Sedimentology (1981) 28,107-114

Particulate matter grain-size characteristics and flocculation in a partially mixed estuary

K A T E K R A N C K

Atlantic Oceanographic Laboratory, Bedford Institute of Oceanography, P.O. Box 1006, Dartmouth, Nova Scotia B2 Y 4A2, Canada

ABSTRACT

The concentration and grain size of the natural and deflocculated inorganic suspended particulate matter were measured along the length of the Miramichi Estuary and interpreted with respect to flocculation and transport properties. Changes in particulate matter concentration are associated with regular changes in grain-size characteristics. In the turbidity maximum region of the estuary the suspended matter occurs mostly as large flocculated particles whereas, in the waters with lower particle concentrations, a larger proportion of the material occurs as fine material. At higher concentrations natural floc modes and inorganic grain modes vary simultaneously but at low concentrations the two modes vary inversely. This modal relationship and the variation in organic matter within the estuary is proposed to result from variation in inorganic-organic composition of flocs. Increase in settling rates due to flocculation is believed to increase the trapping effect of the estuarine circulation that produces the turbidity maximum.

INTRODUCTION

The two-layer estuarine circulation causes the trap- ping of sediment within an estuary and greatly re- duces the transport of river-borne sediment to the open sea. The resulting rapid sedimentation occurs where the interaction of tides and river flow, along with density differences, produce intense long- and short-term fluctuations in the physical environment. Shipping channels and harbour construction often conflict with the variable and unpredictable natural sedimentation. Many pollutants are also particles, or occur absorbed to or flocculated with particles. For these reasons, the sedimentological environments and particulate matter dynamics of estuaries have been studied extensively, but mainly based on a mass balance approach in which total concentra- tions of substances were measured. Relatively little is known about the physical characteristics and changes in form of suspended particulate matter within estuaries. The role of flocculation in sedimentation processes, and maintenance of 0037-0746/81/02ocrO107 $02.00 0 1981 International Association of Sedirnentologists

the turbidity maxima especially, is poorly understood. Estimates of its importance have ranged from insignificant (Meade, 1968, 1972; Schubel & Kana, 1972; Schubel, Wilson & Okubo, 1978) to major (Krone, 1978; Kranck, 1979).

This paper describes the concentration, grain size and gross composition of suspended matter in Miramichi Estuary, New Brunswick. Special atten- tion is given to the effect of flocculation and the partitioning of organic and inorganic material on particle size characteristics. The terms flocculation and flocs as used in this paper refer to the formation of aggregates of many smaller grains by collision and adhesion of material in suspension. Collisions result from Brownian motion, local shear of the suspension and different settling velocities of particles (Einstein & Krone, 1962). Adhesion due to physicochemical bonding of clay grains has long been recognized; recent work indicates that all surfaces immersed in seawater have a coating of organic matter (Neihof & Loeb, 1974; Loeb & Neihof, 1977) which forms the

108 K. Kranck

reacting surface. Irrespective of the nature of the attraction, suspended matter in seawater is inherently unstable and readily forms flocs containing both organic and inorganic matter (Kranck, 1975, 1979).

The Miramichi Estuary is a typical funnel-shaped drowned river mouth, separated from the open Gulf of St Lawrence by a system of well-developed sand barrier islands (Fig. 1). The Miramichi River con- sists of two branches; the Southwest and Northwest Miramichi Rivers which together drain 1 . 3 4 ~ lo4 kma with a mean annual runoff of 9.67 x lop ms, close to one-half of which occurs during the spring freshet.

Tides are mixed diurnal-semi-diurnal with an average range of about 1 metre at the mouth of the bay. Bousfield (1955) showed the Miramichi Estuary to be a typical partially mixed estuary where a net seaward drift, stronger on the south side of the estuary, of brackish water near the surface, and a landward counter drift, strongest on the north side, of more saline water near the bottom is super- imposed on the oscillating tidal currents. The estuary is underlain by flat-lying fairly soft sandy and shaly sediments and glacial surficial deposits cover the surrounding land. The land of the drain- age basin is largely wooded, with only minor areas cleared. Several pulp and paper plants at Newcastle and Chatham (located between stations 13 and 16, Fig. 1) emit particulate effluents into the estuary.

