Chapter Nineteen Sediment content of the ice-cover in muddy tidal areas of the turbidity zone of the St. Lawrence estuary and the problem of the sediment budget

Muddy Coastsof the World: Processes, Deposits and Function

T. Healy, Y. Wang and J.-A. Healy (Editors) 9 2002Elsevier Science B.V. All rights reserved.

463

Chapter Nineteen

Sediment content of the ice-cover in muddy tidal areas of the turbidity zone of the St. Lawrence estuary and the problem of the sediment budget JEAN-CLAUDE DIONNE Universitd Laval Quebec, Canada

1 INTRODUCTION Canada's coastline is one of the longest in the World (McCann 1980, Trenhaile 1998, p. 268), but m u d d y tidal flat areas represent only a small percentage of this complex shoreline extending along the Atlantic, Arctic and Pacific Oceans and along the numerous adjacent seas and water bodies such as Hudson Bay, Foxe Basin and the Gulf of St. Lawrence. Wide tidal flat areas occur in the Bay of Fundy, a temperate region with ice effects during 2 to 3 months annually, in southern James Bay, Ungava Bay and in Foxe Basin, subarctic regions dominated by ice during 5 to 8 months, and in the southern region of British Columbia, a temperate region without ice. The role of ice has been little studied in most of these areas and elsewhere in Canada except in the St. Lawrence estuary, for which many papers have been published during the last three decades (Dionne 1988) and the Bay of Fundy (Knight and Dalrymple 1976). The early references to ice action in muddy tidal flats in Canada go back to the end of the last century (Tarr 1897) and the beginning of the present century (Prest 1901; Bancroft 1902). However, few important studies have been made before 1950. The areas which are presently the best known are, on the Atlantic side, the Bay of Fundy (see references in papers by Amos et al. 1991, and by Dalrymple et al. 1991); the middle St. Lawrence estuary (Dionne 1985, 1988, 1992); for

464 the subarctic, the eastern and western area of southern James Bay (Dionne 1978; Martini 1981, 1991); the southern areas of Ungava Bay with tide range 10 to 20 m (Lauriol and Gray 1980); and Frobisher Bay (McCann et al. 1981; McCann and Dale 1986), and Pangnirtung Fjord (Gilbert and Aitken 1981). These macro-tidal areas dominated by ice are mostly characterized by sandy and boulder-strewn tidal flats. On the west coast of Canada, the only tidal flat area well known and studied is the Fraser delta near Vancouver (Luternauer and Murray 1973; Luternauer 1980; Luternauer et al. 1995). However, this temperate region is not effected by shore ice. The geographic distribution of tidal flats and salt marshes in Arctic Canada is still poorly known. It is worth mentioning that most wide tidal flat areas in Canada, particularly in northern Canada and along the St. Lawrence, are levelled shore platforms cut into postglacial marine clay of sandy deposits with only a thin mud cover (a few cm to about 1 m thick). Consequently, these tidal flats differ greatly from those of the Wadden Sea, The Wash (U.K.) and those of Korea and China. Although there is little field data on the sediment content of ice covers in high latitude regions, particlarly for muddy coastal areas, it is relatively well known that in cold regions ice may contain a large volume of coarse to fine grained sediment (Dionne 1984, 1993). The purpose of this paper therefore is to address the role of shore ice in the evolution of a muddy coastline, and to discuss briefly the problem of the sediment budget of the maximum turbidity zone (MTZ) of the middle St. Lawrence estuary, an area dominated by muddy shorelines and large tidal flats, and influenced by a seasonal ice cover.

