10.12 Estuaries

10.12 Estuaries NL Jackson, New Jersey Institute of Technology, Newark, NJ, USA r 2013 Elsevier Inc. All rights reserved.

10.12.1 10.12.2 10.12.2.1 10.12.3 10.12.3.1 10.12.4 10.12.4.1 10.12.4.2 10.12.4.3 10.12.4.4 10.12.5 10.12.5.1 10.12.5.2 10.12.5.2.1 10.12.5.2.2 10.12.5.2.3 10.12.5.2.4 10.12.6 10.12.6.1 10.12.6.1.1 10.12.6.1.2 10.12.6.2 10.12.6.2.1 10.12.6.2.2 10.12.6.3 10.12.6.4 10.12.6.5 10.12.7 References

Introduction Definition and Distribution Characteristics and Dimensions Classification of Estuaries Geomorphic Classification Estuarine Morphodynamics: Physical Factors Sea Level Tides Waves River Discharge Morphodynamics and Evolution Sediment Transport in Estuaries Estuarine Geomorphic and Sedimentary Facies Tide-dominated estuaries Wave-dominated estuaries Mixed wave-tide-dominated estuaries River-dominated estuaries Estuarine Subenvironments Lower Intertidal Tidal flats Bedforms Upper Intertidal Zone Unconsolidated shorelines Cohesive shorelines Geomorphic–Biotic Interactions Human-Modified Estuarine Systems Restoration Practices Future Issues

308 309 310 311 311 312 312 313 314 314 315 315 315 315 316 316 316 316 317 317 317 318 318 319 320 321 322 322 323

Abstract Estuaries are among the most biologically productive and geomorphologically complex environments in the coastal zone. Estuaries vary considerably in terms of geomorphology, tides, drainage basin and fluvial processes, water chemistry, wave characteristics, sediment provenance, upland land use and land cover, biodiversity, and degree of human modification. This chapter presents background information on the definition, classification, and characteristics of estuaries as well as the physical processes that shape morphology. A review of research is presented, focusing on broad-scale estuarine morphology and evolution and an examination of contemporary processes and forms in the intertidal zone. The chapter includes a discussion of current issues in estuarine research, including geomorphic–biotic interactions, human-modified estuaries, and restoration practices. The chapter concludes with a brief discussion of future areas of concern given current attention to climate variability and sea-level rise. Although the focus is on geomorphic research, there are references to key work within biology, chemistry, and hydrology.

10.12.1

Introduction

Estuaries are among the most biologically productive and geomorphologically complex environments in the coastal

Jackson, N.L., 2013. Estuaries. In: Shroder, J. (Editor in Chief), Sherman, D.J. (Ed.), Treatise on Geomorphology. Academic Press, San Diego, CA, vol. 10, Coastal Geomorphology, pp. 308–327.

308

zone. Estuaries are unique ecosystems that provide spawning grounds for many organisms, feeding stops for migratory birds, and natural filters to maintain water quality. Estuaries have value to humans for shipping and boating, fishing, recreation, mineral extraction, and release of waste materials. Estuaries vary considerably in terms of geomorphology, tides, drainage basin and fluvial processes, water chemistry, wave characteristics, sediment provenance, upland land use and

Treatise on Geomorphology, Volume 10

http://dx.doi.org/10.1016/B978-0-12-374739-6.00285-2

Estuaries

land cover, biodiversity, and degree of human modification. Research on estuaries is extensive and the subject of several textbooks, scholarly monographs, and edited volumes over the past 20 years (e.g., Nordstrom, 1992; Nordstrom and Roman, 1996; Perillo, 1995a; Dyer, 1997; Pye and Allen, 2000; Fitzgerald and Knight, 2005; Hardisty, 2007). The research on estuaries can be broadly categorized into three areas: physical studies related to mixing and circulation (Dyer, 1997; Uncles, 2002), biological studies on species and habitats (Little, 2000), and geomorphological studies on morphodynamics and evolution (Pye and Allen, 2000; Heap et al., 2004; Fitzgerald and Knight, 2005; Anderson and Rodriguez, 2008). Although estuaries constitute a significant portion of the world’s coast, the published research focuses largely on only a small subset of systems in Africa (e.g., Mgeni), Asia (e.g., Pearl and Yangtzee), Australia (e.g., Batesman, Murray, and Swan), Europe (e.g., Mersey, Severn, and Siene), North America (e.g., Chesapeake, Columbia River, Delaware, Narragansett, Puget Sound, San Francisco, and St. Lawrence), and South America (e.g., Bahia Blance and Rio de la Plata). Much of the research is based on empirical studies of individual estuaries and includes regional comparisons of estuarine systems and classifications of types of estuaries. Recent advances in instrumentation have led to better quantification and modeling of these systems (Fitzgerald and Knight, 2005). This chapter presents a review of research on broad-scale estuarine morphology and evolution and an examination of contemporary processes and forms in the intertidal zone. The chapter concludes with a discussion of current issues in estuarine research and future areas of concern given current attention to climate variability and sea-level rise. Although the focus is on geomorphic research, there are references to key work within biology, chemistry, and hydrology.

10.12.2

Definition and Distribution

The word ‘estuary’ is a derivation of the latin word aestus, meaning ‘of the tide’. Estuaries are generally considered areas where saltwater from the ocean mixes with freshwater from land drainage; but there are numerous definitions for the term (Perillo, 1995b) reflecting the complex physical, geomorphological, and biological processes present. Among the earliest definitions is that of Pritchard (1952), who defined an estuary as ‘‘a semi-enclosed coastal body of water having a free connection with the open sea and containing a measurable quantity of sea water.’’ This definition was later modified by Cameron and Pritchard (1963) to ‘‘a semi-enclosed coastal body of water having a free connection with the open sea and within which sea-water is measurably diluted with fresh water derived from land drainage.’’ As Pritchard (1967a) explained, the definition is based on the physical processes of circulation, salinity, and density. Estuaries are commonly identified as one of four types based on vertical stratification and mixing: highly stratified, fjords, partially mixed, and well mixed. These processes define not only the physical and biological attributes of an estuary but also the geomorphology of the estuary insofar as basin geometry influences circulation and salinity. The circulation patterns common to estuaries are controlled by the

309

lateral boundaries of the basin and the connection to the sea that establishes the density gradients that drive circulation (Dyer, 1997). The definition of an estuary, based on Cameron and Pritchard (1963), is the most frequently cited among geomorphologists (Kench, 1999) and includes four types of estuaries that meet the criteria: drowned river valleys, fjords, bar-built estuaries, and tectonically controlled estuaries. Researchers have found the need to develop a more universal definition to include basins with different geomorphic attributes from those common to the temperate regions of the world that informed the definition of Cameron and Pritchard (1963). The multidisciplinary dialog (Day, 1981; Kjerfve, 1994; Elliott and McLusky, 2002; Tagliapietra et al., 2009; Potter et al., 2010) has expanded the geomorphic range of what constitutes an estuary to include intermittently open water bodies (Day, 1981), rivers flowing into nontidal water bodies (Herdendorf, 1990), and lagoon environments (Kjerfve, 1994). In policy circles, countries have developed independent, and at times, scientifically conflicting definitions for the term estuary. For example, in Europe the term ‘transitional waters’ is used to provide an overarching term for implementation of policy directives (McLusky and Elliott, 2007), and in the USA the ‘estuary type’ areas of the Great Lakes are included in the regulatory definition of the National Estuary Program (United States Environmental Protection Agency (USEPA)). A universally acceptable definition has yet to surface and it is likely that the discussion will continue. In this chapter, estuary will include systems as defined by Cameron and Pritchard (1963) but will also provide discussion on systems that are not ‘classical’ estuaries, such as lagoons. The intent is to present the reader with a broad picture of research and discussion in the field. The geographic distribution of estuaries is, in part, influenced by how these systems are defined, but inherited characteristics of the continental shelf, coastal lithology, tectonic activity, and sea-level fluctuations also influence the location of these systems (see Nichols and Biggs, 1985, Figures 2, 3; Perillo, 1995b, Figures 1, 2). The morphology of estuaries around the world is highly variable and attributed to physical factors such as climatic variation (tropical, humid, and temperate), fluvial discharge (seasonal and episodic), wave energy (locally generated, ocean generated), tidal regime (micro, meso, and macro), and sediment availability and characteristics (including textural properties of size and sorting) from upland and marine sources. For example, in Australia more than 738 estuaries occur (Bucher and Saenger, 1991) and include drowned river valley and bar-built estuaries in the south, and delta-front estuaries in the north (Kench, 1999). In North America, the estuaries include both drowned river valleys and bar-built estuaries along the Atlantic and Gulf coasts, and also include fjords and tectonically influenced estuaries on the Pacific coast (Pritchard, 1967a). In South Africa where most micro-tidal estuaries are incised within bedrock valleys, the estuaries are predominantly river dominated or tide dominated (Cooper et al., 1999). In Denmark, two large estuarine-fjord systems (Limfjord and Isefjord–Roskilde Fjord) and numerous smaller estuaries (bays and coves) are located on the 7300 km coastline (Conley et al., 2000).

310

Estuaries

10.12.2.1

Characteristics and Dimensions

Estuaries are shaped by fluvial, estuarine, and marine processes that give rise to a complex array of tidal channels, tidal flats, marshes, beaches, barriers, and deltas. These processes can act in concert such as where marine processes close an inlet by bar formation and fluvial processes later reestablish the opening to the sea by bar breaching. Sediment occurring in an estuary is a product of all three processes and can range from silts and clays to sand and gravel (Guilcher, 1967). Where an estuary begins and ends is a function of the physical criteria used (salinity, tidal influence, and origin of sedimentary facies) but the limits of an estuary fluctuate over time (Figure 1). Pritchard (1967a) defined the upper and lower boundary of an estuary based on salinity. The upper boundary is where salinity is 0.1% and the lower limit is where salinity is 32%. Dalrymple et al. (1992) employed geomorphic criteria and defined the upper limit as the facies boundary between tidal and fluvial influenced sediments. The lower limit of the estuary is the facies boundary between marine and estuarine sediments. The seaward boundary may be delimited by geomorphological features such as a delta at the mouth of a drowned river valley estuary or a barrier fronting a bar-built estuary. Hopkinson and Hoffman (1984) extended the seaward limit beyond the coastal land masses to the nearshore where seawater is diluted from land drainage. Estuaries consist of one or more tidal channels and intertidal flats and a grading of sediment occurs from the

fine-grained sediment in the upper tidal flats to sand-size sediments in the main tidal channel. The upper sector of the estuary is generally dominated by fluvial processes and a small delta may exist at the head of the estuary where deposition occurs due to a reduction in flow. The sediment in this region may be composed of coarser material depending on basin geology. The lower sector of the estuary is where marine processes (waves and tides) have their greatest imprint on estuarine morphology. A tide-constructed delta may be present at the mouth of the estuary, or a wave-constructed sand barrier may be present. The middle sector of the estuary is a mixed regime of fluvial and marine processes, and bedload sediment is generally the finest in this location. The presence and extent of these regions vary, depending on type and stage of evolution of the estuarine system (Dalrymple et al., 1992). Many estuaries in the world exhibit similar size and shape relationships (Dyer, 1997). The most commonly used empirical relationship is that of O’Brien (1931), who proposed that the cross-sectional area of the entrance of an estuary at mean tide elevation (A) is a function of the tidal prism (P), so that A ¼ cPn, where c and n are scale and shape coefficients. A larger tidal prism will result in greater velocities, increased sediment transport, and cross-sectional area. Where velocities decrease below the sediment threshold, there will be no further increases in the cross-sectional area. This relation has been used to classify estuaries in New Zealand (Hume and Herdendorf, 1993) and the UK (Townend, 2005) and has shown a good fit

Tidal limit

Process gradient River dominant

Salinity gradient

Freshwater dominant

Fluvial/tidal sediment Head

Upper

0.1

River and tidal processes Estuary

Middle

Mixed Estuary

Mixed river and marine

(fresh- and saltwater)

Lower Marine dominant (waves and tides) Marine sediment Mouth 32.0 Ocean

Saltwater dominant

Figure 1 Generalized process and salinity gradients in a typical estuary. Modified from Pritchard, D.W., 1967a. What is an estuary: physical view point. In: Lauff, G.H. (Ed.), Estuaries, Publication No. 83. AAAS, Washington, DC, pp. 3–5 and Dalrymple, R.W., Zaitlin, B.A., Boyd, R., 1992. Estuarine facies models: conceptual models and stratigraphic implications. Journal of Sedimentary Petrology 62, 1130–1146.

Estuaries

to estuarine morphology (i.e., drowned river valley, fjord, and bar-built estuary). The relative contribution of freshwater and saltwater to an estuary can produce salinities that are lesser or greater than the ocean inputs when losses due to evaporation are considered. Positive estuaries are defined by conditions where the resultant salinity is less than the salinity of the ocean. Freshwater derived from land drainage, groundwater, or precipitation exceeds evaporation losses. Negative estuaries are defined by conditions where the resultant salinity is greater than the salinity of the ocean. In this case, evaporation losses are greater than freshwater inputs (Pritchard, 1952). Estuaries where the freshwater drainage is in balance with losses from evaporation are referred to as neutral estuaries. Cameron and Pritchard (1963) classified estuaries based on their salinity structure and included four types: highly stratified estuaries (salt wedge and fjord type), partially mixed estuaries, and well-mixed estuaries. Salinity is important not only to estuarine circulation but also to biological productivity. Species abundance and diversity decrease with decreasing salinity (Day et al., 1989), and the relationship between circulation and biological productivity has been documented for different estuarine mouth morphologies.