MET H 0 D S

The field work was carried out during a period of low runoff in September and October 1973 simultane- ously with a physical oceanographic study (Krauel, 1975). Water for particulate matter analysis was collected mainly along a central line through the estuary (Fig. 1). Each station was sampled at least twice at different stages of the tide and odd spot stations were sampled when convenient, in order to give a more random and representative distribution to the central line data. Stations 2, 9 and 22 were sampled over a 13-h tidal cycle. Normally a sample was collected from I m below the surface (surface) and from 1 m above the bottom (near-bottom) and sometimes from intermediate depths.

The water samples were collected using 5 1 Niskin bottles. An RS-5 induction salinometer and a Guild- line STD were used to measure water temperature and salinity and an OTT Arkansas current meter to measure currents. A model T Coulter Counter was used for all grain size analyses. Two types of analysis were carried out; analysis of the natural suspended particle distribution including organic particles and flocs (these are referred to as natural or floc distribu- tions or spectra), and analysis of suspended matter as single inorganic grains after oxidation of organic matter and deflocculation (called inorganic or grain). The natural distributions were analysed as soon as possible after sample collection and are believed to reflect the in situ size distributions, although some

Fig. 1. Miramichi estuary: location, bathymetry and sampling stations.

Particulate matter characteristics 109

S T A T I O N S 20 15 10 5 I

. i !

I!i . I *. . .

i. * ,TURBIDITY MAXIMUM

. .

. . : - ,

. * . . . * . . . . * . ": .. . .. . *

v . . . ..! . . . .I . * .

! I f

: 1 .

I . . . * * . . ' . I . . '

: l .

40

I 20 40 60 80

I D ISTANCE ( k m )

) * I !

20 15 10 5 S T A T 1 0 N S

Fig. 2. Particulate matter variables and salinity along length of the estuary at all depths sampled. Values of in- dividual samples shown for total natural concentration and ash loss (dots); range of values shown for natural mode and salinity (shading). High values at anchor Station 22 not shown.

deflocculation may have resulted from removing the sample from the in situ environment and subjecting it to normal sample handling. All size analyses are plotted as smoothed frequency histograms or frequency spectra (Sheldon & Parsons, 1967). Natural particle distributions are drawn as a broken line and inorganic as a solid line. Details of the analytical techniques are described and discussed in Kranck & Milligan (1979).

0 B S ER VAT I 0 N S

Concentrations

The highest total particulate matter concentrations occur in a well-developed turbidity maximum in the central portion of the estuary (Fig. 2). Upstream and downstream from the maximum, concentrations decrease to much lower values typical of the Mira-

michi River and the Gulf of St Lawrence. The double peak shown in the concentrations of the turbidity maximum may be a reflection of the spacing and timing of the stations rather than a true representa- tion of turbidity distribution.

Variations in concentration recorded at the anchor stations (Fig. 3) reflect changes in the position of the turbidity maximum and their location relative to it. At Station 2, well away from the turbidity maximum, concentrations remain constantly low as the clear oceanic water is advected into and out of the estuary entrance. At Station 9 in the turbidity maximum, surface concentrations are also almost constant, but much higher than at Station 2. The highest con- centrations occur near the bottom, associated with the flood current which persists throughout much of the tidal cycle. Station 22 was located at the upstream limit of the turbiditymaximum and salinity intrusion; concentrations in the river water are low but increase as the saline turbid water is brought in by the flood current. Concentrations decrease again at high water slack, presumably due to settling of particulate matter. Concentrations increase again with the maxi- mum ebb current but are lower than those reached during the flood.

Particle size

The pattern shown in Fig. 4 is typical of the grain size variability observed within the estuary. At the two ends of the estuary, where total concentrations were low, the grain size spectra had flat or slightly undulating irregular traces with almost the same volume concentration of particles of all sizes. These spectra are similar to the 'flat' or 'low concentration' spectra described for samples from the St Lawrence Estuary and elsewhere (Kranck, 1975, 1979).

Samples from the turbidity maximum have a broad unimodal peak or hump extending over most of the size range. Only at the fine-particle end is the distribu- tion usually flat, forming a 'tail' of fine particles. Similar spectra in other studies have been described as 'well flocculated' or ' unimodal' (Kranck, 1975, 1979) and are believed to be typical of flocculated particulate matter with high concentrations of in- organic sediment. The spectra with intermediate concentrations from above and below the turbidity maximum complete a continuous sequence, from flat-low spectra to unimodal-humped. This means that with increase in concentration progressively more of the sample occurs as larger, near modal size particles and less as fine tail-size material (Fig. 5) .

