Deltas

Definition

A delta is a discrete shoreline sedimentary protuberance formed where a river enters an ocean, a semi-enclosed sea, an estuary, a lake, or lagoon and supplies sediment more rapidly than it can be redistributed by basinal processes (modified after Elliott, 1986).

Deltas in an estuarine environment

A delta is often closely associated in time and space with an estuary (Figure 1), but frequently in the literature the two are not adequately separated, particularly for tide-dominated estuaries. For the same riverine outlet, a delta is a geomorphic and sedimentologic feature, while an estuary is a hydrochemical one where riverine freshwater flowing into a bay, a lagoon, or semi-enclosed coastal body of water mixes with seawater (Cameron and Pritchard, 1963; Pritchard, 1967; Day, 1981). Deltas may have either a perennial or a seasonal freshwater flow and hence a perennial or seasonal freshwater-to-seawater transition resulting in some parts of them being estuarine.

Deltas, Figure 1

Idealized diagram showing the gradation from a relatively narrow v-shaped open estuary with minimal or no sedimentary fill to sediment-filling estuaries to a coastal delta where sedimentary accretion has prograded into the marine environment. Intra-estuarine deltas are present and more clearly evident in estuaries where sediments have not fully occluded them.

To a large extent, all deltas can be estuarine in the sense that some part of them will have a freshwater-to-seawater transition, and large estuarine environments whose basin has not been filled with sediment may contain small-scale deltas along their margins or in their headwaters (Figure 2). Geomorphologists and sedimentologists, focused on landforms and stratigraphy, generally do not deal with the hydrochemical estuarine components of deltas, and conversely, researchers of estuarine ecology, hydrochemistry, or hydrodynamics generally have focused on deltas in an estuary only in terms of geomorphology, sedimentology, or stratigraphy. This difference of emphasis becomes important here because the deltas described in this contribution are those occurring in the context of a larger estuarine setting: as such, a “delta within an estuary” is distinguished from an “estuary within a delta” (Figure 3). This contribution focuses on the “delta within an estuary.”

Deltas, Figure 2

Idealized diagram showing a range of estuary types, from an incised single valley to rias, a flooded valley on a coastal plain, a barred estuarine coastal lagoon and a compound estuary, and the occurrence of intra-estuarine deltas (black) therein.

Deltas, Figure 3

Idealized diagram showing the dual concepts of an estuary within a large delta and a delta within a large estuary (or an intra-estuarine delta). In each example, the field of salinity is freshwater = black, brackish water = gray, and marine water = white.

Deltas within estuaries generally are relatively small sedimentary accumulations compared to the size of their estuarine setting (Figure 2). They have been variably termed as “bayhead deltas” (cf. van Heerden and Roberts, 1988; Dalrymple et al., 1992; Kindinger et al., 1994), “river deltas” (Hayes, 1975), and “intra-estuarine deltas” (Semeniuk et al., 2011). As not all of them are located in “bayheads,” the term “intra-estuarine delta” is used here for those deltas occurring within estuaries.

In contrast, deltas in open coastal settings generally are large sedimentary accumulations but are relevant to smaller deltas that occur within estuaries in that the principles involving hydrodynamics, geometry/morphology, mechanisms of construction, sedimentology and facies, and stratigraphy are similar. The Mississippi Delta complex, Nile Delta, Niger Delta, São Francisco Delta, Klang Delta, and Fly Delta are examples of large open coastal deltas (Allen, 1970; Coleman et al., 1970; Gould, 1970; Summerhayes et al., 1978; Dominguez, 1996; Baker et al., 2009). Such deltas have been classified as to their plan geometry in response to their hydrodynamic setting as fluvial-dominated deltas, tide-dominated deltas, and wave-dominated deltas (Galloway, 1975) or by their depositional architecture and facies (Postma, 1990). For completeness in the descriptions of deltas, the reader is referred to geomorphic and stratigraphic descriptions of such open coastal deltas in Scruton (1960), Morgan (1970), Wright and Coleman (1973), Galloway (1975), Coleman (1976), Reineck and Singh (1980), Elliott (1986), Nemec (1990), Postma (1990), and Hori and Saito (2003).