10.12.3

Classification of Estuaries

The wide variability of estuaries throughout the world has generated considerable research on methods for classifying Table 1 Selected classification systems for estuaries Classification

Types

References

Geomorphology

Drowned river valley Ria Fjord (Fjard) Bar-built Faulting or local subsidence Blind Delta-front

Pritchard (1967a) and Fairbridge (1980)

Degree of lnfilling

Drowned river valley Barrier Saline coastal lakes

Roy (1984)

Evolution

Wave-dominated Tide-dominated Mixed River-dominated

Dalrymple et al. (1992) and Cooper (1993)

Tidal range

Micro-tidal Meso-tidal Macro-tidal

Hayes (1975)

Stratification and salinity structure

Stratified Partially mixed Mixed

Cameron and Pritchard (1963), and Pritchard (1967b)

311

them into distinct groups (Kurtz et al., 2006). Estuaries may be classified or described on the basis of numerous criteria (Table 1) but some of those important to geomorphology include physiography and morphology (Pritchard, 1967a; Fairbridge, 1980), stage of development and degree of infilling (Roy, 1984), tidal range (Hayes, 1975), and morphologic evolution (Dalrymple et al., 1992; Cooper, 1993). Many of the early classification systems were descriptive rather than functional and current work has focused on the integration of physical, geomorphic, and biological attributes as a basis for analysis (Kurtz et al., 2006; Hume et al., 2007). The following section presents an overview of general estuarine morphology based on the work of Pritchard (1967a) and Fairbridge (1980).

10.12.3.1

Geomorphic Classification

From a geomorphic standpoint, estuaries can be broadly classified as drowned river valleys and rias, fjords, bar built, and those formed by faulting or local subsidence (Pritchard, 1967a; Fairbridge, 1980) (Figure 2). Drowned river valley estuaries are generally funnel shaped and occur in many locations, including the east coast of the USA (e.g., Delaware Bay), England (e.g., Thames and Mersey estuaries), France (e.g., Seine), and Australia (e.g., Batesman Bay) (Perillo, 1995b). This type of estuary is the result of rivers eroding deep V-shaped valleys during the last glacial period that were subsequently inundated when melting ice sheets caused a rise in sea level. The planform and cross section of these estuaries are generally triangular or funnel shaped. In systems where sedimentation rates are less than rates of sea-level rise, the river valley topography is maintained. Rias are drowned valleys that are generally deep, elongated, and narrow (Castaing and Guilcher, 1995). Rias may not be estuarine along their entire lengths but are characterized by a narrow zone at the head of the estuary where circulation produces salinity values lower than the adjacent ocean (Evans and Prego, 2003). Sedimentation is generally restricted to the upper reaches near the headwaters (Healy et al., 1996), and the accommodation space (the area available for infilling to occur) within the basin may increase with increasing sea-level rise leading to upward and outward accretion (Psuty and Morriera, 2000). Ria-type estuaries occur in New Zealand (Healy et al., 1996), Portugal (Psuty and Morriera, 2000), and Spain (Castaing and Guilcher, 1995). Fjords are glacially scoured, U-shaped valleys that were inundated by a rising sea level (Syvitski and Shaw, 1995). Fjord-type estuaries occur in the upper latitudes of North America, Europe, and South America. Most fjords possess a shallow rock sill near the mouth that forms an estuarine basin. The sill may be the result of deposition or basin deepening (Woodroffe, 2003). Fjords receive a small proportion of their sediment from marine inputs. Most sedimentation is the result of glacial and/or fluvial processes but the sediment volume is often small relative to the size of the basin. Bar-built estuaries have a geologic history similar to that of drowned river valleys, but recent marine sediment transport (alongshore or cross-shore) results in creation of a barrier or spit across the mouth. The inlet at the mouth is small relative to the size of the shallow estuary created behind the barrier. In some cases, the barrier may restrict exchange of water between

312

Estuaries

Head

Head

Head

High relief Sill Mouth

Mouth Ocean

Mouth Ocean

Fjord

Ria

Longshore transport

Spit

Ocean

Ocean

Ocean

Funnel shaped

Ocean

Barrier island

Bar-built

Lagoon

Tectonic

Figure 2 Geomorphic types of estuaries. Modified from Fairbridge, R.W., 1980. The estuary: its definition and geodynamic cycle. In: Olausson, E., Cato, I. (Eds.), Chemistry and Biogeochemistry of Estuaries. Wiley, Chichester.

the ocean and estuary except during high tides. This type of estuary is dominant along the coastal plain of the southeast United States (Dame et al., 2000) and the Pacific coast (USA). This type also occurs in Australia (Roy et al., 2001) and Africa (Anthony et al., 2002). Some estuaries formed in valleys that were created by tectonic processes, which is the case on the Pacific coast where the San Andreas fault contributed to the development of San Francisco and Tomales Bays (Emmett et al., 2000). Bar building can contribute to estuarine evolution on tectonically active coasts by helping to enclose ocean bays. Lagoons are not considered to be estuaries according to the classic definition of Cameron and Pritchard (1963) but are considered estuaries by many scientists and managers (Potter et al., 2010). Lagoons may be considered an end member of the estuarine continuum, situated between estuaries and strandplains and tidal flats (Boyd et al., 1992). They can occur in micro-, meso-, and macro-tidal environments (Cooper, 1994a) but not in strongly tidal-dominated areas (Boyd et al., 1992). They generally form on transgressive coasts with low-gradient continental shelves and in regions where the rate of sea-level rise is slow (Cooper, 1994a). Lagoons may possess flood tidal deltas that provide substrate for subsequent marsh development and a fluvial delta in cases where a river discharges to the lagoon. Several models have been developed to describe the evolution of lagoon systems due to sediment accumulation and sea-level rise (Roy, 1984; Nichols, 1989; Cooper, 1994a). The stage of a system on the continuum is influenced by sediment input (fluvial and marine), characteristics of the basin, and rates of barrier transgression.

10.12.4

Estuarine Morphodynamics: Physical Factors

Research on estuarine morphodynamics has focused on the development of models that predict behavior at timescales that are both long (geologic such as Holocene evolution) and short (individual events such as tidal cycles and storms) and spatial scales that are large (estuary) and small (marsh, mangrove, beach, mudflats, and offshore bars). The forcing functions that create, maintain, or change system behavior at different temporal and spatial scales include sea-level rise, episodic floods, seasonal river discharge, and waves generated over a tidal cycle.

10.12.4.1

Sea Level

Estuaries are geologically young features on the Earth’s surface, formed in response to sea-level stabilization over the past 6000 years. The vertical and horizontal positions of estuaries are the result of inherited lithology and fluctuations in sealevel rise, with sea-level elevation at or above current levels during interglacial periods and up to 150 m below present levels during glacial periods. Sea level is the result of changes in the volume of ocean water due to thermal expansion and contraction and glacial advance and retreat as well as changes in the elevation of the Earth’s surface. These processes may operate in isolation or in concert and their relative influence changes with both spatial scale and temporal scale. Postglacial, global sea-level change was strongly influenced by the exchange of mass between glacial ice sheets and the ocean. More recent regional sea-level histories have revealed both a

Estuaries

lowering and rising of sea level from still stand levels (Edwards, 2007) due to departures in climate and isostatic changes (Pirazzoli, 1993). Lambeck and Chappell (2001) reported sea-level histories from several locations where the data have been adjusted to account for tectonic activity. These data, as well as data reported in Pirazzoli (1993), reveal the effects of local conditions on sea-level trends across the globe. Falling sea levels have occurred in areas where former ice sheets existed (i.e., Hudson Bay, CA) due to isostatic rebound. Areas located beyond the former ice margins reveal a steep sea-level rise curve followed by a deceleration in the rate of rise beginning approximately 6000 years BP (i.e., southern England, Atlantic coast of USA). In locations at considerable distances from the ice margins (i.e., Australia), sea level reached an elevation close to the present-day position approximately 6000 years BP with little variation since that time. For example, in areas such as northern Australia that have been relatively stable tectonically, estuaries have experienced similar sea-level-rise histories (Chappell and Woodroffe, 1994). Radiocarbon dates indicate that the maximum extent of the ice sheets during the last glaciation occurred approximately 17 000–18 000 years BP (Woodroffe, 2003). Postglacial, sea-level rise began soon after this peak in glacial extent. Estuaries and lagoons are transgressive features (Cattaneo and Steel, 2003) and they form under conditions where relative sea-level rise exceeds the rate of sediment infilling (Boyd et al., 1992). The development of estuaries coincided with sealevel rise and inundation of coastal lowlands following the last glacial period and the sea-level still stand that began approximately 6000 years ago. Deltas and coastal plains on the outer edge of the present continental shelf were the first to be inundated during the most recent sea-level rise (Russell, 1967). Estuaries increased in complexity with sea-level rise and inundation of the tributaries of fluvial valley systems or narrow resistant bedrock formations. Pethick (2001) outlined a conceptual model for estuary transgression as a result of relative sea-level rise based on the ‘rollover’ model (Allen, 1990). Changes that occur to the estuary are a result of the redistribution of sediment within the estuary as well as inputs from the marine environment. The estuary maintains its position in the tidal and wave energy frame by a vertical (upward) and horizontal (up-estuary) movement in response to sea-level rise. An increase in water depth in the lower estuary results in the propagation of ocean waves into the estuary and erosion of the upper intertidal sediment that displaces the marsh/tidal flat boundary inland. This sediment will be deposited landward to the inner estuary and subsequently redeposited in the upper intertidal zone. The redistribution results in an increase of the surface elevation of the marsh and tidal flat in the new location and transgression of the marsh edge. The salt marsh/tidal flat boundary of these inner estuarine areas continues to erode as the fetch length and depth for local wave generation increase because of the rise in sea level. The migration rate of the landward marsh boundary is a function of the upgradient slope and the vertical accretion rate of the marsh surface. Accommodation space, or the volumetric area available for sediment infilling or water volume in an estuary, will influence estuarine transgression and is a function of relative sea-level

313

changes, freshwater flow, hypsometry, and sediment supply (Cooper, 1993). Slagle et al. (2006) found evidence of regional erosion, or nondeposition, with intervening periods of deposition in the Hudson River Estuary (USA) that they attributed to limited accommodation space. Townend et al. (2007) found periods of sedimentation and erosion of the Humber Estuary with export of sediment occurring during periods when extensive tidal flats were present. Where fluvial sediment supply is limited or where postglacial sea-level rise and isostatic adjustment have increased the volume of the basin, the accommodation space is increased such as in the case of Laguna Madre, USA (Morton et al., 2000). In cases where sediment infilling is high, the estuary will become shallow and the result is a decrease in the rate of transgression (Woodroffe, 2003). Antecedent morphology of the land that becomes inundated also influences available accommodation space and estuarine evolution (Rodriguez et al., 2008; Heap and Nichol, 1997). A flat, passive margin coastal plain will generate an estuary with a large volume compared to a steep, active margin coast given similar increases in sea-level rise (Boyd et al., 1992).

10.12.4.2

Tides

Estuaries are located in micro-, meso-, and macro-tidal environments, with tidal ranges generally defined by the classification of Davies (1964) where the micro-tidal range is o2 m, meso-tidal is 2–4 m, and macro-tidal is 44 m. Planform morphology is an important control on the variation of tidal range and the magnitude of the tidal currents within the estuary (Nichols and Biggs, 1985). Convergence of the lateral boundaries of a funnel-shaped estuary causes the tidal wave to compress laterally. In the absence of substantial bed friction, the tidal range will increase. In the presence of substantial friction, the tidal range will decrease. Thus, the relationship between convergence and friction controls the amplitude of the tide within the estuary. In cases where convergence is greater than friction, the tidal range and the strength of the tide will increase toward the head of the estuary (hypersynchronous estuaries). Tidal-dominated estuaries are generally hypersynchronous with stronger tidal velocities in the midestuary and weaker tidal velocities near the head and mouth of the estuary (Dalrymple and Choi, 2007). In cases where convergence is less than friction, the tidal range will decrease throughout the estuary (hypsosynchronous estuaries). The transition of an estuary to different morphologic states may be explained, in part, by the relationship of tidal asymmetry to net sediment transport. An estuary reaches an equilibrium state when the quantity of sediment transported during flood tide is balanced by the quantity of sediment transported during ebb (Dyer, 1997). The conditions that lead to equilibrium are a function of tidal velocity, the depth of the channel, and intertidal area of the estuary (Dronkers, 1986). The distortion of the tidal wave and asymmetry that develops (ebb–flood duration) upon entering the estuary is the major influence on the net sediment transport direction in the estuary (Postma, 1967). Under flood-dominated conditions, high-velocity, short-duration flood flows are dominant over low-velocity, long-duration ebb flows. The greater

314

Estuaries

flood-directed bed shear stress, relative to the threshold for the sediment size, increases the sediment volume in transport, and the direction of net transport will result in deposition or infilling of the estuary. Friedrichs and Aubrey (1988), in a study of US estuaries, found that flood dominance was dependent on the ratio of the tidal amplitude (M2) to the hydraulic depth. Ebb dominance was influenced by the volume of storage over the intertidal zone, as a proportion of the carrying capacity of the main channel. Estuaries that are deep tend to be ebb dominated and estuaries that are shallow tend to be flood dominated (Wang et al., 2002). Assessing the phase difference in the M2 (principal semi-diurmal lunar constituent) and M4 (quarter-diurnal nonlinear harmonic over-tide of M2) tidal constituents of the macro-tidal Dee Estuary, Moore et al. (2009) found the estuary to be generally flood dominant and experiencing sediment infilling which agreed with long-term historical data. Townend (2005) used a sample of 188 estuaries in the UK and found the majority to be flood dominant also due to a high ratio of tidal amplitude to hydraulic depth. The transition to an ebb-dominated system has been attributed to the physical extent of tidal flat development in the estuary (Friedrichs and Aubrey, 1988; Kang and Jun, 2003). Brown and Davies (2010) classified the Dyfi Estuary as ebb dominant based on observations in the main entrance channel but found the upper estuary to experience flood-asymmetric flows they attributed to tidal channel and tidal flat distribution that resulted in net transport in the up-estuary direction. More accurate determination of whether an estuary is a source of, or a sink for, sediment requires consideration of additional factors, such as sediment characteristics, fluvial processes, estuarine stratification, and sea-level changes.