110 K. Kranck

STATION 22 STATION 9

h

3ot 301

STATION 2

----.... :I-- --.- 2oL. " ' ' " '

Fig. 3. Changes over a tidai cvcle at three anchor stations. Near-surface measurements shown with broken line. Near- bottom ( < 2 m off) shown &th solid Iine. Total depths at stations 2, 9 and 22 are 13, 10, and 3-5 m respectively. For suspended matter concentration and mode the trace with higher values refers to natural materia1 and lower values refer to deflocculated inorganic distribution.

Most of the large particles appeared to be flocs. This was confirmed by ashing and deflocculation which changed the spectra to broad poorly sorted distribu- tions with less pronounced modes (Fig. 4). The in- organic grain mode in general increased with increase in concentration (Fig. 6) .

The natural floc mode of a sample was generally larger than its inorganic mode. Only the most con- centrated samples plot close to the regression line which described samples from open marine areas of high inorganic sediment (Kranck, 1975). Less con- centrated samples deviate increasingly from the line. Freshwater and low-salinity samples have a relatively narrow range of fairly large natural floc modes with no apparent relationship to the grain modes.

In general, over a tidal cycle when the total con- centrations are high, natural and inorganic modes vary simultaneously, often in response to changes in current speed. However, in low concentrations of particulate matter the two modes vary inversely and show no clear relationship to dynamic conditions. At Station 2 natural modal sizes are slightly higher during ebb and flood than during slack tides (Fig. 3). At this station inorganic modes were analysed only in surface samples and had minimum values when the natural modes were large. In the turbidity maxi- mum (Station 9) both the natural and inorganic modes in the bottom water vary directly with the

current speed (Fig. 3). At the surface, modal sizes remain relatively constant except at high tide, when sizes increase briefly to values similar to those of the bottom water. At Station 22, however, the highest natural modes occurred simultaneously with the low- est inorganic modes (Figs 3 and 4). The latter were often below 1 um, or too poorly defined to be deter- mined. Withtheinfluxofsalty water the natural modes decrease slightly and inorganic modes increase. Both modes decrease at high water slack, when the con- centrations also decrease. At this station the samples from high water show a second mode in the inor- ganic spectra. This anomalous bimodality was ob- served only at this station and in a small number of the samples from this general vicinity.

Composition

Microscopic examination of the natural water samples using an inverted phase-contrast microscope shows the particulate matter to consist of a hetero- geneous collection of particles including detrital material, plankton cells and a number of distinctive particle types which are probably effluent from pulp and paper plants. Most particles are compound particles or flocs. Inorganic and organic grains in the flocs cannot be distinguished except for the plankton cells frequently included in the flocs. There were

Particulate matter characteristics 111

B

D I A M E T E R (,urn)

Fig. 4. Particle spectra from (A), selected stations along the length of the estuary (stations 2-14 and 14-27) were sampled consecutively during separate 5 h runs) and (B), anchor station 22. Broken line, natural distribution; solid line, inorganic grain distribution after ashing and defloc- culation.

relatively few optically discernible differences be- tween samples with different types of spectra. Slightly more planktonic material is found in the inner bay whilst larger flocs are found within the turbidity

8

maximum. The inorganic fraction of the samples consisted of detrital mineral grains and diatom tests. The fine sediment fraction in the Miramichi Estuary is composed of quartz, mica, chlorite, feldspars and minor clay minerals (D. Abbott, New Brunswick Productivity Council, personal communication).

All the samples decrease in total volume after ash- ing and deflocculation due to the oxidation of organic particles. This ash loss by volume may be used as a rough measure of the organic content of the sus- pended particulate matter (Kranck & Milligan, 1979). It varied from 26 to 87 04 and formed a com- plex pattern of change along the estuary (Fig. 2). The ash loss values are low for river samples and increase into the turbidity maximum simultaneously with total concentrations and salinity. From Station 15, in the turbidity maximum, organic content de- creased seaward but increased again in the wide part of the estuary to a second maximum at stations at the entrance and outside the estuary.

DISCUSSION Floculation dynamics

Agglomeration of particles by the feeding of organ- isms has been suggested as a major factor in produc- tion of compound particles (Schubel & Kana, 1972; Haven & Morales-Almo, 1972; Zabawa, 1978). This no doubt occurs, but the smooth trace and regular progressive change in shape of the natural spectra suggest that the predominant control of particle size in the Miramichi Estuary is physical in nature.

