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1.
Seismic reflection profiles (3.5 kHz) were obtained along more than 3500 km of shiptrack in Lake Superior within the last 2 yr. The acoustic character of profiles is categorized as: (I) a single, strong reflector at the lake floor, (II) a thick, acoustically transparent layer overlying a strong reflector, and (III) relatively thick sediment with internal acoustic reflectors. These profiles, in conjunction with sediment cores from the area, reveal that varved glacial-lacustrine sediment settled out preferentially in a trough between Isle Royale and the north shore, and to a lesser extent in other topographic depressions; bottom currents generated by storm waves prevent clay accumulation on till or bedrock in the open lake wherever the bottom is shallower than 100 m; bottom currents prevent deposition or erode bottom sediment in certain deep-water (> 200 m) valleys; and lacustrine sediment is disturbed by creep or slumping off Grand Portage, Minnesota, and by other processes such as dewatering in many other areas. These factors complicate sedimentation in Lake Superior, and must be considered when investigating any aspect of the lake sediment.  相似文献   

2.
A high-resolution side-scan sonar survey of the lake bed off the Keweenaw Peninsula, Lake Superior, demonstrates that bottom currents are affecting lake bed morphology at depths up to 240 m. Numerous lineations which run parallel to the shore appear to be sand ribbons. A field of sedimentary furrows which occurs in one area demonstrates the long-term directional stability of the near-bottom flow. Large (100–300 m in diameter, 2–5 m deep), unusual ring-like or arcuate depressions are common throughout the western half of Lake Superior. These rings themselves do not appear to have been formed by bottom currents, but may have developed as water was released by the rapid compaction of glacial sediments which underlie the lake bed. Off the Keweenaw Peninsula the forms of the rings have been modified by bottom currents. The bottom currents which have modified the lake bed are probably generated when storms cross the lake at times when it is poorly stratified.  相似文献   

3.
A dense grid of very-high resolution seismic profiles on Lake Villarrica provides a quasi-3D view on intercalated lenses of low-amplitude reflections, which are connected by acoustic wipe-out patches and fractures to an underlying voluminous mass-wasting deposit. The lenses are interpreted as being created by earthquake-triggered liquefaction in this buried mass-wasting deposit and subsequent sediment fluidization and extrusion at the paleo-lake bottom. These sediment volcanoes are mapped in detail. They have a rather uniform circular geometry and show a linear relationship between apparent width and maximum thickness on a seismic section. The largest sediment volcanoes are up to 80 m wide and 1.9 m thick. Their slope angles designate a syn- to post-depositional sagging of most sediment volcanoes. Sediment volcano detection and mapping from nearby Lake Calafquén further strengthen the revealed geometrical relationships. Locally, some of the sediment/fluid escape structures extend to a higher position in the stratigraphy, which points to a polyphase escape process associated with multiple multi-century spaced strong earthquakes. Thickness and morphology of the source layer seem to exert a dominant control in the production of sediment/fluid extrusions. This study shows that reflection seismic profiling allowed recognizing 4 different seismic events in the studied stratigraphic interval, which are evidenced by mass-wasting deposits and/or fluidization features.  相似文献   

4.
Upper and Middle Waterton lakes fill a glacially scoured bedrock basin in a large (614 km2) watershed in the eastern Front Ranges of the Rocky Mountains of southern Alberta, Canada and northern Montana, U.S.A. The stratigraphic infill of the lake has been imaged with 123 km of single-channel FM sonar (‘chirp') reflection profiles. Offshore sonar data are combined with more than 2.5 km of multi-channel, land-based seismic reflection profiles collected from a large fan-delta. Three seismic stratigraphic successions (SSS I to III) are identified in Waterton Lake resting on a prominent basal reflector (bedrock) that reaches a maximum depth of about 250 m below lake level. High-standing rock steps (reigels) divide the lake into sub-basins that can be mapped using lake floor reflection coefficients. A lowermost transparent to poorly stratified seismic succession (SSS I, up to 30 m thick) is present locally between bedrock highs and has high seismic velocities (1750–2100 m/s) typical of compact till or outwash. A second stratigraphic succession (SSS II, up to 50 m thick), occurs throughout the lake basin and is characterised by continuous, closely spaced reflectors typical of repetitively bedded and rhythmically laminated silts and clays most likely deposited by underflows from fan-deltas; paleo-depositional surfaces identify likely source areas during deglaciation. Intervals of acoustically transparent seismic facies, up to 5 m thick, are present within SSS II. At the northern end of Upper Waterton Lake, SSS II has a hummocky surface underlain by collapse structures and chaotic facies recording the melt of buried ice. Sediment collapse may have triggered downslope mass flows and may account for massive facies in SSS II. A thin Holocene succession (SSS III, <5 m) shows very closely spaced reflectors identified as rhythmically laminated fine pelagic sediment deposited from interflows and overflows. SSS III contains Mt. Mazama tephra dated at 6850 yr BP.  相似文献   