10.12.4.3

Waves

Waves may be generated within the estuary by local winds or they may be ocean waves that enter the mouth of the estuary. In tide-dominated estuaries, characterized by a relatively open connection to the ocean, waves generated in the ocean are capable of traveling into the estuary, although the magnitude of energy is reduced as a result of diffraction and dissipation caused by shallow water depths and bottom friction. Thus, the lower estuary, at and near the mouth, is subject to more ocean wave influence compared to the upper estuary (Dalrymple and Choi, 2007). Ocean waves and currents can form a spit or sand bar across the mouth when river flow is weak or episodic (Cooper, 1993). The formation of the bar may be seasonal and associated with depositional swell or it may be the result of longshore sediment transport (Ranansinghe et al., 1999; Ranansinghe and Pattiaratchi, 2003). The dimensions of the bar may provide a partial or complete barrier to closure. Breaching of the bar and reestablishment of the connection between the estuary and marine environment are dependent on the strength of river flow or waves and surge. There is interest in understanding the morphologic response to ocean-generated waves entering the mouth of the estuary. Ocean waves entering an estuary have been observed in a number of locations, including Delaware, Chesapeake, and Newport Bay, USA (Ludwick, 1987; Moory and Osborne, 1992), Lough Foyle, Northern Ireland

(Carter, 1980), Port Stephen Bay, Australia (Vila-Concejo et al., 2010), and Bay of Plenty, New Zealand (deLange and Healy, 1990). High-magnitude, low-frequency events can transport significant quantities of sediment from the ocean to the estuary such as reported by Vila-Concejo et al. (2010) and cause erosion to estuarine beaches, but ocean-generated storm waves can also result in depositional conditions on the foreshore (Jackson, 1995). Interest also exists for understanding the effect of waves (ocean and locally generated) in estuaries from a biological standpoint. The horseshoe crab (Limulus polyphemus) is considered a keystone species in Delaware Bay (USA) and spawns annually on the foreshores of the sandy beaches in the estuary. Waves generated in the estuary during storms as well as those entering the mouth of the estuary from the Atlantic Ocean are capable of suppressing spawning activity. Smith et al. (2002) found that horseshoe crab spawning was correlated to wave activity, with greater spawning densities associated with lower wave heights (o0.12 m). Vessel wakes can influence sediment suspension in an estuary (Schoellhamer, 1996) and sediment transport on beaches (Curtiss et al., 2009). Schoellhamer (1996) found that sediment suspended by vessel wakes were more susceptible to resuspension by tidal currents after deposition compared to undisturbed sediment. Curtiss et al. (2009) found vessel wakes contributed to sediment mobilization during nonstorm conditions on a mixed sand and gravel beach in Puget Sound (USA).

10.12.4.4

River Discharge

Sediment flux from rivers to the coastal ocean is episodic and has been associated with seasonal (Tamura et al., 2010) and decadal (Inman and Jenkins, 1999) timescales. Delivery of sediment from rivers to an estuary is a function of basin geology (size, relief, and erodibility of sediments), water flow, and human modifications to the drainage basin (change in land use/land cover) or the river channel (damming, diversion, and concretization) (Inman and Jenkins, 1999). Drainage basins with steep slopes and erodible material create conditions for rivers to deliver greater quantities of sediment to the estuary compared to drainage basins with low gradients and dominated by vegetated uplands. Increased impervious cover can increase streamflow due to runoff and result in increased streambed erosion, as described by Inman and Jenkins (1999). Episodic floods are an important process in estuarine dynamics; they may increase the quantity of sediment delivered to the estuary or result in estuarine scour and sediment export to the nearshore, thus ‘‘resetting the evolutionary clock’’ (Cooper, 1994b). In a comparison of tide-dominated and river-dominated micro-tidal estuaries, Cooper (2002) found that the morphological response to episodic floods was different based on spatial characteristics of erosion, flood magnitude thresholds to initiate morphologic change, and speed of post-flood sediment reworking. The episodic nature of river flow may reduce the capacity of the river to maintain a continuous connection with the sea, and the low magnitude of sediment discharge limits delta

Estuaries

development (Jennings and Bird, 1967). Many estuarine systems have intermittent connections to the sea (Roy, 1984; Cooper, 2001). Closure can be the result of low river discharge or is the result of marine sediment transport processes that create a wave-constructed berm (Carter et al., 1992; Elwany et al., 2003; Weir et al., 2006). These systems are dependent on periods of high river discharge to flush the inlet and reestablish a connection (Elwany et al., 1998). When the river flow is highly sporadic, there is a need to rely on human intervention to artificially open the inlet (Elwany et al., 2003; Weir et al., 2006; Morris and Turner, 2010).

10.12.5

Morphodynamics and Evolution

In addition to broad-scale, long-term processes of climate change and sea-level rise that influence rates of discharge and inundation, wave, tide, and river processes drive estuarine hydrodynamics and sediment transport and account for the distribution of marine and fluvial-derived sediment in the estuary and the morphology of sedimentary deposits. The morphodynamics of estuaries occur over different timescales ranging from short-term, episodic weather events that transport sediment, to longer-term changes that occur as a result of geologic and sea-level history.

10.12.5.1

Sediment Transport in Estuaries

Entrainment, transport, and deposition of sediment in an estuary are the result of a complex suite of river, tide, and wave processes operating over different temporal scales (Nichols and Biggs, 1985). Traditional research on hydrodynamics and sediment transport in estuaries was focused more on mean and residual or subtidal flows but current interest is focused on shorter-term processes such as turbulence and intratidal flows (Uncles, 2002). The textural properties of the sediment available to be entrained and transported is a controlling factor in the likelihood that erosion or deposition will occur. Entrainment will occur when the bed shear stress exceeds the critical value for the sediment sizes available. Once entrained, the coarser sediment will be transported via bedload and the finer sediment will be transported via suspended load. Sediment transported by river flow may exhibit a bimodal population of coarse and fine sediment. The coarser fraction will settle to the bed, whereas the finer fraction will remain in suspension (Dyer, 1995). The finer fraction may aggregate via flocculation that will influence the settling velocity and deposition on the bed. In meso- and macro-tidal environments, a turbidity maximum exists; a zone where suspended sediment concentrations are greater than those occurring in either the river or estuarine flow. The location of the turbidity maximum is a function of both river and tidal processes. High river discharge will move the zone seaward, and on shorter timescales the zone will shift in response to the rise and fall of the tide (Dyer, 1995). The development and maintenance of the turbidity maximum have been explained by residual circulation in partially mixed estuaries that returns fine sediment to the

315

upper reaches of the estuary. In macro-tidal estuaries, the turbidity maximum has been attributed to tidal asymmetries that increase sediment transport potential toward the head of the estuary (see Section 10.12.4.2).

10.12.5.2

Estuarine Geomorphic and Sedimentary Facies

Estuaries are generally considered ephemeral features. Their continued existence requires a balance between relative sealevel fluctuations (eustatic and isostatic) and sedimentation from river discharge (Knebel, 1986) and oceanic inputs (Woodroffe et al., 1993). Periods of progradation occur when the quantity of sediment deposited from fluvial or marine processes exceeds the rate of inundation due to sea-level rise. Progradation may also occur when there is a fall in sea level with no deposition, and the net result is the development of tidal flats and strand plains (Boyd et al., 1992). In the first case, the endpoint is the development of a delta (Dalrymple et al., 1992) characterized by the lack of a channel meander pattern and net bedload transport in the seaward direction. In the second case, the endpoint may be a strandplain or tidal flat, depending on the ratio of the wave height to the tidal range. Intermediate states exist between these two endpoints based on the process regime present. The type and distribution of morphologic and sedimentary facies have been used as diagnostics for determining the evolutionary stage of an estuary along the estuary–delta continuum (Roy, 1984; Dalrymple et al., 1992; Heap et al., 2004). Conceptual models have identified the relative contribution of fluvial and marine processes to estuarine morphology and evolution (see Roy, 1984; Dalrymple et al., 1992; Cooper, 1993) and are based, in part, on regional studies that characterize one estuary or compare multiple estuaries. These studies include tide-dominated, macro-tidal estuaries (Woodroffe et al., 1989; Allen, 1990; Dalrymple et al., 1990), tide-dominated, micro-tidal estuaries (Cooper, 2002), and micro-tidal estuaries in wave-dominated (Roy, 1984; Anthony et al., 1996) and riverdominated (Cooper, 1994b) environments.

10.12.5.2.1

Tide-dominated estuaries

Tide-dominated estuaries occur in macro-tidal environments (tidal range 44 m). They are generally funnel shaped with wide mouths and high current velocities. Tidal currents are the dominant process of transport of sediment compared to river or oceanic processes (Allen, 1990; Dalrymple and Choi, 2007). In tide-dominated estuaries, river flow decreases and tidal currents increase in the seaward direction (Dalrymple and Choi, 2007). The middle sector of the estuary (Figure 1) represents a zone of sediment accumulation with contributions from both fluvial and marine sources (Dalrymple and Choi, 2007). Dalrymple et al. (1990) characterized the sedimentary characteristics of the macro-tidal Cobequid Bay–Salmon River Estuary, in Canada. The axial sediments are characterized by the presence of elongate tidal sand bars in the lower sector of the estuary that trend parallel to the dominant current direction. Sand flats and braided channels are located in the middle sector of the estuary and a single channel is located in the river-dominated head of the estuary. Tidal currents are at a

316

Estuaries

maximum in the inner part of the estuary. Sediments decrease in size from the mouth to the head. Dominant direction of sediment transport is landward with accumulation in the upper sector at the head of the estuary. These characteristics stand in stark contrast to those that occur in the Kennebec River Estuary (USA). Fitzgerald et al. (2000) revealed that the characteristics of the rock-bound, macro-tidal estuary do not conform to the planform geometry or sedimentary characteristics described by Dalrymple et al. (1992) and they ascribed these differences to structural control of the estuary. The estuary is narrow and the bedrock-cut channel results in the lack of development of extensive intertidal sedimentary deposits (Fenster and Fitzgerald, 1996). The processes that influence the morphodynamics of the system are high-magnitude spring freshets that increase river discharge and deliver sediments to the lower estuary. A circulation cell is present in the lower estuary exchanging sediment among the estuary mouth, nearshore and offshore zones, and adjacent beaches.

10.12.5.2.2

Wave-dominated estuaries

Wave-dominated estuaries generally occur in micro-tidal (tidal range o2 m) environments and their morphologic evolution is tied to the changes that occur at the mouth of the estuary (Roy, 1984). Different types of barrier estuaries occur, based on the characteristics of the mouth of the estuary. A barrier estuary can form where sediment transported from adjacent ocean beaches creates a barrier at the mouth and the magnitude of river discharge controls the reopening. Barrier lagoons and inter-barrier estuaries form on low-relief coastal plains. Intermittent estuaries consist of basins that are separated from the ocean for extended periods of time. These estuaries generally form where river discharge is not competent to maintain an open channel through the barrier created by wave-induced transport of beach sediment (Anthony et al., 2002). High variability in rainfall regimes in some regions is an important factor in the development of highly saline lagoons. High waves associated with storms can breach the depositional barrier at the estuarine mouth and reestablish connection with the ocean or can occur via river flow during episodic floods. The complexity of the facies development in this type of estuary is a function of sediment infilling and barrier development. In general, these estuaries have two regions of high energy: an upper sector near the head, where river processes, sediments, and bedforms dominate, and a lower sector near the mouth, where wave and tidal processes and marine sediments dominate. The middle sector is a location where tidal currents are reduced, locally generated waves and windinduced flows are important, and extensive mudflats and marshes are present (Roy et al., 2001).

10.12.5.2.3

from the marine environment occur via transport through the inlets that separate the barriers as well as overwash transfers during storms. The dominant sand bodies in meso-tidal estuaries are the deltas (ebb and flood) formed by tidal inlet processes. The mixed wave-tide estuaries along the northeast Atlantic of the USA are generally flood dominant and have flood tidal deltas that are larger than their ebb counterparts. The result of this sediment accumulation is an extensive shallow-water zone on the seaward margin of the estuary that deepens in the direction of the mainland. Sediments near the inlet are coarse and sediments near the mainland margin are fine where fluvial processes are dominant (Psuty and Silveira, 2009).

10.12.5.2.4

River-dominated estuaries

Dalrymple et al. (1992) argued that a river-dominated category is not required because fluvial dominance is an indicator of the rate of infilling and not of morphology. Cooper (1993) presented evidence from the South African coast that riverdominated, micro-tidal estuaries do not necessarily display the characteristic downstream facies changes observed in waveand tide-dominated estuaries, and the energy level may remain similar along the axis of the river valley. River-dominated estuaries can range from those completely dominated by river processes (river channels) to those that experience some marine inputs at the mouth. Marine influence is minimized due to high fluvial discharge and steep gradients, and these estuaries do not exhibit coastal progradation. River-dominated estuaries may be in equilibrium when overall sediment volume does not change with time (Cooper, 1993) or in disequilibrium when there is a gradual increase in sediment volume over time (Sondi et al., 1995). Although there is a growing literature on estuarine morphologies from a range of environments and qualitative descriptions of some of the potential morphodynamic pathways that could occur, a need still exists for better quantification of both processes and responses. Heap et al. (2004) pointed out that the development of a quantitative model for estuary to delta transition is hampered by the lack of hard data on the abundance and distribution of sedimentary facies for a representative distribution of estuaries. Advances in instrumentation have enabled collection of high-frequency process data and achieve finer resolution of sedimentary deposits over space and through time. These data can also enable better integration of spatial and temporal scales of change besides enabling analytical approaches that have potential for advanced model development (Fitzgerald and Knight, 2005).

