Hydrodynamics and sedimentation in salt marshes: examples from a macrotidal marsh, Bay of Fundy

Geomorphology 48 (2002) 209 – 231 www.elsevier.com/locate/geomorph

Hydrodynamics and sedimentation in salt marshes: examples from a macrotidal marsh, Bay of Fundy Robin G.D. Davidson-Arnott a,*, Danika van Proosdij a,1, Jeff Ollerhead b, Laura Schostak a b

a Department of Geography, University of Guelph, Guelph, Ontario, Canada N1G 2W1 Department of Geography, Mount Allison University, 144 Main St., Sackville, New Brunswick, Canada E4L 1A7

Received 1 May 1998; received in revised form 16 October 2000; accepted 24 January 2002

Abstract This paper presents an overview of sedimentary processes operating at the time scale of the individual tidal cycle in tidal creeks and on the marsh surface, with examples of data from an ongoing study of a high-macrotidal salt marsh in the Cumberland Basin, Bay of Fundy, Canada. Flow and suspended sediment concentrations were measured with electromagnetic current meters and OBSk probes at stations set-up in the tidal creek system and on the marsh surface. Measured flow velocities in the tidal creek channels and over the marsh surface are generally low ( < 0.2 m s 1), reflecting the relatively short, straight creek geometry and absence of levees along the creek channels. At spring high tide, flow vectors are initially controlled by creek geometry and the topography of the marsh surface, but near high tide, when water depth over the marsh exceeds 1 m, flow vectors are controlled more by the general circulation in the Cumberland Basin and by the effects of winds and waves. Suspended sediment concentrations are in the order of 150 – 250 mg l 1 and are similar in the tidal creeks and over the marsh surface. Suspended sediment concentrations generally decrease over the tidal cycle but show little fluctuation in response to changing hydrodynamic conditions. Deposition on the marsh surface is highest in the low marsh area, except near the marsh cliff margin, and shows a significant positive correlation with distance from the marsh edge. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Hydrodynamic; Bay of Fundy; Salt marsh

1. Introduction Coastal salt marshes are a type of wetland ecosystem that border saline water bodies and are vegetated * Corresponding author. Tel.: +1-519-824-4120; fax: +1-519837-2940. E-mail address: [email protected] (R.G.D. Davidson-Arnott). 1 Present address: Department of Geography, Saint Mary’s University, 923 Robie St., Halifax, Nova Scotia, Canada B3H 3C3.

by herbs, grasses or low shrubs (Mitsch and Gosselink, 1986; Adam, 1990). Salt marshes develop in a wide range of geomorphic environments. They form in the upper intertidal zone in mid and high latitudes and are found in areas that are sheltered from high wave action, thus permitting both the deposition of fine sediments and the establishment of vegetation. Areas of salt marsh development include back barrier lagoons and bays, river mouths, estuaries and deltas, natural embayments and sheltered areas behind

0169-555X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 5 5 5 X ( 0 2 ) 0 0 1 8 2 - 4

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islands and reefs (Jacobson and Jacobson, 1987; Dijkema, 1984; Kelley et al., 1995; Allen and Pye, 1992; Luternauer et al., 1995). They may also develop on open coasts where wave energy is dissipated over a wide, shallow nearshore. Salt marshes are found in latitudes ranging from the Arctic to the subtropics, where mangroves become dominant, and consequently, they develop under a wide range of climatic conditions. By occupying zones of transition between terrestrial and marine ecosystems, marshes play a critical role in sediment exchange with adjacent mud flats and open coastal waters. Salt marshes act as sinks for fine sediments that are brought in both by the tidal waters and by runoff from the adjacent uplands and for the accumulation of organic matter (Gordon et al., 1985a). They also act as a sink for contaminants. Salt marshes are generally areas of high primary productivity. The export of organic matter from the marsh is an important component of the food chain of the adjacent coastal waters and mud flats, ultimately supporting large populations of finfish and shellfish. Salt marshes also provide staging and wintering habitats for a wide variety of shorebirds and waterfowl. Recognition of this ecological significance, and the potential threats to marshes by human activities, has stimulated considerable research over the past few decades. Much of this work has been directed at improving our understanding of the functions of salt marshes and our ability to evaluate the potential impact of human activities on marshes (Mitsch and Gosselink, 1986; Williams, 1990; Allen and Pye, 1992; Reed et al., 1997; Allen, 2000). Although considerable advances have been made in the modeling of salt marshes (Allen, 1990, 1997; French, 1993; Woolnough et al., 1995; French et al., 1995), the relative speed and frequency of geomorphological and sedimentological variability remains to be assessed at a variety of time scales. The ultimate goal is to incorporate this variability into the models (Luternauer et al., 1995). The purpose of this paper is to review briefly the controls on the morphology and dynamics of salt marshes and then to describe the results of a study initiated in 1996 of sedimentary processes operating in tidal creeks and on the marsh surface of a high-macrotidal salt marsh in the Cumberland Basin, Bay of Fundy, Canada (van Proosdij et al., 1999, 2000; Schostak et al., 2000). Salt marsh tidal

creeks facilitate sediment transport in the marsh by acting as conduits between the marsh and its adjacent body of water. Once the water level exceeds the banks of the marsh, a number of physical processes occur as the flow becomes distributed over a much larger surface area and encounters the vegetated marsh surface. Previous studies have suggested that the marsh margin acts as a topographic threshold, modifying hydrodynamic and depositional regimes (Healey et al., 1981; French and Stoddart, 1992; Woolnough et al., 1995). In order to examine these processes, flow and sediment concentrations in the tidal creek channels and over the marsh surface and deposition on the marsh surface were measured in sequential experiments over the spring and summer of 1997 (Schostak et al., 2000; van Proosdij et al., 2000). The focus of these measurements was on processes operating at the time scale of the individual tidal cycle, and examples presented here are drawn primarily from two daytime spring tides when the maximum water depth over the lower marsh surface was about 1.5 m.

