Estuarine, Coastal and Shelf Science (2002) 54, 747759idea

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Rivers discharge fresh water and sediments to theinverse estuary in arid regions requires a net flow ofoceanic water toward its head to compensate for thecoastal ocean. The resulting estuarine environmentsbecome the pathways through which finer sedimentsmay reach the open sea. Within an estuary the finesediment particles are dispersed along paths deter-mined by water motion. Tidal energy continuallyerodes, transports and deposits bed sediments. Thus,within the classic estuary the suspended particulatematter (SPM) concentration is generally high evenwith little new sediment input (Dyer, 1995). Theselow salinity water bodies respond to seasonal fluctua-tions of freshwater discharge. Extensive research inshelf seas has been oriented to determine the fate ofthe high river runo that occurs during the wetseason. In contrast, dry season estuarine dynamics hasonly received attention in recent years. The tropicalestuaries of northern Australia, for instance, are influ-enced by large seasonal fluctuations in freshwaterdischarge (Wolanski, 1988). In arid regions wherelittle precipitation and runo combine with a highevaporation rate, hypersaline water bodies developcalled negative estuaries or inverse estuaries. Large-scale examples of this are the Persian Gulf, the RedSea, the Arabian Sea, the South Australian gulfs and

freshwater loss due to evaporation. The tendency forsalt accumulation inside the inverse estuary requiresan opposing mechanism that removes the high salinitywater. One process involves the sinking of saltier(denser) water in the shallower areas and its flowtoward the open ocean boundary as a near-bed gravitycurrent (Nunes Vaz et al., 1990). Gravity currentevents have been observed in the Australian inverseestuaries and in the Upper Gulf of California duringneap tides (Lavn et al., 1998). Among the smaller-scale examples, coastal lagoons in arid regions canalso exhibit inverse-estuarine behaviour when highevaporation causes a net loss of water. In theseso-called anti-estuarine lagoons, the seaward flowingcirculation near the bottom can carry SPM from thelagoon out to the ocean (Groen, 1967; Postma,1967). Enhancement of the seaward flux of SPM canbe expected since both gravity currents and high SPMconcentration may develop close to the sea bedconcurrently. Sediment transport under these condi-tions has not been widely documented, perhaps as aconsequence of the relatively little attention given toinverse estuaries in the past. The present study reportsdoi:10.1006/ecss.2001.0873, available online at http://www.

Factors Influencing SuspenUpper Gulf of CaliforniaL. G. Alvareza and S. E. Jonesb

aDepartamento de Oceanografa Fsica, CICESE, EnsenabSchool of Ocean Sciences, Marine Science Laboratories, M

Received 17 January 2000 and accepted in revised form 1

Few studies exist of sediment dynamics in inverse estuaassociated gravity currents and arise in arid regions whereObservations of velocity, density and suspended sedimDoppler current profiler, CTD, optical backscatter sensoof California over a spring to neap tidal cycle. These revevents produced significant net near-bed suspended sedimInstantaneous suspended sediment fluxes exceeded 30 gfluxes were near zero. The baroclinic gravity current is ssediment toward deeper waters, at least during quiescenta wide, western along-Gulf channel. This is consistent wsediments to deeper water after becoming unstable due

Keywords: suspended matter; sediment flux; inverse es

Introductionthe Gulf of California (Bowers, 1989; Nunes Vazet al., 1990).

02727714/02/040747+13 $35.00/0library.com on

ed Sediment Flux in the

, Baja California, Mexico 22860nai Bridge, LL59 5EY, U.K.

une 2001

s, which are characterized by hypersaline water bodies andtle precipitation and runo combine with a high evaporation.

concentration profiles have been made using an acousticnd water sampling at a shallow water site in the Upper Gulf

led contrasting dynamic conditions in which gravity currentt fluxes of 25 g m2 s1 out of the Gulf during neap tides.2 s1during spring tides due to tidal resuspension, but netn to be the dominant mechanism for net flux of suspendedmer conditions. This flux is proposed to be confined withinevidence that the Colorado River delta system is exportingwitching o of river discharge since 1930.