The increase in mode/tail ratio (Fig. 5) represents a shift of the suspended matter to larger size particles and is evidence of flocculation. Its increase with concentration probably results from increased particle collisions at higher concentrations (Einstein & Krone, 1962). The relationship between natural modal size and inorganic modal size also indicates an increase in flocculation with increase in concentra- tion, if it is assumed that the regression line from Kranck (1 975) represents fully flocculated conditions and that samples are more flocculated the closer to this line they occur (Fig. 6). According to this criterion the samples with low salinities ( < 2 %,) are least flocculated, supporting laboratory evidence that salt promotes flocculation (Gripenberg, 1934; White- house, Jeffrey & Debrecht, 1960; Van Olphen, 1977). Samples from the turbidity maximum were, however, more flocculated than the samples with higher salinity but lower suspended matter concentrations

S E D 28

112 K. Kranck

a - sloe t z t U

2 W 0 z 0 0 -.I U [r

0

: 10-

= I - $

_. . . .

I 10 I00 MODE CONC. / TAIL CONC.

Fig. 5. Mode concentration/tail concentration ratios versus total natural concentration.

CONCENTRATTION . c 2 5 0 2 5 - 5 0 5.1-10 0 10.1 - 2 0 0 >20

/

I . 15 T I .

2 5 5 10 25

N A T U R A L I F L O C ) MODE ( p m )

Fig. 6. Size of inorganic grain mode versus size of natural floc mode of samples. Total natural concentration (ppm) indicated by size of circles. Samples to the right of broken line had salinity less than 2%,. Solid line is regression line from Kranck (1 975) defining well-flocculated samples from open marine conditions.

from the Inner Bay. The relative importance of concentration and salinity in promoting flocculation is difficult to evaluate because they increase simul- taneously in the low salinity region of Miramichi Inlet (Fig. 2).

The varying relationship between inorganic grain mode and natural floc mode within the estuary may result from changes in the density of the flocs. In low concentrations of particulate matter, flocs probably consist largely of organic matter and form large particles (large modes) before settling out of sus- pension. Towards the centre of the estuary both

concentration and deflocculated inorganic grain modes increase, presumably as a result of increased turbulence and resuspension. The natural modes correspondingly decrease to compensate for in- creased density as more inorganic grains become in- corporated into the flocs. Thus initially the twomodes vary inversely, as over much of the tidal cycle at Station 22. Only when both inorganic and organic matter is fully flocculated and the density of the flocs stabilizes will the two vary simultaneously as around high water at Station 22 and at all times at Station 9 (Fig. 3).

The bimodal samples from Station 22 (Fig. 4) have a larger modal size, similar to that of the bottom sediment in the turbidity maximum where this material is resuspended. The extra mode is probably a relic of the bottom sediment, and disappears very quickly when the strong flood current slackens.

Changes in ash loss along the estuary can also be explained in terms of flocculation and mixing of river and ocean water. The river water entering the estuary has a relatively low concentration of particulate organic matter. Mixing of river and seawater causes precipitation and flocculation of dissolved organic matter and fine colloidal organic particles (Sholko- vitz, 1976). The process increases the proportion of organic matter in the measured size range, causing the initial increase in ash loss. In the turbidity maxi- mum, rapid sedimentation selectively removes the proportionally more flocculated organic matter resulting in a decrease in ash loss. With decrease of flocculation rate and increase in living material in the outer end of the estuary, the organic content again increases to the high values representative of the open Gulf of St Lawrence. A similar pattern of increase and subsequent decrease of organic matter in suspension was observed in the St Lawrence Estuary (Kranck, 1979). Some of the organic contents in the turbidity maximum are probably derived from pulp and paper effluent produced by industries in Newcastle and Chatham (Kranck, 1974; Rashid & Reinson, 1978).

Formation of the turbidity maximum

If a river enters the sea with little estuarine circula- tion generated, suspended sediment simply settles out as the current decreases. Flocculation increases the rate of settling as described for the Amazon (Milliman, Summerhayes & Barretto, 1975) but with- out a mechanism for resuspension no turbidity maxi- mum will form. Flocculation will also be relatively

Particulate matter characteristics 113

less important without any mechanism for elevating suspended-matter concentrations.

On the other hand in a normal estuarine circula- tion without flocculation, if such were possible, particles would still settle into a landward-flowing bottom current, but with reduced settling rates. The particles retained in the estuary would then be sub- ject to size sorting and consist of a limited size-range as described by Postma (1967) and Festa & Hansen (1978). Both Fig. 4 and the results of other studies (Kranck, 1975) show that flocs contain grains of all sizes from the suspension. Therefore when flocs are sedimented the concentration of all grain sizes decreases and the turbidity maximum retains all particle-sizes supplied by the river including fines, which otherwise would have been swept out to sea. A coarser, better-sorted bottom sediment would also not be as easily resuspended as are the low-density fluffy flocs which are subject to particle break-up and resuspension by the near-bottom shear forces. Thus, although flocculation is not a primary requirement for turbidity maxima formation, it increases their magnitude by increasing the settling and resuspension rates and the size range of the trapped particulate matter.