Mixed wave-tide-dominated estuaries

Mixed wave-tide estuaries (such as those in meso-tidal environments with a tidal range of 2–4 m) can be found behind barrier islands and barrier spits (Hayes, 1979). These estuaries owe much of their morphologic signature to the sediment characteristics of the barrier that forms the marine boundary. The estuaries may be classified as intermediate between tidedominated and wave-dominated systems (Dalrymple et al., 1992). Within the estuary are meandering tidal channels, point bars, and marsh deposits. Transfers of sediment

10.12.6

Estuarine Subenvironments

The cross-shore intertidal gradient of estuaries contains a rich variety of geomorphic assemblages, including tidal flats and barforms, marshes and mangroves, and sandy barriers and coastal bluffs. These subenvironments provide an array of ecosystem functions and services that increase the importance of understanding their morphodynamics.

Estuaries 10.12.6.1 10.12.6.1.1

Lower Intertidal Tidal flats

Tidal flats may be comprised of predominantly mud or sands or they may transition between muds or sands on a seasonal basis. The spatial variability in sediment composition and morphology of tidal flats is influenced by exposure to wave activity (Ryan and Cooper, 1998). The cross-shore profile of tidal flats is generally characterized by either continuously concave or convex morphology that is controlled by the tidal range and the wave climate (Whitehouse et al., 2000; Le Hir et al., 2000). Convex hypsometry has been associated with a large tidal range, long-term accretion, or low waves; concave hypsometry has been associated with a small tidal range, longterm erosion, or wave activity (Roberts et al., 2000; Uncles, 2002). Not all estuarine environments have typical tidal flat morphology. Eliot et al. (2006) described an estuarine beach in the Swan River Estuary, Australia, where the surface of the fronting terrace extends below mean low tide, but fluctuation in exposure and inundation of the terrace is associated with nontidal forcing of water levels in the estuary. Mudflats have been classified based on tidal range, slope, and exposure to wave activity (Dyer et al., 2000). Most current research on mudflats refers to short-term studies of sediment transport and morphologic change (Uncles, 2002). The processes that influence sediment transport and location of erosion and deposition are a function of wave activity of tidal currents (cross-shore and longshore) and wind-induced circulation (Le Hir et al., 2000). Dewatering of flat sediments at low tide and discharge of tidal creeks can also influence tidal flat morphology. In general, tides induce onshore transport of muds, whereas waves and drainage favor offshore transport (Le Hir et al., 2000).

10.12.6.1.2

Bedforms

Irregularities in bed elevation can be the result of physical forcing (waves, tides, and wind) as well as dewatering of surface sediment due to exposure at low tide, bioturbation, and presence of vegetation. The most conspicuous morphological features in the lower intertidal zone include both shoreparallel and transverse bars (Figure 3).

(a)

317

Shore-parallel sand bars occur in micro-, meso-, and macro-tidal environments. Located in the intertidal zone, they are persistent features (Yamada and Kobayashi, 2007). Offshore losses of sediment from the foreshore to these bars are rarely compensated by transport back to the foreshore (Nordstrom and Jackson, 1992). In drowned river-valley estuaries such as Delaware Bay (USA), the bars are of low amplitude (o0.10 m), extend approximately 0.5 km bayward, and are exposed during low spring tide (Botton et al., 2003). Data on vertical erosion and accretion reveal little annual net change, and response to storms is highly variable where multiple sites are compared (Miller et al., 2002). In microtidal environments, the bars can persist as subtidal forms. Dolan and Dean (1985) attributed the formation of subtidal sand bars in the micro-tidal Chesapeake Bay (USA) to multiple waves breaking across the low gradient slope. Ridge and runnel systems can occur in some estuaries (Carling et al., 2009). These bedforms trend parallel to the shoreline and have heights of up to 1.5 m and lengths ranging from 10 to 100 m where observed in estuaries (Carling et al., 2009). Hydrodynamic and sedimentologic data from the Severn Estuary (UK) demonstrated that the development of runnels was associated with fluid erosion and fine sediment suspension. The ridges that formed were accretionary units of silt and sand and not residual forms produced from the noneroded surface incised by runnels (Williams et al., 2008; Carling et al., 2009). Transverse bars are attached to the lower foreshore and are generally oriented perpendicular to the shoreline or at an angle that trends parallel to the dominant wave approach. These barforms occur along micro-tidal, low-energy shorelines of the Gulf of Mexico (USA), the sheltered shoreline of a barrier island on the Atlantic coast (USA), and in the Swan River Estuary in Western Australia (Bruner and Smosna, 1989; Nordstrom et al., 1996; Eliot et al., 2006). They can reach lengths of up to 100 m with amplitudes between 0.2 and 0.75 m (Niederoda and Tanner, 1970). In estuaries, the exchange of sediment between the foreshore and the transverse bars is low. Nordstrom et al. (1996) conducted a tracer experiment and found that cross-shore sediment transport from the foreshore to the transverse bars was concentrated within a

(b)

Figure 3 Photographs showing (a) shore-parallel bars in Delaware Bay (USA) and (b) transverse bars on the landward side of a barrier island in the Gulf of Mexico (USA).

318

Estuaries

short distance offshore from the location of bar attachment to the foreshore. Our understanding of the processes that give rise to the development, growth, maintenance, and possible migration of these forms is rudimentary. The development of transverse bars has been attributed to many processes, but within low-wave-energy environments, like estuaries, wave refraction is the leading driver (Niederoda and Tanner, 1970). This process explanation was later confirmed by Caballeria et al. (2002), who developed a nonlinear model to predict the conditions for formation of both crescentic and transverse bars in the nearshore. Their model qualitatively matches the shape and spacing of these bar forms as reported by field observations in estuaries (Niederoda and Tanner, 1970; Nordstrom et al., 1996), but a quantitative link still requires instrumented field measurements on wave processes and sediment transport.

10.12.6.2

Upper Intertidal Zone

zones, small-scale variations in submergence rates, effects of varying amounts of sediment in eroding formations, and effects of obstacles to longshore sediment transport, such as headlands, that define drift compartments (Nordstrom, 1992). Differences in the gradient of wave energy between the low-energy (upper) and high-energy (lower) shorelines in an estuary and between the high-energy (windward) and lowenergy (leeward) sides of an estuary also contribute to differences in the types of estuarine shoreline environments and their dimensions. Marsh is likely to form on alluvium in the upper reaches of the estuary, on the upwind side of the estuary or on the downwind side of the estuary in the lee of headlands that provide protection from breaking waves. Beaches are likely to form on the downwind side of estuaries, because there is sufficient energy in the locally generated waves to erode coastal formations or prevent vegetation from growing in the intertidal zone.

10.12.6.2.1

The planform configuration of estuaries is exceedingly complex and the location of intertidal subenvironments varies over relatively short, alongshore lengthscales (Phillips, 1986). Estuarine shoreline environments (beaches, marshes, and mangroves) generally occur in small isolated reaches with different orientations and with great variability in morphology, vegetation, and rates of erosion. This variability results from regional differences in fetch and water depth, exposure to dominant and prevailing winds, variations in subsurface stratigraphy, irregular topography inherited from drainage systems, differential erosion of vegetation or clay, peat, and marsh outcrops on the surface of the subtidal and intertidal

Unconsolidated shorelines

Beaches in estuaries form a class of low-energy types (Jackson et al., 2002). Beaches are most common where wave energy can entrain available sediments (Knebel et al., 1988) and they may front small, transgressive barriers (Cooper et al., 2007; Pilkey et al., 2009) or cliff and bluff environments (Jackson et al., 2002; Shipman, 2008) (Figure 4). Beaches may be unvegetated or partially vegetated and composed of sand, gravel, or shell (Nordstrom, 1992). The best development of beaches occurs where relatively high wave energies have exposed abundant unconsolidated sand or gravel in the eroding coastal formations. Adequate source materials occur where these formations are moraine deposits, submerged

Bluff shoreline Poorly sorted sediments

Ac Sand Reworked tiv ef bluff sediment Wa ores ve ho r cu t te e rra Gravel ce

Marsh barrier shoreline Overwash platform

Dune

Fo r

Marsh Well-sorted sands

Peat

es

ho

re

Low tide terrace

Figure 4 Types of upper intertidal environments in estuaries. Modified from Jackson, N.L., Nordstrom, K.F., Smith, D.R., 2002. Geomorphic–biotic interactions on beach foreshores in estuaries. Journal of Coastal Research SI36, 414–424.

Estuaries

glacial streams, coarse-grained fluvial deposits, and sand delivered by ocean waves and winds, such as the estuarine shorelines of spits and barrier islands (Freire et al., 2007). The textural properties of the sediments that form these beaches may be predominantly sand (Nordstrom and Jackson, 1993; Jackson et al., 2005), mixed sand and gravel (Curtiss et al., 2009), or predominantly gravel (Shipman, 2008). Beach formation is favored where high ground protrudes into relatively deep water, where wave refraction and wave energy loss through dissipation on the bay bottom are minimal (Nordstrom, 1992). The dominant processes of sediment reworking on beaches in estuaries are generally driven by locally generated waves, although refracted and diffracted ocean waves may be present. Ocean waves that enter the estuary generally create beaches close to the inlets. Sediment transported into the estuary by ocean waves may form spits in the lee of headlands in the estuary. Beaches created by waves generated within estuaries are most common in shoreline re-entrants, where sediments can accumulate over time. Other beaches occur where sand is plentiful on the bayside of barriers enclosing the estuary, particularly on former recurves, subaerial overwash platforms, and former oceanside dunes (Nordstrom, 1992). Beaches may form on the bayside of eroding marshes from coarse-grained sediment removed from the eroding substrate. Beaches may precede and favor marsh growth by creating spits that form lowenergy environments landward of them. Both processes create a beach-ridge shoreline that combines features characteristic of beach shorelines and marsh shorelines. Peat, representing the substrate of former marsh, is commonly exposed in outcrops on eroding beaches transgressing marshes. The outcrops are resistant because of the presence of fine-grained materials that have been trapped by upward growth of the marsh and by the binding effect of vegetation (Nordstrom, 1992). Sandy beaches in estuaries are characterized by a narrow backshore (o10 m), steep planar foreshore (6–91), and a broad, relatively flat, bay bottom, low-tide terrace (Nordstrom, 1992; Fenster et al., 2006; Freire et al., 2009) or subtidal terrace (Eliot et al., 2006). Reported heights of locally generated waves range from 0.15 to 0.50 m, with periods of 2–5 s, and they break directly on the foreshore and convert immediately to swash. The width of the swash zone can be only 3 m during small storms (Jackson and Nordstrom, 1993). Ocean waves entering estuaries during storms can result in increased wave periods (45.0 s) and swash runup widths (up to 9.0 m) (Nordstrom et al., 2006). The low wave energies on estuarine beaches limit reworking by storm erosion and post-storm deposition. Nordstrom (1992) reported upper foreshore losses during storms of more than 3.0 m3 m–1 and landward displacement of the foreshore of over 4 m, with breaking wave heights of 0.4–0.8 m and periods of 3.4–4.5 s. The cycle of offshore transport to a break-point bar, followed by post-storm deposition by swash-bar migration that is common on ocean beaches, is absent (Nordstrom, 1992), and the bayward extent of sediment exchange is generally o10 m. Potential for longshore sediment transport occurs because short-period estuarine waves undergo little refraction and may break at a relatively sharp angle to the shoreline (Nordstrom et al., 2003). Waves, tides, and currents are significant geomorphic agents on sandy estuarine beaches that account for temporal

319

variation in beach morphology and spatial variation in textural properties of sediments on the surface and down to the depth of wave reworking (Rosen, 1980; Sherman et al., 1994; Malvarez et al., 2001). Storm surge and low-frequency changes in ocean water level may play an important role in beach dynamics on micro-tidal environments (Armbruster et al., 1995; Eliot et al., 2006). Nordstrom and Jackson (1992) presented a profile change model for estuarine beaches based on differences in wave characteristics observed on 21 sand beaches with a tidal range of nearly 2.0 m. Two types of beach profile response were identified, depending on whether crossshore or longshore sediment transport dominated. In both cases, sediment exchange was limited to a zone between the upper limit of swash at high water and the break in slope separating the foreshore from the low tide terrace. On beaches where cross-shore transport is the dominant process, sediment exchange between the upper and lower foreshore is accomplished by changes in the locally generated wave energy level due to variations in wind speed. An increase in the wave height causes erosion of the upper foreshore and deposition on the lower foreshore, whereas a decrease in wave height returns sediment to the upper foreshore. Changes in wind direction and wave angle are responsible for inducing longshore sediment transport and may be responsible for parallel foreshore retreat (and advance) that is not accompanied by a conspicuous change in slope. Dominance of cross-shore or longshore processes is a function of shoreline orientation to the dominant winds and fetch, and the presence of shorenormal obstacles (such as shore protection structures) that act as sediment traps. Aeolian dunes occur in estuaries and may be relict dunes resulting from aeolian transport from the oceanside, or may be true estuarine landforms, the result of aeolian sediment transport in the estuary (Nordstrom and Jackson, 1994; Varnell et al., 2010). Dunes created by aeolian transport in the estuary occur where beaches are sufficiently wide to provide a viable sediment source or where the shoreline is stable enough to allow ample time for slow accretion or to prevent wave erosion (Nordstrom and Jackson, 1992). Estuarine beaches are narrow, even at low tide, so substantial source widths for aeolian transport occur only over a portion of each tidal cycle and only when the wind blows at an oblique angle to beach orientation (Nordstrom and Jackson, 1994). Wave energy must be sufficient to prevent colonization of intertidal vegetation, but erosion cannot be too great for aeolian forms to survive. Onshore aeolian transport occurring between moderate-intensity storms may create only a thin aeolian cap on top of the backbeach or overwash platform.