2002 Elsevier Science Ltd. All rights reserved.

ries; gravity currents; Gulf of California

In contrast with the classic estuary model, themeasurements of currents and SPM concentrations ata site in the Upper Gulf of California, a shallow

2002 Elsevier Science Ltd. All rights reserved.

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SanFelipe

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748 L. G. Alvarez and S. E. Jones30.9

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10 km114.1115.0W 114.9 114.8 114.7 114.6 114.5 114.4 114.3 114.2macrotidal region with inverse estuarine features.The aim of this work was to investigate theprocesses influencing sediment fluxes using intensiveobservations made for the first time in this region.

Area of study

The Upper Gulf of California (henceforth called theUpper Gulf) is located in the northernmost part of theGulf of California, bounded by the arid lands ofSonora and the Baja California peninsula (Figure 1).It is one of the most dynamic shallow environments inMexico due to its large tidal range (7 m) and fasttidal currents (1 m s1) during spring tides. Before1930, the Colorado River supplied freshwater(20109 m3 yr1) and sediments (180106tons yr1) to the northern end of the Upper Gulf(Thompson, 1968). Estuarine conditions may haveprevailed then, before human tampering with the riverdrainage, as suggested by numerical modelling andrecent surveys made during unusual river discharge(Carbajal et al., 1997; Lavn & Sanchez, 1999).

The region is less than 40 m deep with irregular

troughs aligned parallel to the Gulfs axis. The sea-bedsediments consist of unconsolidated deposits suppliedby the Colorado River before dams were built.Textural classification of the bottom surface sedi-ments yields three groups: silt-clay, sand-silt-clay andsand. Silt-clay predominates o Baja California, northof San Felipe and between the ridges in the interven-ing troughs (Thompson, 1968). Finer sediments ofmean grain size 5 7 predominate over the westernhalf of the Upper Gulf and coarser sediments of2 4 are found over most of the eastern half(Carriquiry & Sanchez, 1999). Observations acrossthe gulf by Garca de Ballesteros and Larroque (1974)indicated that surface SPM concentrations werehigher near the river mouth with a persistent horizon-tal gradient to the southeast. This trend was describedearlier by Thompson (1969) based on turbidity pat-terns. He suggested that tidal currents were the mainconveyor of sediments in this region and that the slightrotary motion of these currents induced a westwarddrift of sediments in suspension.

A sea breeze wind regime prevails near thecoast with magnitude less than 5 m s1. Maximum

1

F 1. The Upper Gulf of California and the bottom configuration in the area under study, o Baja California. The fixedsampling sites (J and W3) and two CTD along-gulf transects (A and B) are indicated31.5N

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temperature and backscattered acoustic signalstrength were recorded by the ADP. The site (W3) is

o the coast of Baja California, and 25 km northwestof San Felipe, as shown in Figure 1. The mooring was

Suspended sediment flux in the Gulf of California 749northwesterly direction in winter and southeasterlydirection in summer. These monsoon-type winds mayreach 13 16 m s1 in winter and 510 m s1 insummer (Gayman, 1969; Green 1969; Delgado-Gonzalez et al., 1994). This wind regime generates thelow, steep, short-period waves typical of the northernGulf. Significant wave height is only 03 10 m witha period of 24 s due to limited fetch and windduration. Southern swells may arrive 210 times ayear, mainly in late spring, with wave height 115 mand period 4 7 s. These events last only 624 h(Gayman, 1969).

Numerical modelling studies, supported by hydro-graphic observations indicate that tidal mixing isresponsible for the generally vertically homogeneouscharacter in the shallow areas of the Upper Gulf(Argote et al., 1995). The vertically mixed watersextend southeast to depths between 30 and 60 mwhere a thermal front has been regarded as theboundary (Durazo-Arvizu, 1989; Argote et al., 1998).

Indirect evidence suggests that a cyclonicresidual circulation is generated in the Upper Gulf(Hendrickson, 1973; Hernandez-Ayon et al., 1993). Asimilar pattern was reported by Marinone (1997),through numerical modelling of the tidal residualcurrents, and also by Carriquiry and Sanchez (1999),based on calculated sediment transport pathways. Thepatterns of suspended sediment plumes observed insatellite images have been attributed to the eect ofthis residual circulation. Carbajal et al., (1997) havespeculated that the transport of sediments to deeperbasins takes place through the mechanism of residualcurrents.