2. Salt marsh morphology and dynamics Morphologically, salt marshes consist of a gently sloping vegetated platform, dissected by a network of tidal creeks that increase in width and depth seaward (Orme, 1990; Pethick, 1992). Salt marshes tend to be areas of net sediment accumulation and they grow both vertically and horizontally. Deposition in salt marshes may range from dominantly organogenic, resulting primarily from belowground organic accumulation, which produces thick beds of peat, to dominantly minerogenic from the accumulation of fine sediments deposited on the marsh surface (Allen, 2000). Under a stable sea level, vertical growth of salt marshes is limited by tidal range. The rate of vertical growth of salt marshes, and of mineralogic salt marshes in particular, has been shown to be asymptotic, with rapid growth in the early phase and very low growth rates once a mature marsh surface is established near or above the mean high tide level (Pethick, 1981; Allen, 1990; French and Spencer, 1993; Jennings et al., 1993). Under rising sea level, however, thick deposits can accumulate as long as the

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vertical growth can keep pace with the rate of sealevel rise (Allen, 1990, 1997; French, 1993; Jennings et al., 1995). The characteristics of salt marshes are determined by a wide range of physical and biological controls and processes (Fig. 1), including climate, shoreline configuration and wave climate, tidal range, sediment sources and volume of sediment input, sea level history and vegetation characteristics and dynamics

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(Chapman, 1960; Frey and Basan, 1985; Jacobson and Jacobson, 1987; Adam, 1990; Allen and Pye, 1992; Luternauer et al., 1995; Chmura et al., 1997; Allen, 2000). Climatic factors in middle and high latitudes control the occurrence and distribution of the species of vegetation that colonize the marsh (Beeftink, 1977; Jacobson and Jacobson, 1987; Chmura et al., 1997) and influence physical processes such as marsh hydrology and salinity, wave climate

Fig. 1. Schematic diagram of factors controlling salt marsh sedimentation at three time scales.

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and wave-induced sediment transport and the occurrence of ice and factors associated with below freezing temperatures (Gordon and Desplanque, 1983; Dione, 1989). The shoreline configuration influences the local wave climate, the slope and nature of the intertidal substrate and the circulation pattern of tidal currents. Salt marsh development is commonly pictured as beginning with the colonization of intertidal sand or mud flats by vascular plants that are both halophytic and tolerant of repeated submergence for periods of up to several hours. The establishment of plants then encourages the deposition of fine sediments and the accumulation of organic matter leading to the vertical growth of the developing marsh surface and the integration of the tidal creek network into the salt marsh tidal creek network (Ganong, 1903; Redfield, 1972). On macrotidal coasts, such as those in the Bay of Fundy, the creek system consists of generally straight, steep channels that are perpendicular to the shoreline, whereas microtidal and mesotidal marshes have more complex creek patterns (Frey and Basan, 1985; Luternauer et al., 1995; Steel and Pye, 1997). Salt marsh vegetation is usually divided into a low marsh community, which is often dominated by a single species such as Spartina alterniflora, and a more diverse high marsh community, which grades into upland plant communities (Thanneiser, 1984; Gordon et al., 1985a). Vegetation growth occurs down to about the mean tide level (Frey and Basan, 1985; Allen and Pye, 1992). Plants in the low marsh are subject to inundation by almost every tide and for durations up to six or more hours. High marsh plants may only be submerged for brief periods during spring tides or at the upper end only a few times a year during extreme astronomical tides and/or storm surges. Tidal range and the tidal regime (semidiurnal, mixed or diurnal) influence the hydrodynamics of flow in tidal creeks and over the marsh surface, as well as the extent and duration of inundation (Healey et al., 1981; Adam, 1990; French and Stoddart, 1992; Allen, 2000). In turn, all of these influence the vertical and horizontal extent over which salt marsh development takes place. On the basis of spring tidal range, salt marshes can be divided into microtidal ( < 2 m), mesotidal (2 – 4 m) and macrotidal (>4 m) (Davies, 1980). Areas of extreme tidal range (>10 m) such as

the Severn Estuary and the upper Bay of Fundy are sometimes distinguished as hypertidal (Allen, 2000). Alternatively, macrotidal salt marshes can be subdivided into low macrotidal (4 –6 m) and high macrotidal (>6 m). As the tidal range increases, the relative significance of water level fluctuations due to storm surge tends to diminish, particularly for the low and middle marsh areas, and the depth of water over the vegetated surface at high tide increases. The nature of sediment sources and the volume of sediment supply will influence the characteristics of the marsh substrate, sedimentation patterns and the potential rate of sediment accumulation, vertical accretion and vegetation development (Cahoon et al., 1996). In estuaries and deltas, sediments may be derived primarily from fluvial inputs, whereas in embayments and in barrier/lagoon systems, they may be derived largely from shoreline erosion and longshore sediment transport by waves (Reed and Cahoon, 1992; Roman et al., 1997). Sediment-laden waters may reach the marsh directly in the case of marshes located on open coasts or embayments or indirectly through estuaries and inlets. In areas where the supply of mineral sediment is low, organic accumulation both belowground and aboveground assumes much greater significance (Heminga et al., 1996). Conversely, in areas such as the Bay of Fundy, large volumes of sediment are supplied through coastal erosion of fine-grained sediments or rocks (Amos and Tee, 1989). The possibility of an increase in global mean sea level as a result of global warming (Gornitz, 1995) has stimulated considerable interest in the controls on the rate of vertical growth of salt marshes and whether vertical growth can keep pace with the predicted rate of sea-level rise (DeLaune et al., 1978, 1983; Orson et al., 1985; Stevenson et al., 1985b, 1986; Reed, 1990, 1995; Allen, 1991; French, 1993; Woolnough et al., 1995; Cahoon et al., 1996; Roman et al., 1997). Attention has been focused largely on the ability of salt marshes to trap sufficient sediment and/or to produce sufficient belowground organic matter so that vertical growth can keep pace with sea-level rise. This has led to more studies of the processes by which vertical accretion on marshes occurs. It has also stimulated interest in the sediment dynamics of salt marshes and in the short- and long-term controls on sediment budget. Empirical studies suggest that ver-