5.
The discharge of taconite tailings into Lake Superior at Silver Bay, Minnesota, produces turbidity current flow. The silty fine-sand tailings fraction transported to the deepest part of the lake has formed a small fan with valleys similar in gross morphology to a submarine fan. Current meters anchored 5 m above the lake floor over the wall and over the levee of a distributary valley on the fan recorded intermittent turbidity current flows during 30 weeks in 1972–73. At least twenty-five discrete periods of observation of turbidity current flow were obtained; single episodes lasted 4?328+ h. Only flows thick enough to overflow the eastern levee of the valley could be observed, and this accounts for the intermittent nature of our observations, as flow within the valleys is expected to be continuous as long as tailings are discharged. Flow velocities were higher near the valley axis where the flow is thicker. Velocities measured over the valley wall averaged 10.8 cm/s for eleven episodes; velocities measured over the levee, more than 1/2 km from the valley axis, only 3.3 cm/s. The maximum velocity during 1300 h of observation did not exceed 31 cm/s. This agrees reasonably well with velocities calculated from channel properties, as commonly done for turbidity currents on deep-sea fans. Current meters tethered above the bottom meters indicate that lake currents normally parallel the shore throughout the water column. With the onset of a turbidity current, currents higher in the water column remain unchanged but velocities near the bottom go to zero, currents then change azimuth by 90° to parallel the downslope (down-valley) direction of the fan, then increase in velocity. During a turbidity current episode, the direction of bottom flow stays relatively constant (± 20° of the down-valley trend) but the velocity oscillates (commonly with 10 cm/s amplitude), periods being of 1/2 h or less to several hours. Turbidity currents generated on Reserve Mining Company's delta are effective in carrying essentially all tailings discharged into the lake into deeper water, where they are deposited.  相似文献   

6.
Approximately 8000 km of continuous seismic reflection profiles throughout Lake Superior were examined for evidence of recessional moraines and other ice-margin deposits associated with the retreat of late Wisconsin ice. These features are correlated with the record of glacial-lake evolution in western Lake Superior. An offlapping sequence of glacial and glacial-lacustrine dediments overlying bedrock is recognized in west-central Lake Superior that is progressively younger to the northeast. The sequence underlies more recent glaical-lacustrine and postglacial sediments. Four facies are recognized on the basis of geomorphologic and acoustic properties and are interpreted to represent a southwest-to-northeast assemblage of: proglacial stratified drift (facies A), drift in major end moraines (facies B), till deposited as glacial retreat resumed, or possibly late-stage ablation till (facies C), and basal till (facies D). The prominent moraines of facies B are unusually thick and are believed to mark the ice-margin shorelines of successive major proglacial lakes that formerly occupied parts of western Lake Superior. The moraines are tentatively correlated with Glacial Lake Duluth (unit 1), Glacial Lake Washburn (unit 2), and Glacial Lake Beaver Bay (unit 3), the most prominent of lakes drained via the progressively lower outlets via the Moose Lake/ Brule-St. Croix Rivers, the Huron Mountains, and the Au Train-Whitefish regions, respectively.  相似文献   