Hydrographic surveys and current measurementsreported by Lavn et al. (1998) have revealed that theUpper Gulf normally behaves like an inverse estuary,in which salinity and density increase toward the headthroughout the year, despite reversal of the tempera-ture gradient from summer to winter. As in otherinverse estuaries, the horizontal density gradients inthe shallow coastal areas of the Upper Gulf aresustained by intense vertical mixing during springtides. During neap tides the decrease in mixing allows,given the horizontal contrast of density, the onset ofgravity currents: the warmer but saltier and denserwater (t=02) in the shallow, well-mixed regionsflows underneath the colder, fresher and lighter o-shore water with a down-slope component and justnear the bed. The same authors reported that thevelocity at 1 and 6 m above the bed was to thesoutheast, flowing out of the Upper Gulf at01 m s1, similar to geostrophically adjusted

speeds of gravity currents observed in Australian Gulfs(Nunes & Lennon, 1987).set inside an 8-km wide trough with the shallowcoastal shelf on the west and a narrow ridge rising810 m on the east. This long, gently sloping channelreaches Wagner Basin 40 km to the southeast of themeasurement site.

Vertical profiles were obtained with a Sea-BirdCTD fitted with a Sea-Tech 10-cm path length trans-missometer and a Seapoint optical backscatter sensor(OBS). Casts were made on board the R/V Franciscode Ulloa along transects and every half an hour nearthe ADP (site W3) over fixed periods. These timeseries lasted 48 h during spring tides and only 15 hduring neaps due to poor weather. Water samplesfrom CTD-mounted bottles were used to measureSPM concentration by filtering a known volume of thewater sample and by dierential weighing of the driedfilters. Forty-two samples were collected less than09 km from the ADP site, and the directly measuredSPM concentration range appeared to encompassmost of the spring to neap tide variation observed inthis area.

Additional CTD and OBS profiles were measuredduring a preliminary survey in June 1996 in the samearea (site J shown in Figure 1), under spring andneap tide conditions. The hydrography and currentmeasurements at 1 and 6 m above the sea bed havebeen described by Lavn et al. (1998).

Analysis of data

Velocity

After rejection of near surface ADP data due to signalreflection or signal-to-noise ratio reduction, usefulvelocity bins started at 12 m above the sea bed andextended up to 16 m. This range represents 60% ofthe water column at mean sea level. The velocitycomponents were projected along the principallocated on the western side of the Upper Gulf, 20 kmObservations

The main data set consists of velocity measurementsmade during a 15-day deployment of a bottom-mounted SonTek Acoustic Doppler Profiler (ADP)in waters 25 m deep. The velocity record consists ofconsecutive 5-min averages from 05 m depth bins .In addition to currents, the sea level, near-bottomaxes corresponding to the directions of maximumvariance and its perpendicular. These directions

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neap tides.

tions measured from water samples collected at cor-

estimating SPM concentrations and fluxes. Instan-

C

1.6

750 L. G. Alvarez and S. E. Jonesresponding times and depths. A linear regressionprocedure yielded the calibration curve:Acoustic signal strength

The ADP stored the raw acoustic signal strengthbackscattered from particulate matter in the water.After conversion to decibels, the signal strength wascorrected for water sound absorption and geometricspreading of the acoustic beams (SonTek TechnicalNotes, 1997). The corrected signal strength from thethree beams was averaged and normalized by theminimum value of the data set. Corrected signalstrength from selected range cells between 3 and 15 mabove the transducers was extracted at suitable times.These values were compared to SPM concentra-coincide with the along-Gulf and across-Gulforientations, respectively.