Factors determining types of deltas in estuaries

Depending on the size and shape of the estuary, a delta within an estuary can be variable in terms of plan geometry (morphology), landforms within the delta, sedimentary facies, and stratigraphy. The main factors determining the morphology and landforms of deltas in estuaries are (1) seasonality and strength of river flow, (2) the salinity of the receiving estuarine basin, (3) the magnitude of tides, (4) wind and wind waves, and (5) the shape of the estuary and where the river(s) is/are located. A number of these factors are interrelated and combine to produce a given delta type or delta form. For instance, the seasonality and strength of river flow can affect the salinity of the receiving basin in that strong perennial river flow will ensure that the receiving basin is perennially brackish, particularly where the tidal regime is microtidal. Similarly, the extent to which wind and wind waves can influence delta morphology and landforms can be dependent on the external shape of the estuary and the location of the river(s) in relation to the wind field and wave field.

The seasonality and strength of the river flow determines whether the delta will be fluvial dominated, tide dominated, or wave dominated. Perennial rivers, deriving from large drainage basins in humid climates, with strong river flow (and, commonly, concomitant strong sediment transport), produce fluvial-dominated conditions at the river outlet. In this setting, delta morphology, controlled by fluvial conditions, tends generally to be fan-shaped varying to elongate and digitate. With fluvial-dominated conditions, the salinity of the estuarine-receiving basin also can play an important part in determining the style and course of river flow into the estuary and hence the shape of any deltaic sedimentary accumulation. In this context, it should also be noted that the dynamics of sediment-laden river flow with its various amounts of traction load and/or suspension load entering and interacting with an estuarine-receiving water body of different density (ranging from fresh to brackish to marine) will result in different types of deltaic depositional morphology. Bates (1953), Wright (1978), and Orton and Reading (1993) describe this variability of depositional styles and resulting delta forms in relationship to three situations: (1) hypopycnal flows in which density of the suspended sediment flow is less than that of the receiving estuarine water body, (2) homopycnal flows in which density of the suspended sediment flow is equal to that of the receiving estuarine water body, and (3) hyperpycnal flows in which density of the suspended sediment flow is more than that of the receiving estuarine water body. Hypopycnal, homopycnal, or hyperpycnal flows also determine the nature of river mouth dynamics as to whether buoyant, inertial, or frictional factors are dominant in distributing and shaping the sediment plume and sand bars (Bates, 1953; Coleman, 1976; Wright, 1978) and the shape of any freshwater jet as it enters a more saline estuary (Wright, 1978).

Riverine freshwater flowing into an estuarine basin of denser brackish water or marine salinity will exhibit hypopycnal flow, with freshwater overlying the denser estuarine water. Riverine freshwater flowing into an estuarine basin of similar freshwater, or turbid freshwater flowing into brackish water, will exhibit homopycnal flow, with the river water invading the estuarine water of similar density in a turbulent mixing front. At the other extreme, sediment-laden turbid riverine freshwater flowing into an estuarine basin of freshwater or weakly brackish salinity will exhibit hyperpycnal flow, with the denser sediment-laden river flow (comprising sediment in suspension and transported in traction) forming a base flow under the less-dense estuarine water.

However, where riverine input is seasonal, or where the sediment-transporting river flow is inter-annual, tides and wind waves will predominate as the formative agents in delta type and in the development of its plan geometry. The flux of tides on a daily or semidiurnal basis can have a prevailing influence on determining delta shape, and river sediment delivered to the mouth of the river in a mesotidal or macrotidal estuary will be redistributed and sculptured by tidal currents and shaped into tidal-current-elongated shoals. In regions with strong winds, wind waves are generated on estuarine water bodies and impinge on delta fronts. Depending on fetch, and particularly if the river mouth is downwind in an estuary with a large fetch, deltaic sediments deposited at the river mouth will be subject to prevailing wind waves. As a result, wave-dominated deltas will develop.