7.
High-resolution seismic imaging and piston coring in Lake Lucerne, Switzerland, have revealed surprising deformation structures in flat-lying, unconsolidated sediment at the foot of subaqueous slopes. These deformation structures appear beneath wedges of massflow deposits and resemble fold-and-thrust belts with basal décollement surfaces. The deformation is interpreted as the result of gravity spreading induced by loading of the slope-adjacent lake floor during massflow deposition. This study investigated four earthquake-triggered lateral mass-movement deposits in Lake Lucerne affecting four sections of the lake floor with areas ranging from 0·25 to 6·5 km2 in area. Up to 6 m thick sediment packages draping the subaqueous slopes slid along the acoustic basement. The resulting failure scars typically lie in water depths of >30 m on slopes characterized by downward steepening and inclinations of >10°. From the base-of-slope to several hundred metres out onto the flat plains, the wedges of massflow deposits overlie deeply (10–20 m) deformed basin-plain sediment characterized by soft sediment fold-and-thrust belts with arcuate strikes and pronounced frontal thrusts. The intensity of deformation decreases towards the more external parts of the massflow wedges. Beyond the frontal thrust, the overridden lake floor remains mostly undisturbed. Geometrical relationships between massflow deposits and the deformed basin-plain sediment indicate that deformation occurred mainly during massflow deposition. Gravity spreading induced by the successive collapse of the growing slope-adjacent massflow wedge is proposed as the driving mechanism for the deformation. The geometry of fjord-type lakes with sharp lower slope breaks favours the deposition of thick, basin-marginal massflow wedges, that effectively load and deform the underlying sediment. In the centre of the basins, the two largest massflow deposits described are directly overlain by thick contained (mega-)turbidites, interpreted as combined products of the suspension clouds set up by subaqueous mass movements and related tsunami and seiche waves.  相似文献   

8.
Near Williams Lake, in the central interior of British Columbia, the Fraser River exposes long sections of late Pleistocene glaciolacustrine sediments selectively preserved within a bedrock trough. The dominant facies types are thick, normally graded gravels and sands that occupy steeply dipping multistorey channels up to 300 m wide and several tens of metres deep. Channels appear to have been simultaneously cut and filled by high density turbidity currents in a glacial lake floored by stagnant ice. Fining upward sediment gravity flow sequences up to 50 m thick may be the product of quasi-continuous ‘surging’ turbidity flows triggered by catastrophic meltwater discharges into the trough or retrogressive failure of ice-cored sediments. Large-scale post-depositional deformation structures, such as synclinal folds, normal faults, sedimentary dyke swarms and dewatering structures, record gravitational foundering of sediment and pore-water expulsion caused by the melt of underlying glacier ice. Melting of buried ice masses along the floor of the trough appears to have controlled the flow paths of turbidity currents by producing sub-basins within the overlying sediment pile. An idealized model of ‘supraglacial’ lacustrine sedimentation is developed that may be applicable to other glaciated areas with similar bedrock topography.  相似文献   

9.
四川西昌邛海的浊流沉积初探   总被引：4，自引：0，他引：4  
余斌  章书成  王士革 《沉积学报》2005,23(4):559-565
鹅掌河位于邛海南岸,在近15年来泥石流和洪水不断地将泥沙从鹅掌河带入邛海,在邛海底部大量淤积泥沙,形成一由南到北(长2km),由低到高(平均高2m),由窄到宽(南岸宽200m,北岸宽600m)的水下堤,在邛海湖中连接鹅掌河河口堆积扇处还发现了的水下冲沟。湖底扇,水下堤和水下冲沟对于研究湖相和海相浊流沉积有一定的研究和应用价值。浊流特别是高浓度浊流在湖中的沉积模式研究对于湖底和海底地貌的变迁,湖盆和深海油气藏的发现都有一定的价值和意义。  相似文献   

10.
We analyze both new and previously published paleomagnetic records of secular variation (PSV) from Lake Superior sediment cores and compare these records to correlated rhythmite (varve) thickness records to determine post-glacial sedimentation rates and to reassess the termination of glaciolacustrine varves in the basin. The results suggest that offshore sedimentation rates have exhibited considerable spatial variation over the past 8000 years, particularly during the mid-Holocene. We attribute offshore, mid-Holocene sedimentation changes to alterations in whole basin circulation, perhaps precipitated by a greater dominance of the Gulf of Mexico air mass during the summer season. Nearshore bays are characterized by high sedimentation rates for at least 1000 years after varve cessation and during a period between around 4500 and 2000 cal. BP. After 2000 cal. BP, sedimentation rates subsided to earlier rates. The increases between 4500 and 2000 cal. BP are probably due to lake level fall after the Nipissing II highstand.The older glaciolacustrine varve thickness records suggest that the influx of glacially derived sediment ended abruptly everywhere in the lake, except near the Lake Nipigon inlets. Multiple sediment cores reveal 36 anomalously thick varves, previously ascribed to the formation of the Nakina moraine, which were deposited just prior to varve cessation in the open lake. The PSV records support the observation that the cessation of these thick varves is a temporally correlative event, occurring at 9035±170 cal. BP (calibrated years before 1950, ca 7950–8250 14C BP). This date would correlate to the eastern diversion of Lake Agassiz and glacial meltwater into Lake Ojibway.  相似文献   