Optical backscatter

The voltage V of the CTD-mounted OBS was con-verted to SPM concentration C (mg l1) by calibra-tion against filtered water samples. These sampleswere obtained throughout the Upper Gulf in theAugust 1997 intensive cruise. During the preliminarycruise in June 1996, a near-bottom water sample wasbrought on board for calibrating the OBS underlaboratory conditions. Results of the least squares fitsare given in Table 1. Time series of the verticalprofiles of SPM concentrations were then obtainedevery half-hour at sites W3 and J during spring and

T 1. Calibration of the optical backscatter sensorsagainst filtered SPM samples. C (mg l1), V (volts)

Date Linear fit equation R2 Data points

June 1996 C=072+9200 V 099 5August 1997 C=344+1603 V 092 371log10C=111860070+(0024500040)A,

Close to the transducers, within the so-callednear-field range, a complex acoustic beam patterndevelops. It deviates from the spherical spreading ofthe far-field, in which the acoustic signal strength isinversely proportional to the range (r) from the trans-ducers. The boundary between the near-field and thefar-field backscatter regions is at a distance r=2/(Sontek Technical Notes, 1998), where is the trans-ducer radius and is the acoustic wavelength. For theADP used, r is 125 m. Thorne et al. (1996) haveapplied a factor of 2 which in our case yielded the firstthree bins within the near-field. Since the suspendedsediment samples for calibration were collected in thefar-field region, only signal strength values measured27 m from the sea-bed and above were used for

130

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F 2. Measured SPM concentration vs ADP signalstrength (circles). The regression line (solid line) is shownwith the 68% confidence interval (dotted lines).but the most frequent were southerly winds, blowingwhere C is the concentration of SPM in mg l1 and Ais the corrected return signal strength in decibels. Theregression parameters are given with 95% confidenceintervals. Figure 2 shows the SPM concentrationvalues and the regression line that accounted for 80%of the variance in the data.taneous horizontal fluxes between 27 and 107 mabove the sea-bed were obtained as the product ofvelocity times SPM concentration (C) at 05 m incre-ments. Vertically integrated instantaneous fluxes were

z2calculated by numerically integrating Cdz, where

z1z1=27 m and z2=107 m above the sea-bed, with05 m increments. Further calculations yielded instan-taneous and time averages of along-gulf (principalaxis) and across-gulf fluxes during spring and neaptides and over the full fortnightly cycle.

Results

Variable winds less than 6 m s1 prevailed during the15-day deployment period (1227 August, 1997).Direction changed from NW to SE within a few hours2log C = 1.1186 + 0.0245 AR2 = 0.80

1.9

1.8

1.7over 80% of the time. Moderate southerly wind events

most of the daylight hours, possibly the eect of solarheating. This increase in temperature reflects a tran-

tidal resuspension signal in which periodic concen-

CTD transects

the point of detachment.

SPM fluxes

Suspended sediment flux in the Gulf of California 751tration peaks coinciding with maximum flow extendedover most of the water column. A dierent pattern isevident in the 15-h time series during neap tides: thehigh SPM concentration layer was constrained to anear-bottom layer, 58 m thick, showing only a weaksient buoyant layer near the surface, as revealed by thesigma-t structure.

The neap-tide time series, despite being limited to15 h, shows contrasting patterns especially close to thesea bed (Figure 4). A denser near-bottom layer 58 mthick was overlaid by an almost homogeneous lessdense water. The near-bed water was up to 05 Cwarmer and 06 more saline than the upper layer. Thedensity changed 0203 sigma-t units across a 2-mthick interface that displayed a wave-like pattern. Atthe end of the series the interface spread upward overmost of the water column. The weakening of thevertical density gradient at the end coincided with theonset of strong southerly winds and rough seas thatprevented longer observations.

SPM concentration

High SPM concentrations were found near the sea-bed, decreasing upwards, as shown in Figures 3 and 4.This indicates that the source of SPM was the sea-bedsediment resuspended by the tidal currents. Themaximum concentration during spring tides reached85 mg l1 close to the sea-bed (Figure 3), while nearsurface values were 5 mg l1. The highest near-bedconcentrations were only 25 mg l1 during neap tides(Figure 4). The spring-tide time series show a clear610 m s1, lasting 1 day, occurred during neaptides on 12 August, and again on 14, 23 and 24August. Maximum period and wave height, visuallyestimated, were 45 s and 1 m respectively, duringthese events.