10.12.6.2.2

Cohesive shorelines

Coastal marshes are valuable as open space, as breeding grounds and food sources for marine and estuarine animals, as traps for sediments and nutrients from rivers and runoff, as natural filters in maintaining water quality, and as buffers to runoff from uplands (Mitsch and Gosselink, 1993). Marshes are a conspicuous subenvironment in estuaries and occur predominantly in temperate climates, whereas mangroves occur in subtropical and tropical environments (Woodroffe, 2003). The occurrence of marshes, like beaches, depends on their environmental setting and mode of origin, defined by factors

320

Estuaries

such as bedrock geology, availability of sediments, and recent sea-level rise history. Marshes occur in a range of estuarine basins, including drowned river valleys, bar-built estuaries, lagoons, and rias (Allen, 2000). Their location in the estuary can include the upper estuary at the margins of rivers, at the toe of coastal bluffs, and on the landward side of barrier islands and spits (Wood et al., 1989). The estuaries along the Atlantic coast of the USA include both drowned river-valley estuaries to the north and bar-built estuaries to the south. Drainage basin characteristics (size and land cover) and resultant sediment load are influential to the presence, size, and substrate characteristics of marshes and submerged vegetation (Roman et al., 2000). Marshes may develop within small embayments or at the mouth of stream channels (Knebel et al., 1988). Sediment in marshes tends to be fine grained but coarser sediments can occur where fluvial inputs are low and marine sources are high (Allen, 2000). Marshes occupy the upper intertidal zone and are affected by wave action, storm surge, and tidal flows that affect sediment transport and cycles of erosion and accretion. The ability for a marsh to maintain its position in the tidal frame is a function of the elevation of the marsh surface elevation in relation to sea level. Increase in surface elevation is the result of accumulation of sediment as well as biomass production (Reed, 2002), but subsurface processes are also important contributors to surface elevation changes (Cahoon, 2006). Marshes in the same estuary can undergo different levels of vulnerability to increases in sea level. In Chesapeake Bay (USA), submerged upland marshes on the eastern shore are characterized by a lack of a well-integrated, tidal creek network. Tidal creeks that do develop tend to be ebb dominant, resulting in export of sediment from the marsh. These marshes are more vulnerable than those that form in the meanders of the major tributaries and receive ample sediment inputs from flood events (Stevenson and Kearney, 1996). Mangroves are most commonly associated with tropical climates and are located in broad intertidal areas characterized by low wave energy and where there is a source of fine-grained sediment (Woodroffe, 1992). Mangroves exist in a range of environmental settings, including river-dominated, tidedominated, and wave-dominated environments (Thorn, 1982), and their morphology and evolution are influenced by characteristics of substrate and rates of sedimentation (Woodroffe, 1992). Tidal flow in mangroves is influenced by vegetation density, evapotranspiration, and groundwater flow (Wolanski, 1992). Mangroves generally exhibit a zonation of vegetation species (Thom et al., 1975), and the type of vegetation present is a function of inundation (frequency and duration), waterlogging of substrate and pore-water potential, and salinity (Woodroffe, 1992).

10.12.6.3

Geomorphic–Biotic Interactions

Great interest occurs concerning the interactions between geomorphology and biology (Black et al., 1998; Naylor, 2005), particularly in regions where species are threatened. Estuaries are one of the most ecologically productive environments, offering habitat for spawning, development of juveniles, and

foraging. Maintaining ecosystem functions and services will depend on our ability to synthesize multidisciplinary data into a format that can be used in a management context (Cooper et al., 1994). The relationship between geomorphic and biotic processes in estuaries has been examined at several scales to determine whether there is a link between biological pattern and estuarine morphology (Dethier and Schoch, 2005). At the macro-scale, the morphodynamics of the estuary influence species abundance and diversity. Species are sensitive to temperature and salinity gradients within an estuary, and these gradients are a function of shape and the connection of the estuary to the sea (Pritchard, 1967a). In a study of estuaries in southeastern Australia, Roy et al. (2001) identified four zones (marine flood-tidal delta, central mud basin, fluvial delta, and riverine channel/alluvial plain) that are common to these estuaries and ecologically important to estuarine species. The relative value of these zones is a function of the salinity gradient, the rate of infilling, and the influence of human development. They found that bar development at the mouth of the estuary influenced the ability of species to migrate into the estuary. The timing of opening and closure of the inlet of an estuary also have important ecological implications. Whitfield et al. (2008) found that successful recruitment of certain species of fish and invertebrates was influenced by the timing of the opening and closing of the estuarine inlet. The magnitude of breaching and depth of the inlet increased tidal exchange and salinity in the estuary prior to closure of the inlet that was beneficial for recruitment but the magnitude of river flow associated with breaching could also lead to declines in species in the estuary. One question that remains is whether estuarine classifications can be used to indicate biologic function. Edgar et al. (2000) classified 111 estuaries in Tasmania based on physical and salinity characteristics (seaward barrier, tidal range, salinity, estuary size, and river runoff) and statistically assessed their ability to serve as surrogates of biological pattern derived from invertebrate and fish data. Results revealed that classification of estuaries by a combination of salinity and physical variables was a more successful indicator than using a classic geomorphologic grouping of the estuaries. At the micro-scale, fauna alter sedimentary characteristics by mechanically activating and transporting sediments, bonding them by chemical secretions, or altering boundary layer conditions by changing bedform conditions (Rhoads and Stanley, 1965; Nowell et al., 1981; Jumars and Nowell, 1984; Meadows and Tait, 1989; Fries et al., 1999; Statzner et al., 2003). The relative roles of physical and faunal processes differ depending on the amount of wave and current energy, the grain-size characteristics of sediments, and species type. Intertidal species can decrease or increase the likelihood for entrainment of sediment by altering the sediment or bed properties (Widdows and Brinsley, 2002). Species can stabilize the bed by enhancing sediment cohesiveness and thus increase the critical erosion threshold (Andersen, 2001). Species can increase surface roughness and enhance erosion rates by bioturbation of the sediment (Wright et al., 1997). In lowwave-energy environments (such as estuaries), the biological processes can have an influence on sediment mobilization and transport of a magnitude comparable to that of waves and tidal currents alone (Wood and Widdows, 2002; Jackson et al.,

Estuaries

2005). Field investigations and development of models to predict sediment transport in estuaries need to account for these interactions (Uncles, 2002).

10.12.6.4

Human-Modified Estuarine Systems

Estuaries have experienced significant morphological and ecological changes from human modifications that have eliminated many intertidal environments, impaired water, and sediment quality, and threatened many of the species that depend on the estuary for all or part of their life history (Kennish, 2002). In some regions of the world (e.g., the midAtlantic coast of the US), the imprint of humans has had serious consequences for other species that occupy the estuary (Paul, 2001) but there are instances where these species have adapted to human-induced stresses (Botton et al., 2006). Remote-sensing and geographic information system (GIS) technologies have provided a platform for assessing estuarine parameters such as sedimentation rates (Zheng et al., 2010), morphology (Mason et al., 2010), and integrating data that can be utilized in a management framework for enhancing estuarine water quality (Ng et al., 2010). Humans can affect the geomorphology of estuaries by changes in land use and land cover of the upland or by armoring and diking the shore (Figure 5). These activities can alter sedimentation rates in an estuary relative to those that would occur under natural conditions. Historical changes in land use that eliminate forest cover can contribute to sedimentation (Willard et al., 2003), and modification via reclamation and embankment can reduce the intertidal area, tidal prism, and current velocities, and increase sedimentation (van der Wal et al., 2002). The leading methods of shoreline armoring in estuaries are installing bulkheads and performing beach nourishment. Despite the prominence of bulkheads on estuarine shorelines, few process-based studies of their effects have been conducted (Nordstrom et al., 2009). Many inferences concerning the physical effects of bulkheads in estuaries are based on other vertical structures in ocean environments (Kraus, 1988; Kraus and McDougal, 1996; Miles et al., 1997). Findings from studies on exposed shorelines reveal that the interaction of waves with structures results in an increase in wave reflection and

(a)

321

turbulence, nearshore current velocities, sediment activation, and longshore sediment transport at the base of the structure (Kraus, 1988; Plant and Griggs, 1992; Kraus and McDougal, 1996; Miles et al., 1997). Empirical field studies note the formation of scour pits directly in front of shore-parallel structures after storms (Morton, 1988), causing a lowering of the profile (Birkemeier et al., 1991), narrowing of the beachface (Hall and Pilkey, 1991), and slower recovery of the profile after storms (Nakashima and Mossa, 1991). Support for these findings in estuaries remains uncertain without further assessment. Beach nourishment is most generally associated with exposed coasts with intensive levels of development or great recreational value, but it is also important in estuaries where it can potentially provide beach habitat as well as shore protection (Nordstrom, 1992; Shipman, 2001; Jones and Hanna, 2004; Fenster et al., 2006; Andrade et al., 2006). Written documentation of appropriate volumes, sediment composition, and purposes of beach nourishment is lacking for many operations in estuaries, and few studies assess the effects of beach nourishment in estuarine environments once the fill is emplaced (Shipman, 2001; Jackson et al., 2007; Jackson et al., 2010). Differences in grain-size characteristics introduced in nourishment operations can result in differences in the form and mobility of estuarine beaches and their drainage (Nordstrom, 1992). Estuarine beaches have been nourished with source material dredged from offshore (Douglass and Weggel, 1987) and adjacent creeks or inlets (Fenster et al., 2006) or mined from inland sand and gravel quarries (Shipman, 2001). Use of nonbeach sources can result in departures in sediment size and sorting from native material. Preexisting surface gravel will be buried. Gravel is a prominent and ecologically valuable characteristic of many low-energy beaches (Nordstrom and Jackson, 1993; Rice, 2006; Ciavola and Castiglione, 2009), so this loss may be locally important. Most nourishment operations in estuaries involve placement of fill on the intertidal foreshore and are designed to build wider backshores and higher berm elevations to protect against wave erosion and overwash. Creation of a high berm has the advantage of increasing beach volume without covering the bay bottom. The restriction in the horizontal and vertical extent of reworking on estuarine beaches because of the low wave energies implies that naturalization of fill sediment will be a slow process. The

(b)

Figure 5 Photographs of bulkheading for shore protection in (a) Delware Bay and (b) Puget Sound, USA.

322

Estuaries

implications of these changes to the beach are the potential reduction in ecological value for species that forage, migrate, or transgress the beach environment (Jackson et al., 2010).

10.12.6.5

Restoration Practices

Restoration activities to enhance or create habitat are now widespread in many estuaries but the success of these projects over the long-term requires a better understanding of the linkages between physical, geomorphological, and biological processes. Habitat is defined as ‘‘the kind or range of environments in which a species/population/life history stage can live’’ (McCoy and Bell, 1991). Structurally, coastal habitat is made of sediment and vegetation reworked by the combined effects of waves, tides, wind, and currents. The use of estuarine habitat by species is spatially and temporally variable due to cross-shore and alongshore gradients in wave energy, salinity, temperature, and oxygen. Thus, the geographic location of optimal habitat for a population or species can vary from year to year. The question for many managers and restorationists is whether optimum habitat for a particular species is available in the proper location as physical and chemical changes shift (Peterson, 2003) and as humans manipulate the environment. In areas where geomorphic systems are stressed by high erosion rates or human alteration, restoration activities have attempted to create new habitat or enhance existing degraded habitat. In estuaries, restoration is most generally associated with the rehabilitation of degraded marsh systems (Teal and Weinstein, 2002; Wolters et al., 2005) and numerous case studies have documented the success and failure of restoration sites to track toward target states (Zedler and Callaway, 1999). A growing number of instances are known where beach environments are being restored to enhance habitat (Figure 6; Jackson et al., 2007). Beach nourishment is employed in estuaries primarily for shore protection but great interest exists from federal, state, and private agencies to use beach nourishment projects to enhance habitat while protecting human development. Beach nourishment is likely to preserve habitat value better than bulkheading, but nourishment has the potential to decrease

(a)

habitat value as well as enhance it, depending on morphology (Jackson et al., 2010) and sediment characteristics (Rice, 2006) of the nourished beach. The possibility of decreasing habitat value is of particular concern, because the application of nourishment will be more widespread in the future. Managed realignment, the landward movement of flooddefense structures to increase intertidal area, is a growing practice in Europe. Projects have focused on the re-establishment of tidal flats and marsh environments, but assessment of these types of projects from models and field data reveals the problems of re-establishing tidal exchange, replacing lost habitat and high project costs relative to recovery of ecosystem functions and services (Elliott et al., 2007; French, 2008; Hughes et al., 2009).

10.12.7

Future Issues

How estuaries respond to projected estimates of sea-level rise, including associated changes to habitat assemblages (sandy beaches, benthic areas, and marsh systems), is uncertain (Fitzgerald et al., 2008; Ganju and Schoellhamer, 2010). Increases in storms, coastal flooding, and water temperatures will change the form and function of estuaries in the future. Sediment availability and the geomorphic type of the estuary have been identified as important controls on the morphodynamics of the estuary system in response to these changes (Reeve and Karunarathna, 2009). If sediment influx to the system is continuous, the estuary will likely maintain its morphology. If sediment influx is restricted, morphologies will be reduced or eliminated. Potential impacts based on future climate-change scenarios in Chesapeake Bay (USA) include increased flooding and the elimination of marshes (Najar et al., 2010). Where intertidal zones are modified or eliminated by human alterations, the anticipated response of an estuarine transgression is prevented (Townend and Pethick, 2002). Construction of hard protection structures along the shoreline in response to sea-level rise will prevent the migration of the marsh inland and also sequester sediment that will be needed to maintain both marsh and beach systems.

(b)

Figure 6 Photographs of use of beach nourishment to restore ecosystem functions on human modified shorelines in (a) Delware Bay and (b) Puget Sound, USA.