2 CHARACTERISTICS OF THE AREA

The study area is a section of the St. Lawrence estuary usually called the Middle St. Lawrence estuary, which extends about 200 km from Qu6bec City to the outlet of the Saguenay River. The upstream section of this large estuary comprises the maximum turbidity zone (Figure 1). The volume of suspended particulate matter (spm) varies greatly horizontally and vertically from place to place, depending upon the tide cycle, the weather conditions, and also upon the seasons (d'Anglejan and Smith 1973; Silverberg and Sundby 1979; d'Anglejan 1981; Lucotte and d'Anglejan 1986). However, on average, during the summer the turbidity ranges from 100 to 450 mg/1. There are no data available for winter and spring periods, particularly during breakup of the ice. The suspended matter is composed mainly of very fine sand, silt and clay with a varying percentage of organic matter. According to d'Anglejan et al. (1973, 1974), illite and chlorite are the two main constituents of the clay fraction. The maximum turbidity zone is about 65 km long and 15 km wide, and depth at low tide ranges from zero to 25 m. However, depths over 20 m represent only a small percentage of the area. Mean tide ranges from 4 to 5 m and spring tide from 5.5 to 6.5 m. In the maximum turbidity zone there are large areas covered by tidal flats. Major tidal flats occur on both sides of the channel north of Orleans Island and at

465 Cap Tourmente on the north shore, at the Montmagny archipelago, particularly on the north side of Goose Island, and at Montmagny (on the south shore). The muddy tidal flats range in width from 500 m to 3 km and extend for several kilometers. At Montmagny for example, the tidal flat area is 15 km long, while at Cap Tourmente it is approximately 8 km long. Most tidal flats are composed of a bare m u d d y flat and of a low marsh dominated by American bullrush (Scirpus americanus).

Figure 1 The St. Lawrence estuary turbidity zone (after d'Anglejan et al. 1981) and location map.

466 The ice season extends from December to April with a complete ice cover over tidal flat areas from mid-January to the end of March. In the channel north of Orleans Island, the ice extends from shore to shore, but downstream from Orleans Island the offshore zone is rarely entirely frozen over during the winter because of tide action and ship circulation. However, when strong winds are blowing from the NE, ice floes do accumulate for a few days in that section covering all the water surface. Breakup occurs at different dates from place to place from the last week of March to the second week of April. There are two main periods of erosion and sedimentation for tidal flats located in the maximum turbidity zone. Sedimentation takes place during the summer (June to September) with an average temporary mud deposition of 10 to 15 cm and locally up to 30 to 35 cm. Mud deposition also occurs under the ice cover. Where the ice carpet is not frozen to the bottom, 10 to 15 cm of fresh m u d is usually deposited during the winter months. Erosion prevails at breakup and during the following month, and also in Fall, during October and November. At this time the surface of the low marsh area is boulder-strewn, while during the two seasonal periods of sedimentation small boulders are m u d covered.

3 S E D I M E N T C O N T E N T OF THE ICE COVER

There are several processes by which sediment is incorporated into the ice cover. In the maximum turbidity zone, mud is incorporated into the ice through the following main processes: (i) By adfreezing at the base of the ice cover which is stranded on the mud flat at low tide. At the end of the winter a layer up to 35 cm thick of stratified ice and mud can be observed almost everywhere at the base of the ice cover (Figure 2). (ii) Mud is also introduced at the surface of the ice cover through the numerous tidal cracks and blow holes (Figures 3 and 4). Under pressure at high tide, turbid water under the ice cover is flushed out and spread at the surface over various distances on both sides of fissures. Almost every year large areas are covered by mud (Figure 5). At Montmagny for example, a layer of m u d 2 to 4 cm thick is commonly observed annually. (iii) Another process is through pressure ice ridges and icefoot pustules, two features characterizing the ice cover in most tidal-flat areas (Figures 6 and 7) (Dionne 1985, 1992). (iv) Mud is also commonly brought to the surface during freezeup by sedimentladen ice blocks which turn over and are subsequently incorporated into the ice cover giving a patchy aspect to the ice carpet (Figure 8). (v) The last important process involved is by freezing in situ of turbid water (Figure 9). In the past, this process has been largely ignored because the current opinion is that upon freezing m u d is released in the same way salt is concentrated and rejected in the marine environment.

467

Figure 3 (95.4.14).

M u d introduced at the surface of the ice cover through a tidal crack

468

Figure 5 A large area of the ice carpet covered by a m u d layer up to 3 cm thick (89.3.27).

469

Figure 7 Mud introduced at the surface of the ice cover through an icefoot pustule (92.4.3).

4/0

Figure 9 An ice floe composed of ice cobbles made of turbide water (71.12.4).