Hydrographic setting

Spring-tide time series of temperature, salinity andsigma-t profiles are shown in Figure 3 which onlydisplays a 15 h interval. The temperature, salinity anddensity variation is small (302308 C, 362364,and 223227 sigma-t units, respectively). The nearlyvertical isolines indicate that well mixed conditionsprevailed from near the bottom to near the surface.Slight stratification involving a vertical temperaturerise of 05 C was observed close to the surface duringresuspension signal. Similar conditions prevailed dur-ing the 2-day long neap tide SPM observations of JuneAt levels between 27 and 107 m above the sea-bed,instantaneous along-gulf SPM fluxes based on theADP backscattered signal reached 30 g m2 s1during spring tides and 6 g m2 s1 during neapCurrents

Currents were mainly tidal, semidiurnal and nearlyunidirectional, with the principal axis trending NW toSE. Velocity was modulated fortnightly, reachingamplitudes of 08 m s1 during spring tides, anddecreasing to 01 m s1 during neap tides. Thevelocity gradually decreased toward the sea-bedwithin an oscillatory bottom boundary layer. The tidalcurrent reversal nearly vanished during neap tides atthe beginning and again at the end of the fortnightcycle. The outcome was net outflow from the UpperGulf (negative velocity) within a layer 78 m thick,next to the sea-bed. This outstanding feature of theflow was due to a gravity current flowing along-channel with its core centred 45 m above the bottom.The speed at the core varied between near zero and02 m s1 over at least four semidiurnal cycles.Figure 1 shows the location of two CTD along-gulftransects made in this area, 1 to 2 days after the neaptides. Transect A was made along the depression inwhich the time series were obtained. Transect B wasmade along the next depression further East, todepths of about 100 m, at the edge of Wagner Basin.Salinity, sigma-t and SPM distributions are given inFigure 6. These transects were made at dierentstages of the tide. However, both transects showhigher values toward the shallow depths in the head ofthe Upper Gulf. Denser, more saline and turbid waterremained close to the sea-bed. At a depth of 50 m thiswater left the bottom and intruded into the ambientfluid, with a sigma-t value near 224, as shown inFigure 6(b). The high salinity and turbid water thatbecame detached from the sea-bed could be traced asa subsurface maximum to at least 10 km away from1996, at site J (Figure 5). The high concentrationswere observed within 6 m from the sea bed, with a20 mg l1 maximum near the bed. A weak resuspen-sion is evident when currents were stronger, flowingout from the Upper Gulf.tides. The time series of instantaneous SPM fluxesdisplay a marked springneap modulation at all the

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F 3. Temperature, salinity, sigma-t and SPM concentration fields measured at site W3, in August 1997, during spring

tides. Along-gulf velocity vectors drawn in the lower frame are at 12 m and 15 m above the sea-bed. Arrows pointing to theleft indicate outflow from the Upper Gulf. Casts were made every half-hour.

Suspended sediment flux in the Gulf of California 7531212 August

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F 4. Temperature, salinity, sigma-t and SPM concentration fields measured at site W3, in August 1997, during neap

tides. Along-gulf velocity vectors drawn in the lower frame are at 12 m and 15 m above the sea-bed. Arrows pointing to theleft indicate outflow from the Upper Gulf. Casts were made every half-hour.

26 June

at site J in June 1996, during neap tides. Along-gulf velocitypoi

754 L. G. Alvarez and S. E. Jonesmeasurement levels. This modulation can be seen inthe time series of instantaneous fluxes measured 52 mabove the sea-bed, at the level of the maximum gravitycurrent flow (Figure 7). Spring tide fluxes were nearlysymmetric about zero, except during neap tides, whenfluxes were predominantly out of the Upper Gulf(), reaching up to 6 g m2 s1. Vertically inte-grated instantaneous fluxes yielded maximum trans-port per unit width near 40 g m1 s1 during neap

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F 5. SPM concentration vs time and height measuredvectors are drawn at 1 m and 5 m above the sea-bed. Arrowswere made every half-hour.tides, 290 g m s during spring tides, and an1 1

taneous fluxes. The remarkable contrast betweenspring- and neap-tide fluxes is better shown byaveraging the fluxes under each tide condition. Sincethe neap-tide outflow events lasted around three days,the average over six semidiurnal cycles was obtainedfor each tide condition. It was found that near-zeroresidual SPM fluxes prevailed during spring tides[Figure 9(c)], while during neap tides they reached25 g m2 s1 [Figure 9(a)]. The average fluxesduring spring tides show only a very weak verticalstructure. In contrast, the average fluxes during neap

nting to the left indicate outflow from the Upper Gulf. Caststides display a significant maximum at 4 m above the

average of 12 g m s over the full fortnightlycycle.