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tical accretion in most salt marshes is generally greater than the forecast rates of sea-level rise under various global warming scenarios (Stevenson et al., 1986; Bricker-Urso et al., 1989; Oenema and DeLaune, 1988; Patrick and DeLaune, 1990; Cahoon et al., 1996; Callaway et al., 1996a,b). Such studies, however, tend to concentrate on the ability of the salt marsh to maintain its location. Often, little attention is paid to the fact that salt marshes may also respond to sea-level rise through horizontal translation, and that new areas of salt marsh may develop through the submergence of river mouths and adjacent low-lying areas, provided that the substrate and broader wave climate regime continue to be favorable to marsh development. This response is of particular importance in developing strategies for coastal management and ‘‘managed retreat’’ (e.g. French, 1999). The response of marshes in areas such as James Bay, Canada and parts of Scandinavia to ongoing sea-level fall also seems to have received much less attention. Studies of the sedimentary dynamics of coastal salt marshes using a sediment budget approach can also be seen as part of a broader effort to understand the complex interaction between the biotic and abiotic components of salt marsh ecosystems and the patterns of energy flow, nutrient and sediment cycling within the marsh, and between the marsh and coastal waters (Mitsch and Gosselink, 1986; Adam, 1990). Several different approaches to the study of the sedimentary dynamics of coastal salt marshes can be recognized in recent studies including: (1) measurements of hydrodynamics, sediment dynamics (erosion, transport and deposition) and sediment budgets associated with individual tidal cycles and/or the biweekly spring– neap cycle in tidal creeks (Boon, 1975; Bayliss-Smith et al., 1979; Ward, 1979, 1981; Pethick, 1980; Roman, 1984; Reed et al., 1985; Green et al., 1986; Reed, 1987; French and Stoddart, 1992; Leonard et al., 1995a,b; Shi et al., 1995a) and over the marsh surface (Letsch and Frey, 1980; Stumpf, 1983; Reed et al., 1985; Stevenson et al., 1985a, 1988; Wolaver et al., 1988; Stoddart et al., 1989; French and Spencer, 1993; Wang et al., 1993; French et al., 1995; Leonard et al., 1995a; Christiansen et al., 2000); (2) measurements of sediment accumulation, rates of vertical growth and changes in the areal extent of marshes and tidal creek networks over periods of 10 1 – 103 years using a variety of techniques both for dating

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sediments and for measuring the rate of growth (Jacobson, 1988; Williams and Hamilton, 1995; Pringle, 1995; Callaway et al., 1997; Brown, 1998; Orson et al., 1998) and (3) development of simulation models to explore one or more aspects of the controls on salt marsh evolution at a variety of time scales (Allen, 1990; French, 1993; Woolnough et al., 1995; Callaway et al., 1996b). The research presented here primarily follows the approaches used by studies within category 1, and to a lesser extent, those within the longer time frame of studies in category 2. Measurements at these scales are designed to increase our understanding of the controls on sedimentation and the interaction between the physical and biological controls on salt marsh development and they can serve as input to, or as a test of, simulation models such as those of Woolnough et al. (1995) and Callaway et al. (1996b).

3. Study area The Bay of Fundy is located on the East Coast of Canada between New Brunswick and Nova Scotia and forms the northeast extension of the Gulf of Maine (Fig. 2a). Amplification of the tidal oscillation of about 4 m near the entrance to the bay results in some of the largest tides in the world, with tidal range averaging about 11 m in much of the upper reaches of the bay and exceeding 16 m in places during spring high tides. The Bay of Fundy has been macrotidal for the past 6000 years, and the tidal range continues to increase with increasing sea level (Scott and Greenberg, 1987). The tides are semidiurnal with only relatively small differences between the two tides, though the absolute difference can be nearly 1 m at spring tide. Strong tidal currents are generated in many parts of the bay, most notably in the Minas passage and the entrance to Chignecto Bay. The bedrock of much of the bay consists of Paleozoic sandstones, siltstones and shales, and wave action results in considerable cliff erosion in the outer and central portions (Amos, 1987; Amos et al., 1991). While the lower portions of the Bay of Fundy are characterized by the presence of sandy material, considerable accumulations of fine sediments occur in the upper reaches, notably in Cobequid Bay and other

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Fig. 2. (a) Map of Bay of Fundy and regional setting. (b) Map of Chignecto Bay and Cumberland Basin showing location of major marshes and mud flats.

parts of the Minas Basin, and in Shepody Bay and the Cumberland Basin at the upper end of Chignecto Bay (Fig. 2b). Extensive marshes have developed in these areas, with the total marsh area estimated to be in the order of 395 km2 prior to European settlement. Over the past four centuries, much of this has been dyked and drained, leaving about 65 km2 of natural tidally flooded marsh today (Gordon and Cranford, 1994). Estuarine studies (Amos and Tee, 1989) suggest that reclamation for development and cultivation has reduced the marsh area available for deposition by approximately 70%.

The study reported here was carried out on the Allen Creek marsh, which is located on the northwest shore of the Cumberland Basin in New Brunswick (Fig. 2b). Tidal range in the Cumberland Basin at spring high tide is 12 –14 m. It has a tidal prism of about 1 km3, and about two-thirds of it dries during spring low tide (Gordon et al., 1985a). The extent, depth and duration of flooding over the marsh surface vary over the spring– neap cycle as a result of both astronomical variations over the year and effects of wave action and wind stress during storm events. At neap high tides, water may flood only the tidal creek

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Fig. 3. (a) Oblique aerial view of Allen Creek marsh taken in July 1996. (b) Topographic map of Allen Creek marsh showing creek system, location of boardwalks, instrumentation stations and sediment traps. Coordinates are in meters for UTM zone 20 and elevations are in meters relative to NAD83 datum.