11.
The stratigraphy of the last deglaciation sequence is investigated in Lake Saint‐Jean (Québec Province, Canada) based on 300 km of echo‐sounder two dimensional seismic profiles. The sedimentary archive of this basin is documented from the Late Pleistocene Laurentidian ice‐front recession to the present‐day situation. Ten seismic units have been identified that reflect spatio‐temporal variations in depositional processes characterizing different periods of the Saint‐Jean basin evolution. During the postglacial marine flooding, a high deposition rate of mud settling, from proglacial glacimarine and then prodeltaic plumes in the Laflamme Gulf, produced an extensive, up to 50 m thick mud sheet draping the isostatically depressed marine basin floor. Subsequently, a closing of the water body due to glacio‐isostatic rebound occurred at 8.5 cal. ka BP, drastically modifying the hydrodynamics. Hyperpycnal flows appeared because fresh lake water replaced dense marine water. River sediments were transferred towards the deeper part of the lake into river‐related sediment drifts and confined lobes. The closing of the water body is also marked by the onset of a wind‐driven internal circulation associating coastal hydrodynamics and bottom currents with sedimentary features including shoreface deposits, sediment drifts and a prograding shelf‐type body. The fingerprints of a forced regression are well expressed by mouth‐bar systems and by the shoreface–shelf system, the latter unexpected in such a lacustrine setting. In both cases, a regressive surface of lacustrine erosion (RSLE) has been identified, separating sandy mouth‐bar from glaciomarine to prodeltaic muds, and sandy shoreface wedges from the heterolithic shelf‐type body, respectively. The Lake Saint‐Jean record is an example of a regressive succession driven by a glacio‐isostatic rebound and showing the transition from late‐glacial to post‐glacial depositional systems.  相似文献   

12.
Lake Sapanca in NW Turkey is a fault originated freshwater basin fed by seasonally variable flows of 15 streams. Considerations of lake–river interaction, supported by statistical measures of 47 bottom samples, suggest that sediment transport and deposition within the lake is controlled by two types of human constructed structures in addition to natural factors: (1) the dykes constructed in the front of streams, which feed the lake by strong flows, to prevent the filling of lakebed by coarse-grained sediments; (2) the constructed regulator on the outflowing Çarksuyu stream results in a higher sedimentation rate of clay-sized material on the NE corner of the lake, which is extraordinary in the shelf environment.  相似文献   

13.
Seismostratigraphical studies of the 11.8‐km2‐large and ~140‐m‐deep Lake Bolshoye Shchuchye, Polar Ural Mountains, reveal up to 160‐m‐thick acoustically laminated sediments in the lake basin. Using a dense grid of seismic lines, the spatial and temporal distributions of the sedimentary history have been reconstructed. Three regional seismic horizons have been identified and correlated with the well‐dated 24‐m‐long sediment core retrieved from the lake. Isopach maps constructed from the seismic data show four phases of sedimentation. A contour map of the deepest regional seismic reflector represents the earliest hemipelagic sedimentation in the lake. Three contour maps represent time intervals covering the last 23 cal. ka based on the well‐dated core stratigraphy from the lake. The detailed time constraints on the upper stratigraphical units in the lake allow calculation of the lake's development in terms of sediment fluxes and the denudation rates from the Last Glacial Maximum (LGM) to the present. The sedimentation in Lake Bolshoye Shchuchye has been dominated by hemipelagic processes during at least the last 24 cal. ka BP only locally interrupted by delta progradation and slope processes. A major shift in the sediment accumulation at c. 18.7 cal. ka BP is interpreted to mark the end of the local glacial maximum, greatly reduced denudation and the onset of the deglaciation period; this also demonstrates how fast the glaciers melted and possibly disappeared at the end of the LGM. The denudation rate during the Holocene is only a fifth of the LGM rate. The age of the oldest stratified sediments in Lake Bolshoye Shchuchye is not well constrained, but estimated as c. 50–60 ka.  相似文献   