In addition to the ADP time series, calibrated OBSprofiles from site W3 have provided independentSPM concentrations and instantaneous flux calcula-tions for two short periods during spring and neaptides. Fluxes estimated from both ADP and OBSmeasurements are in good agreement, as shown inFigure 8, which shows time series of fluxes at 52 mabove the bed. Along-gulf fluxes during the 15-hneap-tide series show maximum near6 g m2 s1.These figures are also similar to the neap-tide maxi-mum flux (5 g m2 s1) measured 6 m above thesea-bed at site J during the two-day preliminary surveyin June, 1996.

Figure 9 shows time-averaged instantaneous fluxprofiles. The average over a springneap cycle (147days) yielded a residual flux out of the Upper Gulf, asshown in Figure 9(b). This residual flux is one orderof magnitude smaller than the maximum instan-

sea-bed, which is the same level as the maximumvelocity of the gravity current.

Discussion

There has been considerable interest recently inconverting acoustic signal strength measured byacoustic Doppler current profilers to SPM concen-tration (Jones et al., 1994; Thorne et al., 1996;Holdaway et al., 1999). The calibration methodsadopted have ranged from obtaining simple heightdependent calibration coecients to full considerationof particle size dependence and attenuation of thesignal received from a given layer by SPM in theintervening layers. Most studies have concluded thatthe technique is sensitive to variations in particle size,and it has been shown that SPM attenuation can actto increase apparent concentrations by up to 26%in high concentration environments (Holdaway08040020161208

SPM (mg l1)

16 16 1612 12

20

8

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844

4et al., 1999). This study considers spreading andattenuation by water but not by SPM; nevertheless, a

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pth 20 6.00

Suspended sediment flux in the Gulf of California 755convincing calibration has been found using samplesfrom a range of heights above the bed.

At short ranges (15 m) and low concentrations1

be neglected in estuarine sediment suspensions ifconcentrations are less than 100 mg l1 (Thorneet al., 1993). In our measurements, the range was

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F 6. Vertical distributions of salinity, sigma-t and SPM concentration measured on 14 August, 1997. (a) transect A and(b) transect B shown in Figure 1.067

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36.537.037.0 36.

(b)(a)(

756 L. G. Alvarez and S. E. Jonesreached 85 mg l1 only during short time intervals.Farther up, near the top of the water columnanalysed, concentrations were 50% lower. Hence,attenuation by suspended particles was assumed to besmall. However, uncertainties introduced by changesin the particle size may be important in naturalenvironments. Since bed sediments of the Upper Gulfare poorly sorted, changes in the bed stress due to theoscillatory tidal current are likely to change the sizedistribution of suspended sediments. Moreover, thepresence of fine silts and clays is expected to causeflocculation and therefore increase the eectiveparticle size. Our measurements were not sucientfor a detailed evaluation of these eects; however, acomparison with earlier studies has provided boundsfor the expected uncertainties in concentration.Thorne et al. (1991) have accounted for the eect ofvariable particle size by introducing a constant k0 thatincorporates the scattering properties of the particlesin suspension. In their study, the particle size varied

by a factor of four produced 20% change in k0. Itwas also estimated that a 10% uncertainty in k0translated into a 20% uncertainty in concentrationvalues. During our observations the median particlesize near the bed varied within a factor of four (40 to180 m) throughout the springneap cycle (Joneset al., in preparation). Therefore we can expect thatuncertainties in the acoustically estimated concen-tration were less than 50% in the lower levels of theconcentration profiles, and it can be assumed that theeects of variable particle size and attenuation bysuspended sediment have introduced uncertaintieshaving magnitude similar to the error of the regressionequation. Conversion of ADP signal strength to SPMconcentration has therefore allowed the estimation ofinstantaneous and time-averaged horizontal fluxes ofSPM between 27 and 107 m above the bed undercontrasting flow and SPM concentration conditionsduring a fortnightly cycle. A more detailed calibra-tion may be needed before finer details of the