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channels and the margins of the low marsh surface. At normal spring high tides, the water depth over the marsh surface is greater than 1 – 2 m. During storms, waves greater than 1.0 m are generated by winds

blowing over fetches exceeding 10 km, and these can lead to significant erosion of the exposed marsh margin along the sides of the lower bay. There is evidence of a cycle of marsh erosion and accretion

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Fig. 4. Photographs of the marsh and tidal creek system. (a) Landward view of Middle Creek at the confluence with Main Creek. Station C1 (not shown) was established in the foreground and the bridge built over the channel at station C2 can be seen at the middle right. Note the muddy nature of the channel banks and the small, sinuous notch at the bottom of the channel that carries drainage during the low tide period. (b) View looking northeast across the low marsh surface with two instrumentation stations visible on either side of a small headwater channel. West Creek is in the middle of the photograph and the instrumentation platform is in the background. Note that the instrumentation stations in the photograph are deployed in different locations than for the experiments reported here. (c) Offshore view across the marsh surface during a spring flood tide. The main creek channels are inundated and the tide is rising as sheet flow across the middle marsh surface. At high tide, the bridge visible to the left is completely submerged.

over a period of several decades for marshes in the basin (Scott, 2000). The salt marshes in the Cumberland Basin increase their elevation primarily as a result of the deposition of sediment, with only moderate contributions from belowground organic production. Sediment accumulation is enhanced by the characteristically high suspended sediment concentration consisting of 95% coarse silt, with small amounts of clay (2.5%) and sand (1.5%) (van Proosdij et al., 1999). Preliminary data suggest that the marsh surface around the MHW line at Allen Creek marsh is accumulating sediment at an average rate of 2.2 cm year 1 (van Proosdij et al., 1999). Salt marshes in the basin vary considerably in their width and exposure (Gordon et al., 1985a; Dawson et al., 1999). The salt marshes are found in the upper part of the intertidal zone around the level of mean high water and do not extend appreciably below the high

water level of neap tides (Gordon et al., 1985a). At Allen Creek Marsh, this occurs between 4.8 and 6.2 m above geodetic datum (NAD83). The salt marshes are bordered offshore by extensive mud flats and ultimately graded into sand and gravel near the low water level. Allen Creek marsh forms part of a stretch of marsh several kilometers in length that is undyked and relatively undisturbed. The section of marsh studied is about 200 m wide and is dissected by a series of small tidal creek networks (Fig. 3a,b). The majority of the marsh surface consists of gently sloping low marsh with a margin cliff 1 –2 m in height at the marsh –mud flat transition. Allen Creek, which forms the western boundary of the section of marsh studied, is a small perennial river that is tidal for a distance of about 1 km upstream, though flows are restricted through a culvert underneath the road visible in the middle-left in Fig. 3a. The river provides very little in

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the way of fresh water and sediment inputs, except for a short period during the spring. Landward, the marsh grades abruptly into woodland along a steep slope and to the east, the marsh continues to an outcrop of bedrock, though part of the eastern end has been dyked. Within the primary study area, the tidal creek network consists of three channel systems that are oriented roughly perpendicular to the shoreline and drain into a creek which is oriented nearly parallel to the shoreline. These are termed informally East, Middle, West and Main Creeks (Fig. 3a). Main Creek empties into Allen Creek just at the point where it reaches the tidal mud flats (Fig. 3a,b). The southwest (offshore) side of the Main Creek consists of a narrow strip of marsh surface that is lower in elevation than the main marsh surface, and consequently, it is over-

topped during the rising tide before the rest of the low marsh. This strip will be referred to as the marsh arm. The channel cross-sections at C5 in Main Creek, and C1 and C2 in Middle Creek are incised in the order of 2 –3 m below the marsh surface and are 10 –20 m wide with steep unvegetated channel sides (Figs. 4a and 5b). The channel narrows considerably and steepens between C2 and C3, and essentially becomes a slight depression on the marsh surface at station C4 (Fig. 5a,b). The marsh can be divided into the traditional low and high marsh zones on the basis of vegetation characteristics and contains a plant community referred to as a Bay of Fundy-type marsh (Chapman, 1960). The low marsh zone is flooded frequently and is composed almost entirely of S. alterniflora, with Salicornia sp. in the subcanopy. The high marsh zone

Fig. 5. (a) Long profile of Middle Creek and Main Creek below the confluence with Middle Creek showing location of the instrumentation stations. (b) Channel cross-sections at the instrument locations, including the reference station C5 on Main Creek.

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is flooded infrequently and is dominated by Spartina patens but exhibits a much higher species diversity than the low marsh. The subordinate species in the high marsh include Pucinellia maritima, Plantago maritima, Triglochin maritima, Solidego sempervirens, Limonium nashii, Juncus gerardi, Hierochloe odorata, Glaux maritima, Sueda maritima, Elymus arenarius, Distichilis spicata, and Atriplex sp. (van Proosdij et al., 1999). The sharp topographic division of terrace separating the two zones, which has been identified elsewhere in the Bay of Fundy and in New England (Redfield, 1972; Chmura et al., 1997), does not occur at this location. Vegetation begins to grow in late April or early May and dies off by the end of November. After this time, most of the vegetation is sheared off in the low marsh region and exported into the estuary (Gordon and Cranford, 1994). However, some standing dead vegetation will remain in the mid and high marsh regions, depending on the extent of snow and ice coverage. In northern marshes, thick, shorefast ice develops primarily over the low marsh region (Gordon and Desplanque, 1983; Dione, 1989), and a ridge of ice blocks and slush may develop in the zone between neap and spring tides. Rafted ice floes can be stranded over the whole marsh surface and will remain there over much of the winter (Ollerhead et al., 1999). During the spring, the stranded floes melt and deposit appreciable amounts of coarse and fine sediment, creating patches of bare, fine deposits. This appears to be an important contributor of sediment to the marsh system, particularly into the high marsh (Gordon and Desplanque, 1983; Wood et al., 1989; Ollerhead et al., 1999). Vegetation clasts, composites of sediment, plant and root matter ripped up from the top of the margin cliff by wave and or ice action, are found scattered over both the low and high marsh, particularly near the mean high tide line.