Estuaries

The effects of human alteration on geomorphology and habitat assemblages over short time frames and the influence on estuarine evolution over long time frames have been established in the literature. A need exists to move from shortand long-term temporal scales toward decade and kilometer scales that will assist in solving near-term morphologic problems associated with managing estuarine environments (Uncles, 2002). For example, in some backbarrier environments the reduction in inlet breaching and overwash during storms that occurs as a result of current management of ocean beaches, in turn, reduces the transfer of sediment to the estuary from ocean sources. The restriction in sediment availability alters the sediment budget of the estuarine shoreline contributing to marsh loss and beach erosion (Nordstrom et al., 2009). The alteration, loss, or fragmentation of habitat, whether the result of natural or human processes, can impact resource exchange and ecological function in estuaries (Cloern, 2007). Developing policies or restoration plans based on future change scenarios will require better understanding of these geomorphic–biotic interactions in estuaries (Pethick, 1993) and the role of human actions through cross-disciplinary, collaborative research initiatives.

References Allen, J.R.L., 1990. The Severn Estuary in southwest Britain: its retreat under marine transgression and fine sediment regime. Sedimentary Geology 66, 13–28. 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. Andersen, T.J., 2001. Seasonal variation in erodibility of two temperate, microtidal mudflats. Estuarine, Coastal and Shelf Science 53, 1–12. Anderson, J.B., Rodriguez, A.B., 2008. Response of Upper Gulf Coast Estuaries to Holocene Climate Change and Sea-Level Rise. Special Paper 443. Geological Society of America, Boulder, CO. Andrade, C., Lira, F., Pereira, M.T., Ramos, R., Guerreiro, J., Freitas, M.C., 2006. Monitoring the nourishment of Santo Amaro estuarine beach (Portugal). Journal of Coastal Research SI39, 776–782. Anthony, E.J., Lang, J., Oyede, L.M., 1996. Sedimentation in a tropical, microtidal, wave-dominated coastal-plain estuary. Sedimentology 43, 665–675. Anthony, E.J., Oyede, L.M., Lang, J., 2002. Sedimentation in a fluvially infilling, barrier-bound, wave-dominated, microtidal coast: the Ouene river estuary, Benin, west Africa. Sedimentology 49, 1095–1112. Armbruster, C.K., Stone, G.W., Xu, J.P., 1995. Episodic atmospheric forcing and bayside foreshore erosion: Santa Rosa Island, Florida. Gulf Coast Geological Society Transactions 45, 31–37. Birkemeier, W.A., Bichner , E.W., Scarborough, B.L., McCarthy, M.A., Eiser, W.C., 1991. Nearshore profile response caused by Hurricane Hugo. Journal of Coastal Research SI8, 113–127. Black, K.S., Paterson, D.M., Cramp, A. (Eds.), 1998. Sedimentary Processes in the Intertidal Zone. Geological Society of London Special Publications. The Geological Society Publishing House, Bath, UK, 139 pp. Botton, M.L., Loveland, R.E., Tanacredi, J.T., Itow, T., 2006. Horseshoe crabs (Limulus polyphemus) in an urban estuary (Jamaica Bay, New York) and the potential for ecological restoration. Estuaries and Coasts 29, 820–830. Botton, M.L., Loveland, R.E., Tiwari, A., 2003. Distribution, abundance, and survivorship of young-of-the-year in a commercially exploited population of horseshoe crabs (Limulus polyphemus). Marine Ecology Progress Series 265, 175–184. Boyd, R., Dalrymple, R., Zaitlin, B.A., 1992. Classification of clastic coastal depositional environments. Sedimentary Geology 80, 139–150. Brown, J.M., Davies, A.G., 2010. Flood/ebb asymmetry in a shallow sandy estuary and the impact on net transport. Geomorphology 114, 431–439. Bruner, K.R., Smosna, R.A., 1989. The movement and stabilization of beach sand of transverse bars, Assateague-Island, Virginia. Journal of Coastal Research 5, 593–601.

323

Bucher, D., Saenger, P., 1991. An inventory of Australian estuaries and enclosed marine waters: an overview of results. Australian Geographical Studies 29, 370–381. Caballeria, M., Coco, G., Falques, A., Huntley, D.A., 2002. Self-organization mechanisms for the formation of nearshore crescentic and transverse sand bars. Journal of Fluid Mechanics 465, 379–410. Cahoon, D.R., 2006. A review of major storm impacts on coastal wetland elevations. Estuaries and Coasts 29, 889–898. Cameron, W.M., Pritchard, D.W., 1963. Estuaries. In: Hill, M.N. (Ed.), The Sea, Wiley, New York, NY, Volume 2, pp. 306–324. Carling, P.A., Williams, J.J., Croudace, I.W., Amos, C.L., 2009. Formation of mud ridge and runnels in the intertidal zone of the Severn Estuary, UK. Continental Shelf Research 29, 1913–1926. Carter, R.W.G., 1980. Longshore variations in nearshore wave processes at Magilligan Point, Northern Ireland. Earth Surface Processes 5, 81–89. Carter, R.W.G., Bauer, B.O., Davidson-Arnott, R.G.D., Gares, P.A., Nordstrom, K.F., Orford, J.D., Sherman, D.J., 1992. Dune development in the aftermath of stream outlet closure: examples from Ireland and California. In: Carter, R.W.G., Curtis, T.G.F., Sheehy-Skeffington, M.J. (Eds.), Coastal Dunes: Geomorphology, Ecology, and Management for Conservation. Kluwer Academic Publishers, Boston, pp. 57–70. Castaing, P., Guilcher, A., 1995. Geomorphology and sedimentology of rias. In: Perillo, G.M.E. (Ed.), Geomorphology and Sedimentology of Estuaries, Developments in Sedimentology 53. Elsevier Science, Amsterdam, pp. 69–111. Cattaneo, A., Steel, R.J., 2003. Transgressive deposits: a review of their variability. Earth Science Reviews 62, 187–228. Cloern, J., 2007. Habitat connectivity and ecosystem productivity: implications from a simple model. American Naturalist 169, E21–E33. Chappell, J., Woodroffe, C.D., 1994. Macrotidal estuaries. In: Carter, W.R.G., Woodroffe, C.D. (Eds.), Coastal Evolution: Late Quaternary Shoreline Morphodynamics. Cambridge University Press, Cambridge, pp. 187–218. Ciavola, P., Castiglione, E., 2009. Sediment dynamics of mixed sand and gravel beaches at short time-scales. Journal of Coastal Research SI56, 1751–1755. Conley, D.J., Kaas, H., Møhlenberg, F., Rasmussen, B., Windolf, J., 2000. Characteristics of Danish estuaries. Estuaries 23, 820–837. Cooper, J.A.G., 1993. Sedimentation in a river-dominated estuary. Sedimentology 40, 979–1017. Cooper, J.A.G., 1994a. Lagoons and microtidal coasts. In: Carter, W.R.G., Woodroffe, C.D. (Eds.), Coastal Evolution: Late Quaternary Shoreline Morphodynamics. Cambridge University Press, Cambridge, pp. 219–265. Cooper, J.A.G., 1994b. Sedimentary processes in the river-dominated Mvoti Estuary, South Africa. Geomorphology 9, 271–300. Cooper, J.A.G., 2001. Geomorphological variability among microtidal estuaries from the wave-dominated South African coast. Geomorphology 40, 99–122. Cooper, J.A.G., 2002. The role of extreme floods in estuary-coastal behavior: contrasts between river- and tide-dominated microtidal estuaries. Sedimentary Geology 150, 123–137. Cooper, J.A.G., Lewis, D.A., Pilkey, O.H., 2007. Fetch-limited barrier islands: overlooked coastal landforms. GSA Today 17, 4–9. Cooper, J.A.G., Ramm, A.E.L., Harrison, T.D., 1994. The Estuarine Health Index: a new approach to scientific information transfer. Ocean and Shoreline Management 25, 103–141. Cooper, J.A.G., Wright, C.I., Mason, T.R., 1999. Geomorphology and sedimentology. In: Allanson, B.R., Baird, D. (Eds.), Estuaries of South Africa. Cambridge Academic Press, Cambridge, pp. 5–25. Curtiss, G.M., Osborne, P.D., Horner-Devine, A.R., 2009. Seasonal patterns of coarse sediment transport on a mixed sand and gravel beach due to vessel wakes, wind waves, and tidal currents. Marine Geology 259, 73–85. Dalrymple, R.W., Choi, K., 2007. Morphologic facies trends through the fluvialmarine transition in tide-dominated depositional systems: a schematic framework for environmental and sequence-stratigraphic interpretation. Earth Science Reviews 81, 135–174. Dalrymple, R.W., Knight, R.J., Zaitlin, B.A., Middleton, G.V., 1990. Dynamics and facies model of a macrotidal sand–bar complex, Cobequid Bay-Salmon River Estuary (Bay of Fundy). Sedimentology 37, 577–612. Dalrymple, R.W., Zaitlin, B.A., Boyd, R., 1992. Estuarine facies models: conceptual models and stratigraphic implications. Journal of Sedimentary Petrology 62, 1130–1146. Dame, R., Alber, M., Allen, D., et al., 2000. Estuaries of the south Atlantic coast of North America: their geographical signatures. Estuaries 23, 793–819. Davies, J.H., 1964. A morphogenetic approach to world shorelines. Zeitschrift fur Geomorphologie 8, 127–142.

324

Estuaries

Day, J.H., 1981. The nature, origin and classification of estuaries. In: Day, J.H. (Ed.), Estuarine Ecology with Particular Reference to Southern Africa. Balkema, Rotterdam, pp. 1–6. Day, J.W., Hall, C.A.S., Kemp, W.M., Yanez-Arancibia, A. (Eds.), 1989. Estuarine Ecology. Wiley, New York, NY, 558 pp. deLange, W., Healy, T., 1990. Wave spectra for a shallow meso-tidal estuarine lagoon: Bay of Plenty, New Zealand. Journal of Coastal Research 6, 189–199. Dethier, M.N., Schoch, G.C., 2005. The consequences of scale: assessing the distribution of benthic populations in a complex estuarine fjord. Estuarine, Coastal and Shelf Science 62, 253–270. Dolan, T.J., Dean, R.G., 1985. Multiple longshore sand bars in the upper Chesapeake Bay. Estuarine Coastal and Shelf Science 21, 727–743. Douglass, S.L., Weggel, J.R., 1987. Performance of a perched beach – Slaughter Beach, Delaware. Proceedings of Coastal Sediments ’87. American Society of Civil Engineers, New York, NY, pp. 1385–1398. Dronkers, J., 1986. Tidal asymmetry and estuarine morphology. Netherlands Journal of Sea Research 20, 117–131. Dyer, K., 1995. Sediment transport processes in estuaries. In: Perillo, G.M.E. (Ed.), Geomorphology and Sedimentology of Estuaries, Developments in Sedimentology 53. Elsevier Science, Amsterdam, pp. 423–449. Dyer, K.R., 1997. Estuaries: A Physical Introduction. Second ed. Wiley, Chichester, 195 pp. Dyer, K.R., Christie, M.C., Wright, E.W., 2000. The classification of intertidal mudflats. Continental Shelf Research 20, 1039–1060. Edgar, G.J., Barret, N.S., Graddon, D.J., Last, P.R., 2000. The conservation significance of estuaries: a classification of Tasmanian estuaries using ecological, physical, and demographic attributes as a case study. Biological Conservation 92, 383–397. Edwards, P., 2007. Sea levels: resolution and uncertainty. Progress in Physical Geography 31, 621–632. Eliot, M.J., Travers, A., Eliot, I., 2006. Morphology of a low-energy beach, Como Beach, Western Australia. Journal of Coastal Research 22, 63–77. Elliott, M., Burdon, D., Hemingway, K.L., Aoitz, S.E., 2007. Estuarine, coastal and marine ecosystem restoration: confusing management and science – a revision of concepts. Estuarine Coastal and Shelf Science 74, 349–366. Elliott, M., McLusky, D.S., 2002. The need for definitions in understanding estuaries. Estuarine Coastal and Shelf Science 55, 815–827. Elwany, M.H.S., Flick, R.E., Aijaz, S., 1998. Opening and closure of a marginal southern California lagoon inlet. Estuaries 21, 246–254. Elwany, M.H.S., Flick, R.E., Hamilton, M.M., 2003. Effect of a small southern California lagoon entrance on adjacent beaches. Estuaries and Coasts 26, 700–708. Emmett, R., Llanso, R., Newton, J., et al., 2000. Geographic signatures of North American west coast estuaries. Estuaries 23, 765–792. Evans, G., Prego, R., 2003. Rias, estuaries and incised valleys: is a ria an estuary? Marine Geology 196, 171–175. Fairbridge, R.W., 1980. The estuary: its definition and geodynamic cycle. In: Olausson, E., Cato, I. (Eds.), Chemistry and Biogeochemistry of Estuaries. Wiley, Chichester. Fenster, M.S., Fitzgerald, D.M., 1996. Morphodynamics, stratigraphy, and sediment transport patterns of the Kennebec River Estuary, Maine, USA. Sedimentary Geology 107, 99–120. Fenster, M.S., Knisley, C.B., Reed, C.T., 2006. Habitat preference and the effects of beach nourishment on the federally threatened northeaster beach tiger beetle, Cicindela dorsalis: Western shore, Chesapeake Bay, Virginia. Journal of Coastal Research 22, 1133–1144. FitzGerald, D.M., Buynevich, I.V., Fenster, M.S., McKinlay, P.A., 2000. Sand dynamics at the mouth of a rock-bound tide-dominated estuary. Sedimentary Geology 131, 25–49. FitzGerald, D.M., Fenster, M.S., Argow, B.A., Buynevich, I.V., 2008. Coastal impacts due to sea-level rise. Annual Review of Earth and Planetary Sciences 36, 601–647. FitzGerald, D.M., Knight, J., 2005. High Resolution Morphodynamics and Sedimentary Evolution of Estuaries. Springer, Dordrecht. Freire, P., Ferreira, O., Taborda, R., et al., 2009. Morphodynamics of fetch-limited beaches in contrasting environments. Journal of Coastal Research SI56, 183–187. Freire, P., Taborda, R., Silva, A.M., 2007. Sedimentary characterization of Tagus estuarine beaches (Portugal) – a contribution to the sediment budget assessment. Journal of Soils and Sediments 7, 296–303. French, J.R., 2008. Hydrodynamic modeling of estuarine flood defence realignment as an adaptive management response to sea-level rise. Journal of Coastal Research 24(2B), 1–12.