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14 16 18 20 22 24 26

F 7. Instantaneous horizontal SPM fluxes derived from ADP data, 52 m above the sea-bed (the level of the gravitycurrent core), at site W3. Along-gulf (solid line) and across-gulf (dotted line) fluxes are 5-min averages.40S

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20from 55 to 210 m and the detection range was up to1 m using a 3 MHz system. A change in particle sizetemporal and vertical variation of flux profiles can berelied upon.

s

Suspended sediment flux in the Gulf of California 757The near-bed region could not be calibrated usingthe method described as no concentration sampleswere collected in this near-field region of the transduc-ers. This region may contribute significantly to depth-integrated fluxes as measured concentration profilesoften indicate sharp increases toward the bed. How-ever, currents decrease rapidly in this region and theflux profiles during periods of high flux indicate amaximum at 4 m above the bed, well above the limitof measurement. Therefore, it is likely that the mainfeatures of near-bed flux have been measured in thisstudy.

It is unlikely that the short-crested waves observedduring the survey contributed significantly to theresuspension and transport of sea bed sediments 25 mdeep. Linear theory predicts that the maximum orbitalvelocity due to 15 m waves at 5 s intervals is less than

1

not stir up sea-bed sediments in water depths morethan 14 wavelength. The wave action of a 4 s wave islimited to a depth of 6 m. Waves of longer periods arerare.

The large range of the barotropic tide in the UpperGulf generates fast currents and large horizontal tidalexcursions that, during spring tides, reach 10 km nearthe surface. However, according to numericalmodels, tidal residual flows of barotropic characterin the Upper Gulf are 001 m s1. (Carbajal,1993; Argote et al., 1998; Marinone, 1997). There-fore, residual displacement of a particle due to tides isless than 05 km in one semidiurnal periodand SPM fluxes produced by this mechanism areapproximately 03 g m2 s1. Gravity currents, firstobserved in this region by Lavn et al. (1998), areshown here to produce near-bed fluxes which, despite

2040

Time (h) after 21:00 11 August 1997

SP

M f

lux

(g m

2

0

10

20 3010

0

4040

Time (h) after 14:00 17 August 19970

20

20 3010

0

F 8. Instantaneous SPM fluxes measured 52 m above the bed at site W3, obtained from OBS (dotted line) and ADPbackscatter (solid line): (a), (b) are across-gulf and along-gulf during neap tides; (c), (d) are across-gulf and along-gulf duringspring tides. The vertical axis scale in (d) is twice as large as in the first three.20

1 ) 10

(b)

2040

20

SP

M f

lux

(g m

2 s

1)

0

10

10

20 3010

0

(a)002 m s at this depth. According to Gayman(1969), the prevailing sea breeze waves probably do40

20

(d)

2040

20

0

10

10

20 3010

0

(c)their short duration, are one order of magnitude largerthan those induced by tidal residual currents, at least

Carriquiry and Sanchez (1999) that this shelf is aslowly prograding feature.

758 L. G. Alvarez and S. E. Jonesduring summer. Therefore, the gravity current can beexpected to be the main cause of SPM flux duringeach neap tide, and hence to contribute significantlyto net fluxes during summer. Furthermore, thesalinity observed in Wagner Basin has been explainedin terms of water mass formation occurring through-out the year, in the shallow areas. The dense, high

01

15

SPM horizontal flux (g m2 s1)

Mean sea level at25 m above bed

Hei

ght

abov

e be

d (m

)

3 2.5 2 1.5 1 0.5 0 0.5

5

10

(a) (b) (c)

F 9. Profiles of time-averaged instantaneous along-gulf SPM fluxes measured at site W3 in August 1997: (a)averaged over six semidiurnal cycles in neap tides, (b)averaged over 147 days, and (c) averaged over six semi-diurnal cycles in spring tides. Negative values indicate fluxesout of the Upper Gulf and the dashed lines indicate the 68%confidence interval.salinity water mass flows down-slope toward the basin