4. Methodology A plank boardwalk 0.25-m wide raised 0.5 m above the surface of the marsh was used to provide access to various parts of the marsh surface, tidal creek channels and the seaward edge of the marsh (Figs. 3b and 4c). The total length of the boardwalk was about 300 m, and it crossed the upper portion of

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two of the creek channels. A bridge 9-m long was used to span Middle Creek at one location and to provide a site for sampling of flow (Figs. 3b and 4a). A 3.5  3.5-m platform raised 3.5 m above the marsh surface was constructed between two of the tidal creek channels (Figs. 3b and 4b,c). A local triangulation network of control points was established on the marsh surface in 1996 using a theodolite and EDM unit, and elevation tied to an NB Department of Highways benchmark. Surveys of the marsh surface, tidal creek thalwegs and crossprofiles were made using these instruments or an automatic level. A detailed survey of the marsh surface and marsh margin was carried out at the beginning of December 1997 using a Geotracer 2000 differential GPS system with the base station established over the benchmark. Operating in this mode, the system has a positional accuracy of 0.01 m and an elevation accuracy of 0.02 m. All control points from the triangulation network were tied to the GPS survey and the combined elevation and location data brought into Surfer for Windowsk in order to construct a topographic map. Measurements of flow and sediment concentration in the tidal creek channels and over the marsh surface were carried out separately in a series of experiments in June and July 1997. Flow velocity was measured using four Marsh McBirney Model 512 bidirectional electromagnetic current meters. The current meters were co-located with four OBSk optical backscatterance probes used to measure suspended sediment concentrations. The probes were mounted at the same elevation on ‘H’ frames and separated horizontally by 0.4 m. The instruments were usually located about 0.15 m above the bed and on the marsh surface. Vegetation height on the marsh surface during the experiments was generally < 0.15 m. A permanent reference station (C5) consisting of a Marsh McBirney 555 bidirectional electromagnetic current meter with 0.1-m diameter head, an OBS probe and a Shaevitz pressure transducer, was established in the main tidal creek channel (Fig. 3b) throughout the series of experiments. Tidal stage was also monitored at C2 in both creek and marsh experiments using a Shaevitz pressure transducer. A small instrumentation hut, occupying about one-half of the platform surface, was used to house the computer data logger, power source and instrument electronics (Figs. 3b and 4a).

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Sampling runs were made over individual tidal cycles during the period when the tidal creeks and marsh surface were inundated based on visual observation from the instrument platform. The instruments were sampled at 2 Hz for 8.5 min during each run. Sampling runs were separated by about 10 min resulting in about 10 to 12 runs over the period during which the marsh and creeks were flooded. Point measurements were made with a handheld, single-axis Marsh McBirney Model 2000 electromagnetic current meter mounted on a 4-m wading rod. Measurements were made in the creeks at bridge locations and over the marsh surface from the boardwalk. Data recorded were 30-s averages of 0.2-Hz readings. Water samples were obtained at point locations during most runs using 0.5-l plastic bottles fixed to the end of a 3-m pole and dipped into the water. Suspended sediment concentrations were determined from these by filtration using a 8-Am Magna nylon 47mm filter. These were used as indicators of average suspended sediment concentration in the water column and for checking the calibration of the OBS probes. Sedimentation on the marsh surface was measured on a number of occasions during the field season, together with measurements of potentially important biological, physical and geomorphic parameters such as vegetation height, density, substrate shear strength, water temperature, depth and salinity. Deposition of sediment on the marsh surface over an individual tidal cycle was measured using sediment traps based on the design of Reed (1989). Preweighed glassfibre filter papers were placed on petri dish lids with slots cut in them to allow drainage and held in place by plasticcoated wire pins. At each sampling location, a thin layer of the surface sediment was scraped off and replaced by a layer of fine gravel to allow drainage and to permit the trap to be mounted flush with the surface. Traps were usually deployed in pairs at each location to assess sampling variability at each location and to allow for the occasional loss of a trap. At the end of the inundation period, the filters were removed, placed in another petri dish and transported to the

laboratory for analysis. The filters were washed with deionized distilled water to remove salts, dried at 35 jC and reweighed to determine the total mass of sediment deposited. During the daytime spring tide on June 24, a total of 112 traps were deployed at 56 locations along three profile lines across the marsh surface and in a concentrated 40  80-m grid at 10-m intervals in the low marsh region (Fig. 3b). A total of 103 traps were recovered. Data from two tides (June 8 and June 24, 1997) will be presented as a representative sample of sediment dynamics within the Allen Creek marsh system. In the experiment carried out on June 8, four arrays, each consisting of an electromagnetic current meter and OBSk probe, were established along the thalweg of Middle Creek at stations C1 –C4 in addition to the instruments at the reference station C5 in Main Creek (Figs. 3b and 5). In the second experiment carried out on June 24, four instrument stations (C5, M1 –M3) were deployed along a transect from Main Creek onto the marsh surface (Figs. 3b and 4b).

5. Results 5.1. Patterns of inundation and flow in tidal creeks and over the marsh surface Visual observations show that during the flood tide, water advances across the mud flats in front of the marsh, flows into Allen Creek and then into the main tidal creek inundating the lower portions of the three tributary creeks (Fig. 4a) and extending towards the shallow, dendritic headwater channels on the marsh surface (Fig. 4b). At this point, water first overtops the marsh cliff that forms the seaward margin of Main Creek, and shortly thereafter, the cliff that fronts the landward margin. The topographic lows along the low marsh area are flooded next and the water then progressively shifts landward until joining the expanding waters in the upper creek region. Sheet flow shifts landward across the middle and high marsh once waters overtop the shallow headwater channels,

Fig. 6. Three-dimensional topography of the marsh and creek system, velocity vectors, and tidal elevation recorded during the daytime spring tide on June 24, 1997, and velocity vectors recorded at two elevations at C2 on June 6, 1997. Coordinates and elevations are the same as in Fig. 3b. Note that values for station M3 are for the x-axis, which was aligned parallel to the creek channel. The tide stage curve for the tide on June 6 was almost the same as that for June 24 and is not shown here.

