Taylor Valley, Antarctica, and the streams that flow into them. In all three lakes, the hypolimnetic waters have different 87Sr/86Sr ratios than the surface waters, with the deep water of Lakes Fryxell and Hoare being less radiogenic than the surface waters. The opposite occurs in Lake Bonney. The Lake Fryxell isotopic ratios are lower than modern-day ocean water and most of the whole-rock ratios of the surrounding geologic materials. A conceivable source of Sr to the system could be either the Cenozoic volcanic rocks that make up a small portion of the till deposited in the valley during the Last Glacial Maximum or from marble derived from the local basement rocks. The more radiogenic ratios from Lake Bonney originate from ancient salt deposits that flow into the lake from Taylor Glacier and the weathering of minerals with more radiogenic Sr isotopic ratios within the tills. The Sr isotopic data from the streams and lakes of Taylor Valley strongly support the notion documented by previous investigators that chemical weathering has been, and is currently, a major process in determining the overall aquatic chemistry of these lakes in this polar desert environment. 相似文献
18.
C. Gregory Skilbeck Timothy C. Rolph Natalie Hill Jonathan Woods Roy H. Wilkens 《第四纪科学杂志》2005,20(4):327-347
We have undertaken a comparative study of down‐core variation in multiproxy palaeoclimate data (magnetic susceptibility, calcium carbonate content and total organic carbon) from two coastal water bodies (Myall and Tuggerah Lakes) in temperate eastern Australia to identify local, regional and global‐forcing factors within Holocene estuarine sediments. The two lakes lie within the same temperate climate zone adjacent to the Tasman Sea, but are not part of the same catchment and drain different geological provinces. One is essentially a freshwater coastal lake whereas the other is a brackish back‐barrier lagoon. Despite these differences, data from two sites in each of the two lakes have allowed us to investigate and compare cyclicity in otherwise uniform, single facies sediments within the frequency range of 200–2000 years, limited by the sedimentation rate within the lakes and our sample requirements. We have auto‐ and cross‐correlated strong periodicities at ～360 years, ～500–530 years, ～270–290 years, 420–450 years and ～210 years, and subordinate periods of ～650 years, 1200–1400 years and ～1800 years. Our thesis is that climate is the only regionally available mechanism available to control common millennial and centennial scale cyclicity in these sediments, given the geographical and other differences. However, regional climate may not be the dominant effect at any single time and either location. Within the range of frequency spectral peaks we have identified, several fall within known long‐term periodical fluctuations of sun spot activity; however, feedback loops associated with short‐term orbital variation, such as Dansgaard–Oeschger cycles, and the relationship between these and palaeo‐ENSO variation, are also possible contributors. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
19.
E. Swierczek G. Backe S. P. Holford E. Tenthorey A. Mitchell 《Australian Journal of Earth Sciences》2013,60(1):77-94
Mechanical damage (e.g. faults and fractures) related to tectonic forces and/or variations in formation pore pressures may enable the leakage of fluids through otherwise effective seal rocks. Characterisation of faults and fractures within seals is therefore essential for the assessment of long-term trap integrity in potential CO2 storage sites. 3D seismic reflection data are used to describe a previously unrecognised network of extensive, small Miocene-age faults with displacement of generally <30 m and lengths that vary between ～300 and 2500 m above the Snapper Field, in the Gippsland Basin. The Snapper Field is a nearly depleted oil and gas field that presents an attractive site for potential CO2 storage due its structural closure and because it has effectively retained significant natural hydrocarbon (including CO2) columns over geological time-scales. Volume-based seismic attributes reveal that this fault system is located within the Oligocene Lakes Entrance Formation of the Seaspray Group, which acts as the regional seal to the Latrobe Group reservoirs in the Gippsland Basin. Detailed analysis of fault lengths and linkages suggests that the Miocene faults are non-tectonic, polygonal faults, although the displacement analysis of fault segments reveals strong correlations with the both the structure of the underlying Top Latrobe surface and normal faults that segment the Latrobe Group reservoirs, suggesting that the development of this fault system has been influenced by underlying structures. The geological evidence for long-term retention of hydrocarbons within the Snapper Field suggests that this fault system has not compromised the integrity of the Lakes Entrance Formation seal, although elevated pore pressures during CO2 injection could potentially lead to reactivation of these structures. 相似文献
20.
Current trends in Great Lakes shipping 总被引：2，自引：0，他引：2
Harold M. Mayer 《GeoJournal》1978,2(2):117-122
Shipping on the Great Lakes has changed rapidly in recent years. Internal Great Lakes traffic consists almost entirely of bulk movements, primarily of iron ore, coal, limestone, and grain. Ore shipments are mainly of taconite concentrate. Low-sulfur coal from the western areas of the United States is now moving downbound through the lakes, a movement in the opposite direction from the predominantly upbound movement of Appalachian coal. Opening of a larger lock in 1970 between lakes Superior and Huron has initiated a new generation of lake vessels, which are three times the size of the previous lake ships, and are too large to transit the Welland Canal and the St. Lawrence Seaway; for the first time since 1959 the largest “lakers” are land-locked. General cargo traffic between the Great Lakes and overseas has been declining rapidly since 1970, largely as the result of the rapid development of container ships, and the Interstate highway system, which increases the competitive advantages of salt-water ports. 相似文献