The shape of the estuary and position of the river(s) within it play important roles in determining delta morphology. Relatively simple estuaries, that are v-shaped, narrow linear valley tracts with a single river mouth at the head of the estuary, are subject to interactions of river flow, tidal flux, and wind waves, and the delta developed at the estuary head will tend to be the form indicative of the locally dominant hydrodynamic condition. Complex estuaries and large estuaries with large fetch, on the other hand, can create conditions where there are complicated hydrodynamics of waves and wind-induced currents, and in situations where there is more than one river entering the estuary, each river may be subject to differing hydrodynamics. There will also be differences in the deltas where the rivers have dissimilar flow magnitudes. Within the one estuarine basin with multiple river inflows and multiple deltas, one delta may be fluvial dominated; others may be tide dominated or wave dominated. The deltas in these types of complex estuaries are even more variable if the various contributing rivers arise from different geological provenances in their respective hinterland and are delivering different suites of sediments.

Types of deltas within estuaries

Given that a delta is a sedimentary deposit formed where a river enters an ocean, a semi-enclosed sea, an estuary, a lake or lagoon, the hydrodynamic forces operating on these sediments to distribute, rework, and shape them into various types of deltaic bodies are river flow (fluvial hydrodynamics), waves, and tides. If any of these hydrodynamic forces are dominant, the resulting delta will be fluvial dominated, wave dominated, or tide dominated (Figures 4 and 5). A classification of deltas based on their resultant morphology deriving from the style of hydrodynamic forcing was developed by Morgan (1970), Wright and Coleman (1973), Galloway (1975), and Coleman (1976). While the emphasis on delta classification in the literature has been on open coastal deltas, the classification can equally be applied to deltas wholly contained or confined in semi-enclosed water bodies, estuaries, lakes, and lagoons.

Deltas, Figure 4

The traditional tripartite classification of deltas (Modified from Galloway, 1975; Reineck and Singh, 1980) based on open marine deltas and showing the Mississippi Delta, the São Francisco Delta, and the Fly Delta as examples of fluvial-dominated, wave-dominated, and tide-dominated deltas, respectively. Examples also are shown of intra-estuarine deltas from southwestern Australia to illustrate fluvial-dominated forms (Harvey River Delta), wave-dominated forms (Deep River Delta), partly tide-dominated and fluvial-dominated forms (Preston River Delta) and a delta that shows longitudinal sectors that are fluvial dominated, wave dominated, and tide dominated (Frankland River Delta).

Deltas, Figure 5

The traditional tripartite classification of deltas of Figure 4 with some intra-estuarine deltas from southwestern Australia on the ternary diagram.

Deltas can be subdivided into various geomorphic/sedimentologic units related to location within the delta. These geomorphic/sedimentologic units can vary in size from delta to delta, and not all may be present in every delta. From the river hinterland to the deeper water into which the delta progrades, the units are (Coleman and Wright, 1975; Hart, 1996) alluvial feeder, upper delta plain, lower delta plain, delta front, delta slope, and prodelta. The mechanics of delta formation in different hydrodynamic situations are well summarized by Hart (1996).

Where fluvial processes dominate over the two other hydrodynamic forces, the resulting delta is termed a “fluvial-dominated delta.” Its morphology is determined by river flow transporting sediment loads in traction and suspension, and, depending on the salinity of the receiving estuarine water body and the nature of influx (whether hypopycnal, homopycnal, or hyperpycnal), the delta can be lobate, fan-shaped, elongate, or digitate. The delta progrades into the estuary by subaqueous deposition of a fan of sand or bar-finger sand, shoaling to high-tidal levels and river flood levels. The fans of sand or bar-finger sands are capped by tidal deposits, levee deposits, and floodplain deposits, while interdistributary bays are filled with tidal flat deposits shoaling to floodplain deposits. The overall delta form consists of (1) a subaerial part whose plan shape is lobate, fan-shaped, elongate, or digitate and whose geomorphic/sedimentologic components include levee banks, floodplains, high-tidal to supratidal flats, lagoons, and abandoned channels; (2) a delta slope comprised of sheets, fans or bar-fingers of sand or muddy sand, and laterally intervening bays underlain by sand or mud; and (3) a prodelta usually underlain by mud that forms a peripheral apron around the delta slope.