471 4 M E A S U R E M E N T OF THE S E D I M E N T C O N T E N T

There are two methods to measure the content of debris in an ice cover. The first method is by coring along transects and upon melting the ice collected to measure the volume (weight) of sediment in the cores. Although this method is apparently more scientific, results are not always satisfying because of the small number of cores compared to the size of the area investigated. In addition, this method is inappropriate for measurement of the coarse debris content. The second method is by field surveys, particularly at ice breakup. At this time, hundreds of ice floes are stranded at low tide and can be examined in detail (i.e. from each side, the surface and often the base when ice floes are tilted or turned over). The load can also be evaluated from the margin of the ice cover and from the many w i n d o w s throughout the ice carpet. Direct observations and measurements are thus the best method to estimate adequately the sediment content of the ice cover assuming the observations are representative of the entire ice cover. When one examines the ice cover during the winter, it may happen that one sees very little m u d at the surface because of the snow cover. However, if surveys are undertaken at the end of the winter, particularly at breakup it is observed that in m u d d y coastal areas shore ice plays an important role in the evolution of this type of shoreline.

5 ESTIMATE OF THE MUD C O N T E N T Based on many years of field surveys at several localities, it has been evaluated that, on average, m u d in the ice cover, particularly at Montmagny, is equivalent to a layer 10 cm thick (Dionne 1981b, 1984). As m u d d y tidal-flat areas in the m a x i m u m turbidity zone cover an area approximately 50 km 2, the volume or weight of the sediment in the ice should amount to 6 to 8 million metric tons annually. This volume does not include the load of ice blocks in the offshore zone which are made of turbid water, and which drift downstream throughout the entire winter season.

6 ICE R A F T I N G

What happens to the sediment trapped into the ice cover of tidal flats? At breakup most floes are rafted offshore in a few days. It is estimated that upon melting in situ about 10% of the m u d in the ice cover returns to the tidal flat areas, whereas approximately 35 to 40% is released offshore in the turbidity zone before ice floes escape the area and drift toward the Lower St. Lawrence Estuary and Gulf. Consequently, about 50% of the volume of m u d in the ice cover does escape the m a x i m u m turbidity zone annually. During the ice-free season the suspended particulate matter is kept within the turbidity zone because of hydrodynamic processes related to the tide cycle; only a small volume of m u d is considered to

472 escape the turbidity zone during the ice-free period. Measurements made by d'Anglejan et al. (1973) suggest that less than one million metric tons a year of suspended particulate matter are exported annually. This estimate does not take into account ice drifting.

7 THE S E D I M E N T B U D G E T P R O B L E M

The sediment budget of the maximum turbidity zone of the middle St. Lawrence estuary is based on the annual input and output of suspended material. Unfortunately there are no data available for the bedload. There is much controversy on the input at Quebec City, i.e. in the middle estuary (Table 1). The most conservative estimate is 3.6 to 4 x 106 metric tons per year, while the most optimistic estimate is up to 20-25 x 106 t/y. However, it should be noted that the 3.6 to 4 x 106 t / y estimates for the input at Quebec City mentioned by many authors (Holeman 1968; d'Anglejan et al. 1973; Loring and Nota 1973; Milliman and Meade 1983; d'Anglejan 1990) are inadequate. Quite strangely this value is based on one year's measurement made 50 years ago, at Montreal, and first referred to by Corbel (1959) in a study dealing with dissolution of carbonate bedrock. This estimate has been used without any critical assessment. The most recent estimates range from 6 to 12 x 106 t/y. In a recent report, Frenette et al. (1989 p. 94) stated that on average the input of suspended particulate matter at Quebec City is 6.5 x 106 t/y, with a range from 4 to 12 depending upon hydrological conditions. This value is about twice the estimate mentioned again by d'Anglejan in a recent review paper (1990). If we accept for the input at Quebec City the estimate of 6.5 x 106 t / y as an average, and if we also accept the estimate made by d'Anglejan and others (1973) and d'Anglejan (1990) of about one million tons per year for the output during the icefree period, the balance is 5.5 x 106 t / y of suspended particulate matter available for deposition in the turbidity zone. In these conditions an abundant sedimentation should take place annually, both on the bottom and in tidal-flat areas. Curiously, according to d'Anglejan and Brisebois (1978), very little deposition has been observed on the bottom in the upper section of the middle St. Lawrence estuary during the last centuries. In this case the tidal flat areas should receive mud and increase vertically and laterally, because data do not indicate that the turbidity has increased significantly during the last decades. On the other hand, measurements at a few localities have shown that in the long term most tidal flats are not aggrading (Allard 1981; d'Anglejan et al. 1981; S~rodes and Dub~ 1983; S~rodes and Troude 1984; Dionne 1985; Drapeau 1990; Troude and S~rodes 1990). On the contrary, some are apparently in an equilibrium state, while most are eroding at varying rates, but commonly 100 cm per year for lateral erosion of the high marsh (Dionne 1986). There is, thus, a problem of sediment budget. What happens to the 5.5 x 106 tons of suspended sediment load in excess every year in the turbidity zone?