Wagner Basin. The transect made along the secondchannel o Baja California revealed a down-channelOne can speculate on an asymmetric sedimentdeposition based on rotation eects. The earths rota-tion constrains a gravity current to flow along thewestern boundary of a channel (in the NorthernHemisphere) if time and space scales are sucientlylarge. A crude estimate can be made to test whetherthe conditions in the Upper Gulf would allow for thisadjustment: The shallow head and western side of theUpper Gulf are the sources of heavier water which is02 sigma-t units denser than the water foundfurther oshore (Lavn et al., 1998). By taking a 10 mwater depth, the internal Rossby radius of deforma-tion gives a cross-flow scale of less than 4 km, which isabout half the width of the channels and about onetenth their length. Since the time scale for geostrophicadjustment is 1 day (2/f) and the gravity currentevent lasts at least 3 days, then this current couldevolve into a flow along the western boundary of thechannels. This type of buoyancy driven flow along aboundary has been observed in laboratory rotatingtanks by Wadhams et al. (1979) and Griths andHopfinger (1983), among others. Thus, a plausibleexplanation for the eastward progradation of the west-ern boundary of the channels involves the settling ofsuspended load that the gravity current transportsmainly along the western side of the wide along-gulfdepressions. More observations at adequate scales areneeded before a precise evaluation of this concept canbe made.

Acknowledgements

The authors are grateful to Rafael Ramrez, VctorGodnez, Salvador Sanchez, Neil Fisher and DavidProbert for their assistance with the intensive fieldand laboratory work. The technical support of JuanFrancisco Moreno is also acknowledged. Thanksare given to the crew on R/V Francisco de Ulloa ofCICESE, under the command of Capt. GerardoSanchez. NOAA wind data were made available by T.as sporadic gravity current pulses (Lavn et al., 1998).Thus, sediment fluxes induced by the near-bottomgravity current may occur not only during summer,but all through the year. The present study hasrevealed that a net transport of fine sediments isoccurring out of the Upper Gulf. However, contraryto what has been speculated, the barotropic tidalresidual is not the main cause. The baroclinic gravitycurrent was found here to be the dominant mech-anism for near-bed SPM flux toward deeper waters.

The bathymetry shows several narrow along-gulfsubmarine ridges up to 40 km long and 15 m high.The ridges o Baja California form 5 to 10 km widechannels having an asymmetric cross section withgentler slopes on the western side (Figure 1). Byjoining the head of the Upper Gulf with the WagnerBasin, these channels must play an important role inalong- and across-gulf exchange processes, especiallyin those that take place near the bed. As suggested inFigure 6(b), sediments in suspension seem to beconveyed through the channels to the deeper waters ofdecrease of SPM concentration, in support of thisidea. The horizontal intrusion of turbid water at 50 mdepth indicates that part of the suspended sedimentload reached the edge of the Wagner Basin, whileanother fraction seems to have settled along the cur-rent path. If sediments are settling mainly on thewestern side of the channel, an asymmetric cross-section would develop in the long term. This processcould also explain the seaward growth of the shallowwestern shelf, in agreement with the suggestion byGerrodette. Jose Ochoa and two anonymous reviewersprovided useful comments and suggestions to improve

the manuscript. This study was funded by the ConsejoNacional de Ciencia y Tecnologa of Mexico (grant3007P-T).

Holdaway, G. P., Thorne, P. D., Flatt, D., Jones, S. E. & Prandle,D. 1999 Comparison between ADCP and transmissometermeasurements of suspended sediment concentration. ContinentalShelf Research 19, 421441.

Jones, S. E., Jago, C. F., Prandle, D. & Flatt, D. 1994 Suspendedsediment dynamics: measurement and modelling in the DoverStrait. In Mixing and Transport in the Environment (Beven,Chatwin & Millbank, eds). 183201.

Jones, S. E., Alvarez, L. G., Fisher, N. & Probert, D. In

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Factors Influencing Suspended Sediment Flux in the Upper Gulf of CaliforniaIntroductionFigure 1

Area of studyObservationsAnalysis of dataVelocityTable 1Optical backscatterAcoustic signal strengthFigure 2

ResultsHydrographic settingSPM concentrationCTD transectsCurrentsSPM fluxesFigure 3Figure 4Figure 5

DiscussionFigure 6Figure 7Figure 8Figure 9

AcknowledgementsReferences