Friedrichs, C.T., Aubrey, D.G., 1988. Non-linear tidal distortion in shallow well-mixed estuaries: a synthesis. Estuarine, Coastal and Shelf Science 27, 521–546. Fries, J.S., Butman, C.A., Wheatcroft, R.A., 1999. Ripple formation induced by biogenic mounds. Marine Geology 159, 287–302. Ganju, N.K., Schoellhamer, D.H., 2010. Decadal-timescale estuarine geomorphic change under future scenarios of climate and sediment supply. Estuaries and Coasts 33, 15–29. Guilcher, A., 1967. Origins of sediments in estuaries. In: Lauff, G.H. (Ed.), Estuaries, Publication No. 83. AAAS, Washington, DC, pp. 139–157. Hall, M.J., Pilkey, O.M., 1991. Effects of hard stabilization on dry beach width for New Jersey. Journal of Coastal Research 7, 771–785. Hardisty, J., 2007. Estuaries: Monitoring and Modeling the Physical System. Blackwell Publishing, Singapore, 157 pp. Hayes, M.O., 1975. Morphology of sand accumulation in estuaries: an introduction to the symposium. In: Cronin, L.E. (Ed.), Estuarine Research, II. Academic Press, New York, NY, pp. 3–22. Hayes, M.O., 1979. Barrier island morphology as a function of tidal and wave regime. In: Leatherman, S.P. (Ed.), Barrier Islands. Academic Press, New York, NY, pp. 2–21. Healy, T.R., Russell, C., de Lange, W., 1996. Geomorphology and ecology of New Zealand shallow estuaries and shorelines. In: Nordstrom, K.F., Roman, C.T. (Eds.), Estuarine Shores: Evolution, Environments and Human Alterations. Wiley, New York, NY, pp. 115–145. Heap, A.D., Bryce, S., Ryan, D.A., 2004. Facies evolution of Holocene estuaries and deltas: a large-sample statistical study from Australia. Sedimentary Geology 168, 1–17. Heap, A.D., Nichol, S.L., 1997. The influence of limited accommodation space on the stratigraphy of an incised-valley succession: Weiti River Estuary, New Zealand. Marine Geology 144, 229–252. Herdendorf, C.E., 1990. Great lakes estuaries. Estuaries 13, 493–503. Hopkinson, C.S., Hoffman, F.A., 1984. The estuary extended – a recipient-system study of estuarine outwelling in Georgia. In: Kennedy, V. (Ed.), The Estuary as a Filter. Academic Press, New York, NY, pp. 313–330. Hughes, R.G., Fletcher, P.W., Hardy, M.J., 2009. Successional development of saltmarsh in two managed realignment areas in SE England, and prospects for saltmarsh restoration. Marine Ecology Progress Series 384, 13–22. Hume, T.M., Herdendorf, C.E., 1993. On the use of empirical stability relationships for characterising estuaries. Journal of Coastal Research 9, 413–422. Hume, T., Snelder, T., Weatherhead, M., Liefting, R., 2007. A controlling factor approach to estuary classification. Ocean and Coastal Management 50, 905–929. Inman, D.L., Jenkins, S.A., 1999. Climate change and the episodicity of sediment flux of small California rivers. Journal of Geology 107, 251–270. Jackson, N.L., 1995. Wind and waves: influence of local and non-local waves on mesoscale beach behavior in estuarine environments. Annals of the Association of American Geographers 85, 21–37. Jackson, N.L., Nordstrom, K.F., 1993. Depth of activation of sediment by plunging breakers on a steep sand beach. Marine Geology 115, 143–151. Jackson, N.L., Nordstrom, K.F., Smith, D.R., 2002. Geomorphic–biotic interactions on beach foreshores in estuaries. Journal of Coastal Research SI36, 414–424. Jackson, N.L., Nordstrom, K.F., Smith, D.R., 2005. Influence of waves and horseshoe crab spawning on beach morphology and sediment grain-size characteristics on a sand estuarine beach. Sedimentology 52, 1097–1108. Jackson, N.L., Nordstrom, K.F., Saini, S., Smith, D.R., 2010. Effects of nourishment on the form and function of an estuarine beach. Ecological Engineering 36, 1709–1718. Jackson, N.L., Smith, D.R., Nordstrom, K.F., Tiyarattanachai, R., 2007. Evaluation of a small beach nourishment project to enhance habitat suitability for horseshoe crabs. Geomorphology 89, 172–185. Jennings, J., Bird, E.C.F., 1967. Regional geomorphological characteristics of some Australian estuaries. In: Lauff, G.H. (Ed.), Estuaries, Publication No. 83. AAAS, Washington, DC, pp. 121–128. Jones, K., Hanna, E., 2004. Design and implementation of an ecological engineering approach to coastal restoration at Loyola Beach, Klegberg County, Texas. Ecological Engineering 22, 249–261. Jumars, P.A., Nowell, A.R.M., 1984. Effects of benthos on sediment transport: difficulties with functional grouping. Continental Shelf Research 3, 115–130. Kang, J.W., Jun, K.S., 2003. Flood and ebb dominance in estuaries in Korea. Estuarine, Coastal and Shelf Science 56, 187–196. Kench, P.S., 1999. Geomorphology of Australian estuaries: Review and prospect. Australian Journal of Ecology 24, 367–380.

Estuaries

Kennish, M.J., 2002. Environmental threats and environmental futures of estuaries. Environmental Conservation 29, 78–107. Kjerfve, B. (Ed.), 1994. Coastal Lagoon Processes. Elsevier Publishers, Amsterdam. Knebel, H.J., 1986. Holocene depositional history of a large glaciated estuary, Penobscot Bay, Maine. Marine Geology 73, 215–236. Knebel, H.J., Fletcher, C.H., Kraft, J.C., 1988. Late Wisconsinan–Holocene paleogeography of Delaware bay; a large coastal plain estuary. Marine Geology 83, 115–133. Kraus, N.C., 1988. The effects of seawalls on the beach. Journal of Coastal Research SI 4, 1–28. Kraus, N.C., McDougal, W.G., 1996. The effects of seawalls on the beach: Part 1, An updated literature review. Journal of Coastal Research 12, 691–701. Kurtz, J.C., Detenbeck, N.D., Engle, V.D., Ho, K., Smith, L.M., Jordan, S.J., Campbell, D., 2006. Classifying coastal waters: current necessity and historical perspective. Estuaries and Coasts 29, 107–123. Lambeck, K., Chappell, J., 2001. Sea level change through the last glacial cycle. Science 292, 686. Le Hir, P., Roberts, W., Cazaillet, O., Christie, M., Bassoullet, P., Bacher, C., 2000. Characterization of intertidal flat hydrodynamics. Continental Shelf Research 20, 1433–1459. Little, C., 2000. Biology of Soft Shores and Estuaries. Oxford University Press, Oxford, 264 pp. Ludwick, J.C., 1987. Mechanisms of sand loss from an estuarine groin system following an artificial sand fill. Technical Report 87-2. Old Dominion University Department of Oceanography, Norfolk, VA. Malvarez, G., Cooper, J.A.G., Jackson, D.W.T., 2001. Relationships between waveinduced currents and sediment grain size on a sandy intertidal flat. Journal of Sedimentary Research 71, 705–712. Mason, D., Scott, T., Dance, S., 2010. Remote sensing of intertidal morphological change in Morecambe Bay, UK, between 1991 and 2007. Estuarine, Coastal and Shelf Science 87, 487–496. McCoy, E.D., Bell, S.S., 1991. Habitat structure: the evolution of and diversification of a complex topic. In: Bell, S.S., McCoy, E.D., Mushinsky, H.R. (Eds.), Habitat Structure: The Physical Arrangements of Objects in Space. Chapman and Hall, London, pp. 3–27. McLusky, D.S., Elliott, M., 2007. Transitional waters: a new approach, semantics or just muddying the waters? Estuarine. Coastal and Shelf Science 71, 359–363. Meadows, P.S., Tait, J., 1989. Modification of sediment permeability and shear strength by two burrowing invertebrates. Marine Biology 101, 75–82. Miles, J.R., Russell, P.E., Huntley, D.A., 1997. Sediment transport and wave reflection near a seawall. Proceedings of the 25th Coastal Engineering Conference. American Society of Civil Engineers, New York, NY, pp. 2612–2624. Miller, D.C., Muir, C.L., Hauser, O.A., 2002. Detrimental effects of sedimentation on marine benthos: what can be learned from natural processes and rates? Ecological Engineering 19, 211–232. Mitsch, W., Gosselink, J., 1993. Wetlands. Van Nostrand Rheinhold, New York, NY, 539 pp. Moore, R.D., Wolf, J., Souza, A.J., Flint, S.S., 2009. Morphological evolution of the Dee Estuary, eastern Irish Sea, UK: a tidal asymmetry approach. Geomorphology 103, 588–596. Moory, R., Osborne, R.H., 1992. Causes for the short-term periodic erosion of an artificial beach. Lower Newport Bay, CA. Bulletin of the Association of Engineering Geologists 29, 355–369. Morris, B.D., Turner, I.L., 2010. Morphodynamics of intermittently open-closed coastal lagoon entrances: new insights and a conceptual model. Marine Geology 271, 55–66. Morton, R.A., 1988. Interactions of storms, seawalls and beaches of the Texas coast. Journal of Coastal Research SI4, 113–134. Morton, R.A., Ward, G.H., White, W.A., 2000. Rates of sediment supply and sealevel rise in a large coastal lagoon. Marine Geology 167, 261–284. Najar, R.G., Pyke, C.R., Adams, M.B., et al., 2010. Potential climate change impacts on the Chesapeake Bay. Estuarine Coastal and Shelf Science 86, 1–20. Nakashima, L.D., Mossa, J., 1991. Responses to natural and seawall backed beaches to recent hurricanes on the Bayou Lafourche headland, Louisiana. Zeitschrift fur Geomorphologie, N.F. 35, 239–256. Naylor, L.A., 2005. The contributions of biogeomorphology to the emerging field of geobiology. Palaeogeography, Palaeoclimatology, Palaeoecology 219, 35–51. Ng, S., Wai, O., Xu, Z., Li, Y., 2010. Integrating GIS with hydrodynamic model for wastewater disposal and management: Pearl River Estuary. In: Greene, D.R. (Ed.), Coastal and Marine Geospatial Technologies. Springer, New York, NY, pp. 207–217. Nichols, M.M., 1989. Sediment accumulation rates and relative sea-level rise in lagoons. Marine Geology 88, 201–219.

325

Nichols, M.M., Biggs, R.B., 1985. Estuaries. In: Davis, Jr. R.A. (Ed.), Coastal Sedimentary Environments, Second ed. Springer, New York, NY, pp. 77–186. Niederoda, A.W., Tanner, W.F., 1970. Preliminary study on transverse bars. Marine Geology 9, 41–62. Nordstrom, K.F., 1992. Estuarine Beaches. Elsevier, London. Nordstrom, K.F., Jackson, N.L., 1992. Two dimensional change on sandy beaches in estuaries. Zeitschrift fu¨r Geomorphologie 36, 465–478. Nordstrom, K.F., Jackson, N.L., 1993. Distribution of surface pebbles with changes in wave energy on a sandy estuarine beach. Journal of Sedimentary Petrology 63, 1152–1159. Nordstrom, K.F., Jackson, N.L., 1994. Aeolian processes and dunefields in estuaries. Physical Geography 15, 358–371. Nordstrom, K.F., Jackson, N.L., Allen, J.R., Sherman, D.S., 1996. Wave and current processes and beach changes on a microtidal lagoonal beach at Fire Island, New York, USA. In: Nordstrom, K.F., Roman, C.T. (Eds.), Estuarine Shores: Evolution, Environments and Human Alterations. Wiley, New York, NY, pp. 213–232. Nordstrom, K.F., Jackson, N.L., Allen, J.R., Sherman, D.J., 2003. Longshore sediment transport rates on a microtidal estuarine beach. Journal of Waterway, Port, Coastal, and Ocean Engineering 129, 1–4. Nordstrom, K.F., Jackson, N.L., Rafferty, P., Raineault, N.A., Grafals-Soto, R., 2009. Effects of bulkheads on estuarine shores: an example from Fire Island National Seashore, USA. Journal of Coastal Research SI56, 188–192. Nordstrom, K.F., Roman, C.T. (Eds.), 1996. Estuarine Shores: Evolution, Environments and Human Alterations. Wiley, New York, NY. Nowell, A.R.M., Jumars, P.A., Eckman, J.E., 1981. Effects of biological activity on the entrainment of marine sediments. Marine Geology 42, 133–153. O’Brien, M.P., 1931. Estuary tidal prism related to entrance areas. Civil Engineering 1, 738–739. Paul, R.W., 2001. Geographical signatures of middle Atlantic estuaries: historical layers. Estuaries 24, 151–166. Perillo, G.M.E. (Ed.), 1995a. Geomorphology and Sedimentology of Estuaries, Developments in Sedimentology 53. Elsevier Science, Amsterdam, pp. 17–47. Perillo, G.M.E., 1995b. Definitions and geomorphologic classifications of estuaries. In: Perillo, G.M.E. (Ed.), Geomorphology and Sedimentology of Estuaries, Developments in Sedimentology 53. Elsevier Science, Amsterdam, pp. 17–47. Peterson, M.S., 2003. A conceptual view of environment-habitat-production linkages in tidal river estuaries. Reviews in Fisheries Science 11, 291–313. Pethick, J., 1993. Shoreline adjustments and coastal management: physical and biological processes under accelerated sea-level rise. Geographical Journal 159, 162–168. Pethick, J., 2001. Coastal management and sea level rise. Catena 42, 307–322. Phillips, J.D., 1986. Spatial analysis of shoreline erosion, Delaware Bay, New Jersey. Annals of the Association of American Geographers 76, 50–62. Pilkey, O.H., Cooper, J.A.G., Lewia, D.A., 2009. Global distribution and geomorphology of fetch-limited barrier islands. Journal of Coastal Research 25, 819–837. Pirazzoli, P.A., 1993. Global sea-level changes and their measurement. Global and Planetary Change 8, 135–148. Plant, N.G., Griggs, G.B., 1992. Interactions between nearshore processes and beach morphology near a seawall. Journal of Coastal Research 8, 183–200. Postma, H., 1967. Sediment transport and sedimentation in the estuarine environment. In: Lauff, G.H. (Ed.), Estuaries, Publication No. 83. AAAS, Washington, DC, pp. 158–179. Potter, I.C., Chuwen, B.M., Hoeksema, S.T., Elliott, M., 2010. The concept of an estuary: a definition that incorporates systems which can become closed to the ocean and hypersaline. Estuarine Coastal and Shelf Science 87, 497–500. Pritchard, D.W., 1952. Salinity distribution and circulation in the Chesapeake Bay estuarine system. Journal of Marine Research 11, 106–123. Pritchard, D.W., 1967a. What is an estuary: physical viewpoint. In: Lauff, G.H. (Ed.), Estuaries, Publication No. 83. AAAS, Washington, DC, pp. 3–5. Pritchard, D.W., 1967b. Observation of circulation in coastal plain estuaries. In: Lauff, G.H. (Ed.), Estuaries, Publication No. 83. AAAS, Washington, DC, pp. 37–44. Psuty, N.P., Morriera, M.E.S.A., 2000. Holocene sedimentation and sea level rise in the Sado Estuary, Portugal. Journal of Coastal Research 16, 125–138. Psuty, N.P., Silveira, T.M., 2009. Geomorphological evolution of estuaries: the dynamic basis for morpho-sedimentary units in selected estuaries in the northeastern United States. Marine Fisheries Review 71, 34–45. Pye, K., Allen, J.R.L., 2000. Coastal and Estuarine Environments: Sedimentology, Geomorphology and Geoarcheology. Special Publication 175. Geological Society, London, 435 pp. Ranansinghe, R., Pattiaratchi, C., 2003. The seasonal closure of tidal inlets: causes and effects. Coastal Engineering Journal 45, 601–627.