473 Table I Estimates of suspended particulate matter (SPM) entering the maximum turbidity of the St. Lawrence estuary at Qu6bec City, Canada

Millions of metric tons by year 1. 1,1 2. 3.

.

5.

.

7. 8.

,

10. 11. 12. 13. 14. 15.

3,0 3,6

3,9 4,0

4,2 4,5 5,0

5alO 6,5 (4 a 12) 8~1o lO 11 20 20 ~ 25

References Hranck (1979, p. 164) Meade (1996) Lisitzin (1972, p. 37) Silverberg and Sundby (1979, p. 946) d'Anglejan (1990, p. 118) Loring and Nota (1973, p. 20) Corbel (1959, p. 6) Holeman (1968, p. 739) d'Anglejan, Smith and Brisebois (1973, p. 1395) Milliman and Meade (1983, p. 8) Lucotte and d'Anglejan (1986, p. 86) Milliman and Syvitski (1992, p. 532) Frenette, Sasseville and S~rodes (1974, p. 52) Pelletier and Long (1990, p. 615) Frenette and Larinier (1973, p. 114) CENTREAU (1975, p. 22) Sunby (1974, p. 1530) Soucy et al. (1976) Troude and S4rodes (1985, p. 106) Frenette, Barbeau and Verrette (1989, p. 94) S4rodes (1980, p. 23) Dionne (1981b, p. 281; 1984, p. 160) Cattaliotti-Valdina and Long (1982, p. 173) Frenette, Sasseville and S6rodes (1974, p. 53) Frenette and Verrette (1976, p. 22) Cremer (1979, p. 32)

474 There appear to be three explanations. One is that the estimate for the output in the ice-free period is underestimated. The second is that, in the maximum turbidity zone, the turbidity increases significantly annually. Clearly, there is a need for adequate measurements to validate these two hypotheses. The third explanation is that a large volume of the suspended sediment load is exported outside the turbidity zone through ice rafting. If we consider the volume of mud trapped within the ice cover and in ice floes which drift during the entire ice season, ice rafting should account for 4 x 106 t / y and probably more. Such an explanation accounts for the small amount of permanent mud deposition on the bottom and in tidal-flat areas in the maximum turbidity zone of the middle St. Lawrence estuary.

8 CONSEQUENCES OF ICE RAFTING

What are the consequences of ice rafting on the evolution of the middle St. Lawrence estuary shoreline? The main consequence is that only a few tidal flats are accreting, and at a very slow rate at present, while others are in a fragile equilibrium, and many others are eroding vertically and laterally. In the absence of ice rafting, the turbidity maximum in the upper section of the middle estuary should be much higher than it is at present, and one would expect sedimentation to take place on the estuary bottom and in the tidal flat areas. Such vertical and lateral accretion of tidal flats should reduce the width of the estuary and consequently extend the emerged land seaward because, according to many authors (Dohler and Ku 1970; Vanicek and Hamilton 1972; Dunbar and Garland 1975; Pirazzoli 1986; Carrera and Vanicek 1988; Emery and Aubrey 1991), the St. Lawrence shorelines are still emerging. Just as along the Wadden Sea (Germany and The Netherlands), recently emerged coastal lowlands offer a good potential for agriculture and other purposes giving economic value to mud deposits. Presently this is not the case in Qu6bec because of ice processes.

9 CONCLUSION

In conclusion, although m u d d y shoreline processes in cold regions are poorly known, they are clearly significantly influenced by ice action. It is thus important and necessary to consider this factor in future studies for a better understanding of the evolution trend of this type of shoreline.

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