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and it also reaches the high marsh on the west side as water overtops the banks of Allen Creek (Fig. 4c). Slack tide lasts about 20 min, after which, the water level drops quite quickly. Flows during the period when the marsh itself is submerged are greatly affected by wind and wave conditions, as well as by the general circulation in the basin. During the ebb tide, waters initially receded parallel to the margin until the level falls to an elevation of approximately 5.50 m above geodetic datum (NAD83) and the flow becomes increasingly confined to the tidal creek network. Flow over the marsh surface is exemplified by results from an experiment carried out on June 24 with moderate southeasterly winds producing locally generated waves 0.2 – 0.3 m in height. Three instrument stations (M1 – M3) were deployed along a transect from the main tidal creek onto the marsh surface in addition to the reference station C5 in the Main Creek (Figs. 3b and 4b). Flow vectors over the tidal cycle illustrate the complex interaction of controls by topography, stage and the effects of winds and waves (Fig. 6). During the first 40 min of the flood, tide water entered the main creek system from the Allen Creek channel and flowed into the lower channels of the tributary creek systems, giving rise to flow vectors at C5 in Main Creek that are oriented upstream towards the northeast. However, a little over an hour into the experiment, the rising tide overtopped the arm that forms the south bank of the creek channel, and shortly thereafter, it overtopped the cliff on the north bank and began to inundate the marsh surface. At this point, the Main Creek channel was completely submerged and flow at C5 became directed towards the northwest and west as water flooded over the margin and across the marsh surface (Fig. 6). The direction of flow was probably influenced as well by inland flow up the Allen Creek channel as it flooded the upper tidal reach and by the southeasterly winds. Flows over the low marsh surface at M1 and M2 follow this general trend with flow direction becoming more southwesterly during the ebb tide under the influence of the general circulation towards the entrance to the Cumberland Basin. Flow direction became increasingly influenced by the creek channel as the water depth decreased and as the southern arm was exposed again. Flow in Middle Creek, illustrated from an experiment on June

6, 1997 with current meters located at 0.75 m (V2) and 1.5 m (V3) above the bed, shows a similar pattern (Fig. 6). In the early stages of the flood and late stages of the ebb, flow direction is primarily controlled by the orientation of the channel banks. Once the water level exceeds bankfull, the flow is controlled by the general flow over the marsh surface, with the upper current meter showing more response to this than the lower one (Fig. 6). On the marsh surface at stations M1 and M2, flow was generally onshore and into topographic depressions for 20– 30 min at the beginning and end of the inundation period when the water depth was small and the effect of wind was reduced by the presence of vegetation. During the remainder of the period, when much of the vegetation becomes submerged, flow vectors were oriented toward the west under the influence of the southeasterly winds (Fig. 6). The single axis of current meter M3, located on a headwater tributary whose channel is only about 0.2 m below the main marsh surface, demonstrated a clear flow reversal after slack tide (Fig. 6). 5.2. Velocity and suspended sediment concentrations Examination of data from individual instruments shows that there are fluctuations in speed and suspended sediment concentration at two time scales (Fig. 7). High-frequency fluctuations in flow speed along the long axis of the channel (Fig. 7a) in the order of 0.3– 0.5 Hz are associated with the passage of small, locally generated wind waves. The waves are generated primarily in the Cumberland Basin and propagate up the creek channels, or around high tide, they come over the marsh front. Low-frequency oscillations in the order of 0.1 –0.05 Hz reflect local seiching or Helmholtz resonance within the channels as a result of wind forcing. The fluctuations in current speeds, while small, serve to emphasize the importance of locally generated wave action in the suspension and resuspension of sediments within the channel and this is evident in the fluctuations in sediment concentration recorded in the OBS data (Fig. 7b). Patterns of mean flow velocity and suspended sediment concentrations in Middle Creek and over the low marsh surface are illustrated by results from experiments carried out over spring tides on June 8 and June 24 (Fig. 8). The tidal range on those 2 days

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Fig. 7. Time series of measurements at station V2 located at C2 on Middle Creek during one run on June 6, 1997 near high tide: (a) x-axis current speed (parallel to channel) and (b) suspended sediment concentrations. The instruments are located 0.75 m above the bed. Water depth in the channel at the time of measurement was about 1.75 m.

was similar (Fig. 8A). Mean speeds in the creeks seldom exceed 0.1 m s 1 (Fig. 8B) and they are generally too low to erode the bed and bank of the channels though some resuspension of recently settled material does occur. At station C1, in the lower part of the tidal creek, the flow velocity is relatively high at the beginning of the flood when the lower part of the channel fills rapidly. However, velocity decreases as the water reaches the narrow and relatively steep part of the channel between stations C2 and C3, where there is a relatively small increase in the prism for a given rise in water level. Towards high tide, the flow increases again as the water overtops the banks and spreads over the middle and high marsh. At this time, water in the lower channel is about 3-m deep and forms part of an extensive body that is flooding landward and to the west. Flow velocities in the deep

and very narrow channel at C3 (Fig. 5b) are generally very low, and this may be caused by isolation from the general flows across the marsh surface. Flow magnitude at C4, which is right on the marsh surface, shows an initial increase as the sheet flow moves landward across the mid and high marsh then decreases to near zero around high tide. Flow magnitude increases again as water flows seaward off the marsh surface. Plots of mean current speed measured at station C5 in Main Creek and at three stations set-up on the marsh surface (Fig. 3b) are shown in Fig. 8C. Mean flow velocities over the marsh surface are generally low ( < 0.2 m s 1). Current velocity tends to increase with increasing depths of water over the marsh surface at the stations located closest to the marsh margin (M1 and C5) and demonstrate a flood dominance. This increase may be attributed to the larger tidal circu-

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Fig. 9. Plot of sedimentation over the daytime spring tide, June 24, 1997, recorded for each trap plotted against elevation above datum.