Where wave action is dominant because the delta resides in an estuary with a strong component of wind and wind-generated waves, regardless of the mechanism that delivers sediment to the front of the river mouth (viz., hypopycnal, homopycnal, or hyperpycnal), the sediment deposited at the river mouth is subsequently reworked shoreward into a series of beach ridges, or recurved spits, or bars and their leeward lagoons, all built by waves and wind to levels of the high tide and above. Progradation of the delta thus is by beach ridge accretion, recurved spit accretion, or as a series of bars and lagoons. The delta usually is a lobate complex of prograded beach ridges, a series of beach ridges and/or recurved spits with intervening swales and/or linear lagoons, or a prograded series of bars and linear, oval to circular lagoons. The beach ridges, recurved spits, and bars are underlain by sand and are often accreted to above the level of high tide. The swales and linear, oval to circular lagoons, depending on the style of sedimentary filling, are underlain by sand, muddy sand, mud, or peat. River floods, unable to reach the height of beach ridges, are confined to the distributary channels or flood into the beach ridge swales.

Where tides are the dominant hydrodynamic force, again, regardless of the mechanism that delivers sediment to the front of the river mouth, the sediment is subsequently reworked by tidal currents into tidal-current-aligned (usually shore-normal) subaqueous sand shoals that accrete vertically to levels of the high tide. Progradation of the delta thus is by tidal shoal vertical accretion to a level where the deposits are finally capped by floodplain sediments. The delta front (or delta slope) usually is a crenulate to palmate complex of prograded subaqueous to tidal shoals, and the landward part of the delta is a floodplain.

Locally in estuaries, where river gradients are relatively steep, there may be development of Gilbert-type deltas. These are a specific type of fluvial-dominated delta, usually fan-shaped and coarse-grained, with internal geometry of simple large cross-stratification corresponding to the delta morphology of topset, foreset, and bottomset (Postma, 1990).

While deltas can be classified as fluvial dominated, tide dominated, or wave dominated depending on their hydrodynamic setting, often in estuaries, because of the complexity of the hydrodynamics, an intra-estuarine delta may exhibit different morphology in different parts of the delta or contrasting landforms reflective of hydrodynamic conditions in a specific part of that delta. For instance, the wave-dominated intra-estuarine delta of the Deep River in southern Western Australia (the Walpole-Nornalup Inlet Estuary; Semeniuk et al., 2011) comprises two distinct geomorphic responses reflecting different degrees of wave action and sediment transport/mobility. There are prograded beach ridges on one half of the delta and a prograded bar-and-lagoon complex on the more sheltered other half. In the same estuary, another intra-estuarine delta (the Frankland River) also reflects the variable hydrodynamic forces across the delta depositional environment. It comprises a wave-dominated part in its northern third (prograded beach ridges and inter-ridge swales), a fluvial-dominated part in its central third (prograded and shoaled digitate/palmate sedimentary accumulation), and a tide-dominated part in its southern third (prograded sand platform). Within an intra-estuarine delta in another estuary in southwestern Australia (the Leschenault Inlet Estuary; Semeniuk, 2000), one side of a fluvial-dominated palmate delta faces a 12 km fetch, and during intermittent winter storms deriving from the north-west, waves break on the shore to create a repetition of storm-wave-generated cheniers across the floodplain.