The Miramichi conformed in general to the classical model of turbidity maximum formation (Postma, 1967; Dyer, 1971). At two out of the three tidal-cycle stations the flood current carried appreci- ably more suspended particulate matter than the ebb current, but the sampling data were insufficient for any quantitative calculation of sediment budgets and extrapolation of long-term net transport. Postma (1967) and Inglis & Allen (1957) have demonstrated differences in the direction as well as quantity of residual sediment transport over a neap-spring cycle, and it is probable that even continuous sampling over as long as a month would give misleading results due to even longer cyclic changes such as storms and seasonal factors.

In the Miramichi the turbidity maximum is centred in the region of maximum density stratifica- tion rather than at the limit of salt intrusion. The sediment fluxes associated with the two-layer flow are apparently more effective in concentrating sediment than the less stratified circulation above Station 17, although the particulate matter concentra- tions here are also elevated compared to the river water.

CONCLUSION

In the Miramichi Inlet, concentrations of suspended particulate matter entering the estuary by the river are relatively low. Concentrations increase towards the centre of the estuary where a well-developed turbidity maximum occurs. Concentrations again decrease seaward to the levels typical of the open Gulf of St Lawrence. The particle-size spectra of the particulate matter change concurrently and con- tinuously with concentration. In the river they form distributions with nearly equal volume concentration in equal logarithmic size-classes. Modes are poorly developed or less than 1 wn. The proportions of fine particles decrease seaward until, in the turbidity maximum spectra, most material is concentrated at the coarse end of the distribution as a broad uni- modal hump or peak. Seaward of the turbidity maximum the spectra again flatten.

The changes in natural particle size result from increase in flocculation, as indicated both by the ratio of small particles to flocs and by the relationship between the floc and grain modal sizes. A progressive change in floc densities as a result of more inorganic sediment becoming incorporated into the initially mainly organic freshwater flocs is postulated. The levels of flocculation vary directly with concentration throughout the estuary, pointing to total particulate matter concentration as the principal factor control- ling flocculation. Salinity may be a second less important factor. Where salt and freshwater first start to mix, the relative proportion of organic matter in the suspended matter increases rapidly due to precipitation and flocculation of dissolved and fine colloidal organic substances.

Most of the suspended particles are so small that without flocculation they probably would be carried out of the estuary. Both flocculation and settling of the high concentration suspended sediment, and erosion of the resulting soft unstable bottom, are probably continuous, simultaneous processes. It is concluded that the maintenance of a turbidity maxi- mum of the magnitude that occurs in the Miramichi is dependent both on estuarine circulation and flocculation.

ACKNOWLEDGMENTS

Numerous people have made completion of this study possible and I gratefully acknowledge their help. I would especially like to thank D. Krauel and

8-2

114 K. Kranck

Captain R h o of the Metridia for co-operation in the field. J. Pritchard, F. Ewing and E. Adams pro- vided technical assistance in the field and Sheila Byers and T. Milligan performed sample and data analysis. H. Neu, J. M. Bewers and D. E. Buckley reviewed the manuscript.

REFERENCES

BOUSFIELD, E.L. (1955) Some physical features of the Miramichi Estuary. J. Fish. Res. Bd Can. 12, 342-361.

DYER, K.R. (1971) Sedimentation in estuaries. In: The Estuarine Environment (Ed. by R.S.K. Barnes and J. Green). Applied Science Publishers, London. 133 pp.

EINSTEIN, H.A. & KRONE, R.B. (1962) Experiments to determine modes of cohesive sediment transport in salt water. J . geophys. Res. 67, 1451-1461.

FESTA, F. & HANSEN, D. V. (1978) Turbidity maxima in a partially mixed estuary: a two dimensional numerical model. Estuar. Coastal Mar. Sci. 7 , 347-359.

GRIPENBERG, STINA (1934) A study of the sediments of the North Baltic and adjoining seas. Fennia, 60, 1-231.