lation system of the Cumberland Basin after the marsh arm is submerged. Further up on the marsh surface, at stations M2 and M3, higher velocities were recorded during the initial and latter stages of the tide and exhibit ebb dominance. This may be attributed to flow being restricted by the topography of the marsh surface. Suspended sediment concentrations measured in the creek (Fig. 8B) tend to be highest at the beginning of the flood and to decrease slightly over the course of inundation. There are occasional small fluctuations at the beginning and end of the inundation period that probably reflect resuspension of sediment deposited on the bed and banks of the channel, primarily due to the effects of small waves on the water surface and due to drainage down the unvegetated banks of the lower portions of the creeks during the ebb period. Suspended sediment concentration recorded by OBS probes located at stations M1 and M2 is similar to that recorded in Middle Creek, with the highest concentration occurring during the initial flood stage and tending to decrease at a similar rate (Fig. 8C). The rate of decrease in suspended sediment concentration slowed during slack tide and over the first part of the ebb tide until the depth of water over the marsh

surface was less than 1 m. The higher concentrations at station M1 compared to M2 suggest that suspended material is deposited as tidal waters flow across the marsh surface. Furthermore, the sharp increases in suspended sediment concentration during the initial flood and latter portions of the ebb suggest that sediment is being locally resuspended at the marsh margin by wave activity. 5.3. Sedimentation over the marsh surface The mass of sediment deposited at each sediment station (Fig. 3b) during the June 24 tidal cycle is plotted against elevation in order to assess the effect of depth and duration of inundation on the amount of material deposited (Fig. 9). The average mass of sediment deposited was 2.93 mg cm 2 with a standard deviation of 1.0 mg cm 2. Deposition tends to follow a parabolic form in which the maximum amount of sediment deposition occurs at an elevation of 4.6 –4.8 m and deposition is least at the lowest elevations close to the marsh margin and in the high marsh area where the duration of inundation is shortest. This central region coincides with the location of the creek tributaries. In addition to deposition on the surface, sediment was

Fig. 8. Mean values recorded over spring tidal cycles on June 8 and 24, 1997 for (A) water elevation at station C5; (B) flow magnitude and suspended sediment concentration recorded at stations in the creek channels on June 8, 1997; (C) flow magnitude and suspended sediment concentration recorded at station C5 and stations on the marsh surface on June 24, 1997. Current meter numbers correspond to station locations shown in Fig. 3b. Note that readings for stations C3 and M3 are for the x-axis only, which is aligned parallel to the channel at this location.

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visually observed adhered to low marsh vegetation, particularly within half a meter of the creek and tributaries. During this tidal cycle, there was no significant relationship between deposition and vegetation height, vegetation density or inundation time. Correlation with spatial variation in suspended sediment concentration could not be tested because of the small number of locations at which these measurements were taken, though measurements of suspended sediment concentration over the field season do not suggest that there is a great deal of variability over the low marsh.

6. Discussion and conclusions 6.1. Patterns of inundation and flow velocity Vector analysis and flow visualization suggest that the direction and magnitude of flow within the tidal creeks and over the marsh surface are controlled by the complex interaction of topography, tidal stage and the effect of wind and waves. Nearly identical patterns of tidal inundation were observed on Allen Creek Marsh over each tidal cycle despite differences in wave direction and speed, tidal cycle and vegetation cover. Within the study creeks, the flow speed remains quite low (0.1 m s 1 or less) for most of the period of inundation (Fig. 8B). Small pulses in velocity are observed as the flow encounters topographical ‘steps’ or changes in channel gradient within the upper creek system, rising slowly until it oversteps the topographical boundary and produces a transient surge in velocity. However, because of the absence of pronounced levees along the creek banks, the short distances involved and the relatively rapid rise in water level, which allows water to flood across the marsh margin and banks of the creek at almost the same time, the pulses in velocity observed are not as marked as those noted in other studies (e.g. Bayliss-Smith et al., 1979; Reed, 1988; French et al., 1993; Ayles and LaPointe, 1996; Leonard, 1997). Furthermore, once the water level exceeds bankfull conditions, flow speeds decrease because of the effects of vegetation on the shallow flows over the vegetated marsh surface. Previous studies have indicated that, once the flow of water reaches the vegetated surface, the canopy initially exerts a strong influence on suspended sediment concentration and the deposition rate of sus-

pended sediment through the effects of bed shear stress and turbulence of flow within the canopy (Gleason et al., 1979; Pethick et al., 1990; Leonard and Luther, 1995; Shi et al., 1995b, 1996; Leonard, 1997). The morphology, height and density of marsh vegetation such as Spartina interact strongly with the flow, acting as roughness elements that extract momentum from the fluid via hydrodynamic drag and dissipate turbulent energy (Fonseca et al., 1982; Knutson et al., 1982; Leonard and Luther, 1995; Shi et al., 1995b, 1996; Leonard, 1997). As the water level increases, the marsh vegetation becomes less effective at attenuating tidal flow and wave energy since the relative roughness of the vegetation on the flow diminishes (Fonesca and Fisher, 1986; Pethick et al., 1990; Shi et al., 1996). However, in the Upper Bay of Fundy, given a water depth of approximately 1.5 m at high tide over the marsh surface and an average vegetation height of 15 cm, the vegetated canopy at Allen Creek marsh occupies only about one-tenth of the water column. Consequently, the influence of the velocity sublayer created within the S. alterniflora canopy on sedimentation processes is minimal, particularly within the low marsh. It is anticipated that vegetation plays a greater role in sediment deposition further up in the tidal frame within the high marsh. Once the water level drops to approximately 30 cm above the marsh surface, following topographic lows, a reversal in the orientation of the flow ensues and further increases in flow speeds are observed in the current meter record of overmarsh instruments. Although higher flow speeds (>1 m s 1) during the ebb period have been described in other marsh studies (Green et al., 1986; Stevenson et al., 1988; French and Stoddart, 1992; French and Clifford, 1992), these have been attributed to the funneling of a larger prism of water draining off the marsh surface and being confined within the creeks during the latter stage of the tidal cycle. In the Allen Creek Marsh system, although the water level itself decreases quite rapidly, this flow/ volume is distributed over a large cross-sectional area because most of it drains seaward over the whole width of the marsh front. In effect, the relatively narrow marsh area and steep gradient limit the areas of ponded water or retarded flow, which tend to produce more rapid flows in tidal creeks of microtidal areas. Thus, high velocities were measured in the tidal creeks only at the very end of the ebb and are likely due to the