In hydrodynamically and geomorphically complex estuaries with multiple river entries, there may be a range of intra-estuarine types within the same estuary. For instance, the Peel-Harvey Estuary of southwestern Australia, with three river entries (Semeniuk and Semeniuk, 1990a; Semeniuk and Semeniuk, 1990b), has two wave-dominated deltas (composed of prograded bar-and-lagoon complexes) because they face the prevailing regional summer breezes that generate wind waves on the estuarine water body and one fluvial-dominated delta (composed of prograded fans of sand, levee deposits, and floodplain deposits) that is not subject to these wind waves. The Leschenault Inlet Estuary, with two river entries (Semeniuk, 2000), has one fluvial-dominated delta (a palmate delta) and another delta that is, in part, tide dominated (composed of tidally-aligned shoals) and, in part, fluvial dominated. The Walpole-Nornalup Inlet Estuary of southern Australia, with three river entries (Semeniuk et al., 2011), has two wave-dominated estuaries (composed of prograded bar-and-lagoon complexes, or of beach ridges) because they face the prevailing regional summer breezes that generate wind waves on the estuarine water body and one hydrodynamically complex delta that is one third wave dominated (facing the wind waves generated by sea breezes), one third fluvial dominated, and one third tide dominated (the latter two not subject to wind waves).

Factors determining the stratigraphy of deltas in estuaries

Deltas within estuaries exhibit a variety of stratigraphic sequences, depending on the sediments available, and their hydrodynamic setting. However, given that estuaries as enclosed to semi-enclosed coastal bodies of water where multiple interactions between marine and riverine environments may take place, there are other factors that result in a richness and variety of stratigraphy in intra-estuarine deltas. These differences not only occur between deltas from different estuaries but even between deltas within the same estuary. The main factors determining the stratigraphy within a delta are (1) the provenance of the contributing rivers and types of sediment entering the estuary, (2) seasonality and strength of river flow, (3) nature of tides, (4) contribution of wind and wind waves, and (5) climate.

The provenance of the contributing rivers entering the estuary, that is, the geology of the drainage basins, determines the supply and composition of sediment delivery. Whether the delta is sand, mixed sand and mud, or mud-dominated, how much gravel is present and the type of shoaling stratigraphy that is developed are factors dependent not only on provenance but also on the hydrodynamic factors at the interface of river and estuary. The effects of seasonality and strength of river flow, the magnitude of the tides, and the influence of wind in generating wind waves and circulation currents on intra-estuarine deltaic stratigraphy are expressed in the dominant grain size which results in, by the way in which the gravel, sand, and mud are separated into different deltaic environments, the distance the gravel, sand, and mud are transported from the river mouth, the types of sedimentological mechanisms that occur, and the range of sedimentary structures that are generated. Differences can result in upward shoaling, sediment interlayering, the development of environment-specific sedimentary structures, and the micro-stratigraphic and macro-stratigraphic sequences.

Climate plays a part in the development of deltaic stratigraphic sequences in that rainfall and evaporation can determine the nature of the high-tidal and supratidal lithologies (whether they are vegetated and replete with plant root bioturbation or are mud-cracked and have generated mud chips or contain evaporite minerals) and if organic matter-enriched sediment and/or peat forms the upper part of the stratigraphy. Climate also determines the nature of biota that colonize deltaic environments. The composition of these biota varies according to biogeographic setting, the occurrence of the biota relative to supratidal, tidal zones, and the subtidal and substrate type. The biota influence sedimentation and generation of lithotypes through shell and test production, root-structuring by trees, sedges, rushes and other salt marsh plants, bioturbation by plants and animals, and production of organic matter. Shell material contributes to the gravel and sand fraction in sediments and, through winnowing during wave action and storms, may be concentrated into sheets, lenses, and cheniers in mud-dominated sediments. The range of biota that directly contributes material to the lithotype or alter sedimentary structures by bioturbation and root-structuring include mangroves in tidal tropical environments, rushes, sedges, samphire, and other salt marsh plants in tidal tropical and subtropical environments, various crustacean-polychaete-mollusc assemblages in tidal tropical and subtropical environments, and wetland forests, sedges, and grasses in subaerial deltaic environments. Biota in intra-estuarine deltas are also described in Semeniuk and Semeniuk (this volume) in their description of Estuarine Deltaic Wetlands.