14.
Lake Vättern represents a critical region geographically and dynamically in the deglaciation of the Fennoscandian Ice Sheet. The outlet glacier that occupied the basin and its behaviour during ice‐sheet retreat were key to the development and drainage of the Baltic Ice Lake, dammed just west of the basin, yet its geometry, extent, thickness, margin dynamics, timing and sensitivity to regional retreat forcing are rather poorly known. The submerged sediment archives of Lake Vättern represent a missing component of the regional Swedish deglaciation history. Newly collected geophysical data, including high‐resolution multibeam bathymetry of the lake floor and seismic reflection profiles of southern Lake Vättern, are used here together with a unique 74‐m sediment record recently acquired by drill coring, and with onshore LiDAR‐based geomorphological analysis, to investigate the deglacial environments and dynamics in the basin and its terrestrial environs. Five stratigraphical units comprise a thick subglacial package attributed to the last glacial period (and probably earlier), and an overlying >120‐m deglacial sequence. Three distinct retreat–re‐advance episodes occurred in southern Lake Vättern between the initial deglaciation and the Younger Dryas. In the most recent of these, ice overrode proglacial lake sediments and re‐advanced from north of Visingsö to the southern reaches of the lake, where ice up to 400 m thick encroached on land in a lobate fashion, moulding crag‐and‐tail lineations and depositing till above earlier glacifluvial sediments. This event precedes the Younger Dryas, which our data reveal was probably restricted to north‐central sectors of the basin. These dynamics, and their position within the regional retreat chronology, indicate a highly active ice margin during deglaciation, with retreat rates on average 175 m a?1. The pronounced topography of the Vättern basin and its deep proglacial‐dammed lake are likely to have encouraged the dynamic behaviour of this major Fennoscandian outlet glacier.  相似文献   

15.
《Sedimentary Geology》2007,193(1-4):105-129
The blocking of major river valleys in the Leinebergland area by the Early Saalian Scandinavian ice sheet led to the formation of a large glacial lake, referred to as “glacial Lake Leine”, where most of the sediment was deposited by meltwater. At the initial stage, the level of glacial Lake Leine was approx. 110 m a.s.l. The lake level then rose by as much as 100 m to a highstand of approx. 200 m a.s.l.Two genetically distinct ice-margin depositional systems are described that formed on the northern margin of glacial Lake Leine in front of the retreating Scandinavian ice sheet. The Bornhausen delta is up to 15 m thick and characterized by a large-scale tangential geometry with dip angles from 10°–28°, reflecting high-angle foreset deposition on a steep delta slope. Foreset beds consist of massive clast-supported gravel and pebbly sand, alternating with planar-parallel stratified pebbly sand, deposited from cohesionless debris flows, sandy debris flows and high-density turbidity flows. The finer-grained sandy material moved further downslope where it was deposited from low-density turbidity currents to form massive or ripple-cross-laminated sand in the toeset area.The Freden ice-margin depositional system shows a more complex architecture, characterized by two laterally stacked sediment bodies. The lower part of the section records deposition on a subaqueous ice-contact fan. The upper part of the Freden section is interpreted to represent delta-slope deposits. Beds display low- to high-angle bedding (3°–30°) and consist of planar and trough cross-stratified pebbly sand and climbing-ripple cross-laminated sand. The supply of meltwater-transported sediment to the delta slope was from steady seasonal flows. During higher energy conditions, 2-D and 3-D dunes formed, migrating downslope and passing into ripples. During lower-energy flow conditions thick climbing-ripple cross-laminated sand beds accumulated also on higher parts of the delta slope.  相似文献   

16.
Sedimentation in ice-covered Lake Hoare, Antarctica   总被引：2，自引：0，他引：2  
The sedimentation mechanisms that occur in ice-covered Lake Hoare, Antarctica are examined, to determine how sediment enters the lake, and how the sedimentation pattern affects blue-green algal growth at the lake bottom. The 3 m-thick ice cover contains pebbly sand as much as 2 m below the surface. Sediment with similar texture and mineralogy is found at the lake bottom. This evidence, together with the lack of sediment in the inflowing stream and the markedly different texture of sediment from the other terrains around the lake suggest that most of the sediment at the lake bottom comes in through the ice cover. Sand grains intermittently migrate through porous ice on the surface, water-filled vertical gas-channels penetrating two-thirds of the ice cover, and possibly through cracks in the ice that act as conduits. The algae at the lake bottom are able to survive in part because sediment that comes through the ice cover does not obliterate them.  相似文献   