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drainage of the remaining surface water of the catchment area and to some drainage of subsurface waters from within the marsh itself. 6.2. Suspended sediment dynamics The results from this study demonstrate modest changes in suspended sediment concentrations over a single tidal cycle. There is a slight increase in suspended sediment over the first half of the flood tide as sediment is carried into the creek system from the tidal flat. Within both the tidal creeks and over the marsh surface, the amount of suspended sediment carried in the water column decreases slowly over the remainder of the tidal cycle. However, much of the fine sediment remains in suspension, and the absence of a substantial fraction of coarse sediment with higher settling velocities accounts for the absence of levee formation along the banks of the creek channels. Although considerable variability in suspended sediment concentration over and between tidal cycles has been observed, even at identical velocities and stages of the tidal cycle, low flow speed data within the canopy coupled with a decreasing flux trend suggest that sediment is not resuspended from the marsh surface. Decreases in suspended sediment may result from the reduced sediment availability in the overlying water column (van Proosdij et al., 2000). This may be attributed to material settling throughout the inundation event or remaining within the viscous sublayer within the canopy. This sublayer prevents resuspension and may be isolated from sediment supply in the upper region due to the possible presence of a reversal of the flow velocity gradient (Pethick et al., 1990; Ke et al., 1994; Shi et al., 1996). No peaks in suspended sediment concentration are observed at peak velocities in the deeper water, as have been observed in some previous studies (Ward, 1981; Green et al., 1986; Ashley and Zeff, 1988; Leonard, 1997).

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here reduces deposition near the tidal creek channels and this is reflected in the absence of levee development. Furthermore, the relatively greater depths of water over the marsh surface at spring high tides, compared to micro- and mesotidal environments, enable wave action from the bay to penetrate further up onto the marsh surface, thus inhibiting deposition in the low marsh region. Therefore, in this macrotidal environment, it appears that the dominant control on sediment deposition is wave activity, both in terms of increased suspended sediment concentrations within the water column and interaction with the marsh surface. This study suggests that in order for the highest amount of sediment to be deposited on a marsh surface, both high suspended sediment concentration and low wave activity are required. A high suspended sediment concentration in itself does not guarantee high deposition rates. Low wave energy and a high relative roughness index enhance the depositional process.

7. Conclusions The preliminary results presented here suggest that hydrodynamics and sedimentary processes in a high-macrotidal environment exhibit differences from those found in micro- and mesotidal environments. One of the most significant characteristics of flow in this high macrotidal environment is that a very large proportion of the total marsh tidal prism enters and exits across the marsh margin rather than through the tidal creek channels. Consequently, input and output measurements in the creek alone are not sufficient to fully quantify the marsh sediment budget (e.g. French and Stoddart, 1992). In turn, these hydrodynamic effects also influence the magnitude and spatial patterns of sedimentation on the marsh surface and have implications for modeling vertical marsh development.

6.3. Controls on sedimentation on the marsh surface Acknowledgements At Allen Creek Marsh, the highest amounts of deposition appear in the mid marsh zone. Field studies in other environments suggest that the greatest amounts of deposition generally occur on the creek levees and low marsh region. However, the absence of coarse sediment in the sediment inputs to the marsh studied

We thank Jaimie Dawson, Becky Rush and Marie Tower for their assistance in the field and Don Forbes of The Atlantic Geoscience Center for his assistance with the field survey. Marie Puddister and Mario Finoro of the Geography Department, University of

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Guelph provided considerable cartographic and technical assistance. Support from Mount Allison University is appreciated. The Canadian Wildlife Service in Sackville, New Brunswick offered support for a number of aspects of the work. We thank the New Brunswick Ministry of Natural Resources for giving us permission to carry out the work on Allen Creek Marsh. The study was supported by research grants from the Natural Sciences and Engineering Research Council of Canada to RD-A and JO, and by Postgraduate scholarships to DVP and LS. DVP also acknowledges University of Guelph travel grants from the Latornell Foundation and from the College of Social Sciences Alumni Fund. References Adam, P., 1990. Salt Marsh Ecology. Cambridge Univ. Press, Cambridge, UK, 461 pp. Allen, J.R.L., 1990. Salt marsh growth and stratification: a numerical model with special reference to the Severn Estuary, southwest Britain. Marine Geology 95, 77 – 96. Allen, J.R.L., 1991. Salt marsh accretion and sea-level movement in the inner Severn Estuary: the archaeological and historical contribution. Journal of the Geological Society 148, 485 – 494. Allen, J.R.L., 1997. Simulation models of salt marsh morphodynamics: some implications for high-intertidal sediment couplets related to sea-level change. Sedimentary Geology 113, 211 – 223. Allen, J.R.L., 2000. Morphodynamics of Holocene salt marshes: a review sketch from the Atlantic and Southern North Sea coasts of Europe. Quaternary Science Reviews 19, 1155 – 1231. Allen, J.R.L., Pye, K., 1992. Salt Marshes: Morphodynamics, Conservation and Engineering Significance. Cambridge Univ. Press, Cambridge, UK, 184 pp. Amos, C.L., 1987. Fine-grained sediment transport in Chignecto Bay, Bay of Fundy, Canada. Continental Shelf Research 7, 1295 – 1300. Amos, C.L., Tee, K.T., 1989. Suspended sediment transport processes in Cumberland Basin, Bay of Fundy. Journal of Geophysical Research 94 (C10), 14407 – 14417. Amos, C.L., Tee, K.T., Zaitlin, B.A., 1991. The post-glacial evolution of Chignecto Bay, Bay of Fundy, and its modern environment of deposition. In: Smith, D.G., Reinson, G.E., Zaitlin, B.A., Rahmani, R.A. (Eds.), Clastic Tidal Sedimentology. Canadian Society of Petroleum Geologists, Memoir, vol. 16. Geol. Surv. Canada, Ottawa, Ontario, pp. 59 – 90. Ashley, G.M., Zeff, M.L., 1988. Tidal channel classification of a low-mesotidal salt marsh. Marine Geology 82, 17 – 32. Ayles, C.P., LaPointe, M.F., 1996. Downvalley gradients in flow patterns, sediment transport and channel morphology in a small macrotidal estuary: Dipper Harbour Creek, New Brunswick, Canada. Earth Surface Processes and Landforms 21, 829 – 842.

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