HAVEN, D.C. & MORALES-ALMO, R. (1972) Biodeposition as a factor in sedimentation of fine suspended solids in estuaries. In: EnvironmentalFramework of CoastalP/ain Estuaries (Ed. by B.W. Nelson). Mem. geol. SOC. Am.

ZNGLIS, C.C. 8t ALLEN, F.H. (1957) The regimen of the Thames Estuary as affected by currents, salinities and river flow. Instn civ. EnRrs, Mar. Water. Div. Paper No.

133, 121-130.

38, pp. 827-878. KRANCK. K. (1974) The role of flocculation in the trans-

port of particulate pollutants in the marineenvironment. Proc. Int. Conf. on Transport of Persistent Chemicals in Aquatic Ecosystems, pp. 41-46, Ottawa, Canada.

KRANCK, K. (1 975) Sediment deposition from flocculated suspensions. Sedimentology, 22, 11 1-123.

KRANCK, K. (1979) Dynamics and distribution of sus- pended particulate matter in the St. Lawrence Estuary. Nut. Can. 106, 163-179.

KRANCK, K. & MILLIGAN, T.G. (1979) Methods of particle size analysis using a Coulter counter. Bedford Institute of Oceanography Report Series BI-R-79-7, 48 PP.

KRAUEL, D.P. (1975) The physical oceanography of the Miramichi River Estuary September-October 1973. Fish. Mar. Serv. Env. Can. Technical Report No. 571, 184 pp.

KRONE, R.B. (1978) Aggregation of suspended particles in estuaries. In : Estuarine Transport Processes (Ed. by B. Kjerfve), pp. 177-190. University of South Carolina Press. 331 pp.

LOEB, G.L. & NEIHOF, R.A. (1977) Adsorption of an organic film at the platinum-seawater interface. J . mar. Res. 35,283-291.

MEADE, R.H. (1968) Relation between suspended matter and salinity in estuaries of the Atlantic seaboard. U S A . Int. Ass. Sci. Hydrol., Gen. Ass. Bern 1967, vol. 4 (I.A.S.H. Publ. 78).

MEADE, R.H. (1972) Transport and deposition of sediments inestuaries. Mem.geo2. SOC. Am. 133,91-119.

MILLIMAN, J.N., SUMMERHAYES, C.P. & BARRETTO, H.T. (1975) Oceanography and suspended matter off the Amazon River. February-March 1973. J. sedim. Petrol. 45, 189-206.

NEIHOF. R.A. & LOEB. G.L. 11974) Dissolved organic , - . , ~~~ ~ ~ - matter in seawater and the electric charge of immersed surfaces. J. mar. Res. 32, 5-12.

POSTMA, H (1967) Sediment transport and sedimentation in the estuarine environment. In: Estuaries (Ed. by G.H. Lauff). Am. Ass. Adv. Sci. Pub/. 83,158-179.

RASHID, M.A. & REINSON, G.E. (1978) Organic matter in surficial sediments of the Miramichi Estuary, New Brunswick, Canada. Estuar. Coast. Mar. Sci. 7,23-35.

SCHUBEL, J.R. & KANA, T.W. (1972) Agglomeration of fine-grained suspended sediment in Northern Chesa- peake Bay. Powder Technol. 6, 9- 16.

SCHUBEL, J.R., WILSON, R.E. & OKUBO, A. (1978). Vertical transport of suspended sediment in Upper Chespeake Bay. In : Estuarine Transport Processes (Ed. by B. Kjerfve), pp. 161-175. University of South Carolina Press. 331 pp.

SHELDON, R.W. & PARSONS, T.R. (1967) A continuous size spectrum for particulate matter in the sea. J. Fish. Res. Bd Can. 24, 909-915.

SHOLKOVITZ, E.R. (1976) Flocculation of dissolved organic and inorganic matter during the mixing of river water and sea water. Geochim. cosmochim. Acta,

VAN OLPHEN, H. (1977) An Introduction to Clay Colloid Chemistry, 2nd edn. Wiley, New York. 318 pp.

WHITEHOUSE, V.G., JEFFREY, L.M. & DEBRECHT, J.D. (1 960) Differential settling tendencies of clay minerals in saline waters. In: Clay and Clay Minerals. Proc. 7th Nut, Conf. on Clays and Clay Minerals, Nut. Acad. Sci., Nut. Res. Coun., pp. 1-76.

ZABAWA, C.F. (1978) Microstructure of agglomerated suspended sediments in Northern Chesapeake Bay Estuary. Science, 20, 49-51.

40, 831-845.

(Manuscript received 14 August 1979; revision received 21 February 1980)