17.
We report results of bottom temperature monitoring of 2003–2004 in the deepwater South Baikal basin (Lake Baikal) near active gas-fluid methane vents at lake depths of 1020 and 1350 m. Sediments and water temperatures were measured using an autonomous temperature recorder designed at the Institute of Geophysics (Novosibirsk). Experiments implied short-duration recording and pioneering continuous 350 day-long monitoring near the Staryi vent. Measurements within a 1 m thick layer above and below the bottom showed notable variations in water (up to 0.07 °C) and sediment temperatures and in geothermal gradient. The long temperature records include a relatively steady period (mid-June 2003-early February 2004) with smooth temperature variations (especially in sediments) and two transient unsteady periods. The steady season is the best time for heat flow studies in the South Baikal basin. The 0.04–0.05 °C drop in bottom water temperature during the unsteady periods may result from intrusion of cold surface water. A positive temperature anomaly of ∼0.04 °C recorded in April 2003 may be caused, among other reasons, by active gas venting.  相似文献   

18.
The temporal and spatial variations in the rate of sedimentation and sediment resuspension in Lake Rehtijärvi (southern Finland) were studied by sediment traps deployed in the stratifying and non‐stratifying regions of the lake. Both the gross sedimentation rate and the resuspension rate recorded by near‐bottom traps were significantly higher in the stratifying region (>20 m depth) than in the shallow part of the lake (2 m depth). These rate changes were caused by temporarily elevated hypolimnetic resuspension rates exceeding 40 g dry weight m?2 day?1 in July 2005, whereas in the shallow station no such peak was observed and the resuspension rate remained below 30 g dry weight m?2 day?1 throughout the study. The elevated resuspension rate in the hypolimnion could not be attributed to surface wave action or sediment disturbance by fish, but was due to the occurrence of internal seiches. The seiche amplitude was on average the highest in July, although the wind velocities were low; this was because winds temporarily were blowing along the longitudinal axis of the lake. Due to the steep slopes, the sediments of Lake Rehtijärvi are prone to redistribution and sediment focusing towards the accumulation areas also affected trap catches in the deep stations.  相似文献   

19.
We studied the dissolved silica cycle in the water column of the North basin of Lake Lugano, Switzerland/Italy. Lake Lugano is a meromictic, eutrophic lake, permanently stratified below 100-m depth. A one-box model was used to calculate a silica mass-balance over 1993, based on various lake measurements, such as sediment traps, sediment cores, water analysis and biota countings. We found that the North basin of Lake Lugano is at steady state as far as dissolved silica is concerned. The primary source of dissolved silica in the lake is river input (about 80%), with diffusion from bottom sediments and groundwater input also playing a role. Atmospheric input is negligible. The main export of dissolved silica occurs via biogenic uptake by diatoms and final burial of their frustules in the bottom sediment. Loss of dissolved silica through the lake outflow only represents 15% of the total output. Of the total amount of Si exported to the lake bottom through diatom sinking, less than 20% is re-supplied to the surface water by diffusion. Thus, the lake acts as an important permanent sink for silica. The long residence time of dissolved silica in the lake (7 years) is related to the strong physical stratification of the lake. Only about 10% of the standing stock are available to phytoplankton uptake.  相似文献   

20.
Lake Houston is a man-made reservoir located northeast of Houston, Texas. The purpose of this investigation was to document suspended sediment transport, sedimentation, and resuspension in the lake with a view towards estimating the influence of sedimentation on water quality. Sediment traps were placed in strategic locations in the lake to collect suspended sediments. Samples were analyzed for bulk density, grain size, organic carbon, and a number of trace elements. These data were analyzed along with meteorological data to examine those factors which regulate suspended sediment input and dispersal, and the role of suspended sediments in controlling water quality within the lake. Sediment input to the lake depends primarily on the intensity of rainfall in the watershed. Sediment movement within the lake is strongly influenced by wave activity, which resuspends sediments from shallow areas, and by wind-driven circulation. The increased residence time of suspended sediments due to resuspension allows greater decomposition of organic matter and the release of several trace elements from sediments to the water column. Virtually all samples from sediment traps suspended between 1 and 5 m above the lake bottom contain medium to coarse silt, and even some very fine sand-sized material. This implies that circulation in Lake Houston is periodically intense enough to transport this size material in suspension. During winter, northerly winds with sustained velocities of greater than 5 m/sec provide the most suitable condition for rapid (<1 d) transport of suspended sediment down the length of the lake. Fluctuations in current velocities and the subsequent suspension/deposition of particles may explain variations in the abundance of coliform bacteria in Lake Houston.  相似文献   

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