326

Estuaries

Ranansinghe, R., Pattiaratchi, C., Masselink, G., 1999. A morphodynamic model to simulate the seasonal closure of tidal inlets. Coastal Engineering 37, 1–36. Reed, D.J., 2002. Sea-level rise and coastal marsh sustainability: geological and ecological factors in the Mississippi delta plain. Geomorphology 48, 233–243. Reeve, D.E., Karunarathna, H., 2009. On the prediction of long-term morphodynamic response of estuarine systems to sea level rise and human interference. Continental Shelf Research 29, 938–950. Rhoads, D.C., Stanley, D.J., 1965. Biogenic graded bedding. Journal of Sedimentary Petrology 35, 956–963. Rice, C.A., 2006. Effects of shoreline modification on a northern Puget Sound beach: microclimate and embryo mortality to surf smelt (Hypomesus pretiosus). Estuaries 29, 63–71. Roberts, W., Le Hir, P., Whitehouse, R.J.S., 2000. Investigation using simple mathematical models of the effect of tidal currents and waves on the profile shape of intertidal mudflats. Continental Shelf Research 20, 1089–1097. Rodriguez, A.B., Green, D.L., Anderson, J.B., Simms, A.R., 2008. Response of Mobile Bay and eastern Mississippi Sound, Alabama to changes in sediment accommodation and accumulation. In: Anderson, J.B., Rodriguez, A.B. (Eds.), Response of Upper Gulf Coast Estuaries to Holocene Climate Change and Sea-Level Rise. Special Paper 443. Geological Society of America, Boulder, CO, pp. 13–29. Roman, C.T., Jaworski, N., Short, F.T., Finlay, S., Warren, R.S., 2000. Estuaries of the northeastern United States: habitat and land use signatures. Estuaries 23, 743–764. Rosen, P.S., 1980. Erosion susceptibility of the Virginia Chesapeake Bay shoreline. Marine Geology 34, 45–59. Roy, P.S., 1984. New South Wales estuaries – their origin and evolution. In: Thom, B.G. (Ed.), Developments in Coastal Geomorphology in Australia. Academic Press, New York, NY, pp. 99–121. Roy, P.S., Williams, R.J., Jones, A.R., Yassini, I., Gibbs, P.J., 2001. Structure and function of south-east Australian estuaries. Estuarine, Coastal and Shelf Science 53, 351–384. Russell, R.J., 1967. Origins of estuaries. In: Lauff, G.H. (Ed.), Estuaries, Publication No. 83. AAAS, Washington, DC, pp. 93–99. Ryan, N.M., Cooper, J.A.G., 1998. Wave-mediated spatial variability in intertidal flat morphology, Strangford Lough, Northern Ireland. In: Black, K.S., Paterson, D.M., Cramp, A. (Eds.), Sedimentary Processes in the Intertidal Zone. Geological Society of London, Special Publications, London, vol. 139, pp. 221–230. Schoellhamer, D.H., 1996. Anthropogenic sediment resuspension mechanisms in a shallow microtidal estuary. Estuarine, Coastal and Shelf Science 43, 533–548. Sherman, D.J., Nordstrom, K.F., Jackson, N.L., Allen, J.R., 1994. Sediment mixingdepths on a low-energy reflective beach. Journal of Coastal Research 10, 297–305. Shipman, H., 2001. Beach nourishment on Puget Sound, a review of existing projects and potential applications. In: Puget Sound Research 2001. Puget Sound Water Quality Action Team, Olympia, Washington, pp. 1–8. Shipman, H., 2008. A Geomorphic Classification of Puget Sound Nearshore Landforms. Puget Sound Nearshore Partnership Report No. 2008-01. Published by Seattle District, U.S. Army Corps of Engineers, Seattle, Washington. Slagle, A.L., Ryan, W.B.F., Carbotte, S.M., Bell, R., Nitsche, F.O., Kenna, T., 2006. Late-stage estuary infilling controlled by limited accommodation space in the Hudson River. Marine Geology 232, 181–202. Smith, D.R., Pooler, P.S., Loveland, R.E., Botton, M.L., 2002. Horseshoe crab (Limulus polyphemus) reproductive activity on Delaware bay beaches: interactions with beach characteristics. Journal of Coastal Research 18, 730–740. Sondi, I., Juracic, M., Pravdic, V., 1995. Sedimentation in a disequilibrium riverdominated estuary: the Rasˇa River Estuary (Adriatic Sea, Croatia). Sedimentology 42, 769–782. Statzner, B., Peltret, O., Tomanova, S., 2003. Crayfish as geomorphic agents and ecosystem engineers: effect of a biomass gradient on baseflow and floodinduced transport of gravel and sand in experimental streams. Freshwater Biology 48, 147–163. Stevenson, J.C., Kearney, M.S., 1996. Shoreline dynamics on the windward and leeward shores of a large temperate estuary. In: Nordstrom, K.F., Roman, C.T. (Eds.), Estuarine Shores: Evolution, Environments and Human Alterations. Wiley, New York, NY, pp. 233–260. Syvitski, J.P.M., Shaw, J., 1995. Sedimentology and geomorphology of fjords. In: Perillo, G.M.E. (Ed.), Geomorphology and Sedimentology of Estuaries, Developments in Sedimentology 53. Elsevier Science, Amsterdam, pp. 113–178. Tagliapietra, D., Sigovini, M., Ghirardini, A.V., 2009. A review of terms and definitions to categorise estuaries, lagoons and associated environments. Marine and Freshwater Research 60, 497–509.

Tamura, T., Horaguchi, K., Saito, Y., et al., 2010. Monsoon-influenced variations in morphology and sediment of a mesotidal beach on the Mekong River delta coast. Geomorphology 116, 11–23. Teal, J.M., Weinstein, M.P., 2002. Ecological engineering, design, and construction considerations for marsh restorations in Delaware Bay, USA. Ecological Engineering SI18, 607–618. Thorn, E., 1982. Mangrove Ecology – A Geomorphological Perspective. Australian National University Press, Canberra, p. 1. Thom, B., Wright, L., Coleman, J., 1975. Mangrove ecology and deltaic-estuarine geomorphology: Cambridge Gulf-Ord River, Western Australia. Journal of Ecology 63, 203–232. Townend, I.H., Wang, Z.B., Rees, J.G., 2007. Millennial to annual volume change in the Humber Estuary. Philsophical Transactions of the Royal Society London: Series A 463, 837–854. Townend, I., 2005. An examination of empirical stability relationships for UK estuaries. Journal of Coastal Research 21, 1042–1053. Townend, I., Pethick, J., 2002. Estuarine flooding and managed retreat. Philosophical Transactions of the Royal Society London: Series A 360, 1477–1495. Uncles, R.J., 2002. Estuarine physical processes research: some recent studies and progress. Estuarine, Coastal and Shelf Science 55, 820–856. van der Wal, D., Pye, K., Neal, A., 2002. Long-term morphological change in the Ribble Estuary, northwest England. Marine Geology 189, 249–266. Varnell, L., Hardaway, Jr. C.S., Milligan, D., 2010. Classification of fetch limited dunes in the lower Chesapeake Bay: evidence of morphologic equilibrium. Journal of Coastal Research 26, 663–672. Vila-Concejo, A., Hughes, M., Short, A.D., Ranansinghe, R., 2010. Estuarine shoreline processes in a dynamic low energy system. Ocean Dynamics 60, 285–298. Wang, Z.B., Jeuken, M.C.J.L., Gerritsen, H., De Vriend, H.J., Kornman, B.A., 2002. Morphology and asymmetry of the vertical tide in theWesterschelde estuary. Continental Shelf Research 22, 2599–2609. Weir, F.M., Hughes, M.G., Baldock, T.E., 2006. Beach face and berm morphodynamics fronting a coastal lagoon. Geomorphology 82, 331–346. Whitehouse, R.J.S., Bassoullet, P., Dyer, K.R., Mitchener, H.J., Roberts, W., 2000. The influence of bedforms on flow and sediment transport over intertidal mudflats. Continental Shelf Research 20, 1099–1124. Whitfield, A.K., Adams, J.B., Bate, G.C., et al., 2008. A multidisciplinary study of a small, temporarily open/closed South African estuary, with particular emphasis on the influence of mouth state on the ecology of the system. African Journal of Marine Science 30, 453–473. Widdows, J., Brinsley, M., 2002. Impact of biotic and abiotic processes on sediment dynamics and the consequences to the structure and functioning of the intertidal zone. Journal of Sea Research 48, 143–156. Willard, D.A., Cronin, T.M., Verardo, S., 2003. Late-Holocene climate and ecosystem history from Chesapeake Bay sediment cores, USA. Holocene 13, 201–214. Williams, J.J., Carling, P.A., Bell, P.S., 2008. Field investigation of ridge-runnel dynamics on an intertidal mudflat. Estuarine, Coastal and Shelf Science 79, 213–229. Wolanski, E., 1992. Hydrodynamics of mangrove swamps and their coastal waters. Hydrobiologia 247, 141–161. Wolters, M., Garbutt, A., Baker, J.P., 2005. Salt-marsh restoration: evaluating the success of de-embankments in north-west Europe. Biological Conservation 123, 249–268. Wood, M.E., Kelly, J.T., Bellnap, D.F., 1989. Patterns of sediment accumulation in the tidal marshes of Maine. Estuaries 12, 237–246. Wood, R., Widdows, J., 2002. A model of sediment transport over an intertidal transect, comparing the influences of biological and physical factors. Limnology and Oceanography 47, 848–855. Woodroffe, C., 1992. Mangrove sediments and geomorphology. In: Roberston, A.I., Alongi, D.M. (Eds.), Tropical Mangrove Ecosystems, American Geophysical Union, Washington, DC, pp. 7–42. Woodroffe, C.D., 2003. Coasts: Form, Process and Evolution. Cambridge University Press, Cambridge, 623 pp. Woodroffe, C., Chappell, J., Thom, B.G., Wallensky, E., 1989. Depositional model of a macrotidal estuary and floodplain, South Alligator River, northern Australia. Sedimentology 36, 737–756. Woodroffe, C., Mulrennan, M., Chappell, J., 1993. Estuarine infill and coastal progradation, southern van Diemen Gulf, northern Australia. Sedimentary Geology 83, 257–275. Wright, L.D., Schaffner, L.C., Maa, J.P.-Y., 1997. Biological mediation of bottom boundary layer processes and sediment suspension in the lower Chesapeake Bay. Marine Geology 141, 27–50.

Estuaries

Yamada, F., Kobayashi, N., 2007. Intertidal multiple sand bars in a low-energy environment. Journal of Waterway, Port, Coastal and Ocean Engineering 133, 343–351. Zedler, J.B., Callaway, J.C., 1999. Tracking wetland restoration: do mitigation sites follow desired trajectories? Restoration Ecology 7, 69–73.

327

Zheng, Z., Zhou, Y., Li, X., Kuang, R., 2010. Determination of sedimentation rate of tidal flats at the Yangtze estuary, China, using multi-temporal Landsat TM images. 18th International Conference on Geoinformatics. IEEE, Beijing, China, pp, 1–6.

Biographical Sketch Nancy Jackson is a professor in the Department of Chemistry and Environmental Science at New Jersey Institute of Technology. Her research focuses on coastal processes on beaches and dunes in estuarine and ocean environments. She held the Fulbright Distinguished Chair in environmental science at the Polytechnic in Turin in 2005. She is an associate editor of Estuaries and Coasts. She received her bachelor’s degree from Clark University, her master’s degree from Antioch New England Graduate School, and her doctorate from Rutgers University.