The stratigraphy of deltas within estuaries

Depending on whether deltaic sediments are dominated by sand, mud, mixtures of these sediment types, or gravel and whether the delta is fluvial dominated, wave dominated, or tide dominated, intra-estuarine deltas can exhibit a wide variety of stratigraphic types. Descriptions of the stratigraphy of intra-estuarine deltas are provided in Corner et al. (1990), Semeniuk and Semeniuk (1990a), Semeniuk and Semeniuk (1990b), Dalrymple et al. (1992), Allen and Posamentier (1993), Semeniuk (2000), and Semeniuk et al. (2011). A range of stratigraphic types in various hydrodynamic settings and with various contributions of sediment types is listed below and illustrated in Figures 6 and 7.

Deltas, Figure 6

Simplified and idealized stratigraphy of deltas formed in sand-dominated, mixed sand-and-mud, and mud-dominated settings under hydrodynamics conditions of fluvial dominated, wave dominated, or tide dominated. The lithologies are simplified to sand, muddy sand, and mud.

Deltas, Figure 7

Some case examples of the gross stratigraphy of fluvial-dominated, wave-dominated, or tide-dominated intra-estuarine deltas from southwestern Australia showing array of generalized lithology in terms of sand, muddy sand, and mud (Semeniuk and Semeniuk, 1990a; Semeniuk and Semeniuk, 1990b; Semeniuk, 2000; and Semeniuk et al., 2011).

Figure 6 shows a simplified and idealized gross-shoaled stratigraphy of deltas formed in sand-dominated, mixed sand-and-mud, and mud-dominated settings under hydrodynamics conditions of fluvial dominated, wave dominated, or tide dominated. The complications in stratigraphy due to lateral deltaic morphologic variation are not shown here. The sand-dominated deltas need not be exclusively sandy but may have a minor component of mud, and similarly, mud-dominated deltas need not be exclusively muddy but may have a minor component of sand. Sand in mud-dominated deltas can be exogenic (riverine sources) or endogenic (generated biogenically).

Microstratigraphic details of the various facies and subfacies of deltas in these different hydrodynamic environments can be found in Dalrymple et al. (1992), Allen and Posamentier (1993), and Semeniuk (this volume on “Stratigraphy of Estuaries”).

The sand-dominated delta developed under fluvial-dominated conditions is a wedge of sand prograded into the estuary. The sand-dominated delta developed under wave-dominated conditions is a wedge of sand comprising sediments of the beach-to-beach ridges, stacked and prograded into the estuary. The sand-dominated delta developed under tide-dominated conditions is a sequence of tide-aligned low-tidal to mid-tidal sand shoals, bars, and lenses that have shoaled to the level of high tide and that have been covered by floodplain sand deposits.

The sand-and-mud-dominated delta developed under fluvial-dominated conditions is a wedge of sand overlain by muddy sand and in turn overlain by mud prograded into the estuary. The sand-and-mud-dominated delta developed under wave-dominated conditions is a sheet of mud and muddy sand deposited at levels below the prevailing wave base and capping by a wedge of sand of beach-to-beach ridges, stacked and prograded into the estuary. The sand-and-mud-dominated delta developed under tide-dominated conditions is a sequence of low-tidal to mid-tidal, tide-aligned sand shoals, bars, and lenses that have shoaled to the level of high tide progressing through lithologies of muddy sand and mud and finally capped by floodplain mud deposits.

The mud-dominated delta developed under fluvial-dominated conditions is a wedge of mud prograded into the estuary. The mud-dominated delta developed under (moderate) wave-dominated conditions (i.e., prevailing waves hydrodynamically dominate over tides and river flow) is generally a wedge of mud prograded into the estuary, but with the wave action and intermittent storms, there is local concentration of exogenic sand and biogenic sand and gravel through winnowing. These coarser sediments find expression in sheets of muddy sand, lenses of sand, and in cheniers. Mud accumulates below the prevailing wave base. The mud-dominated delta developed under tide-dominated conditions is a sequence of low-tidal to mid-tidal tide-aligned mud shoals that have shoaled to the level of high tide to form mud sheets that have been covered subsequently by floodplain mud deposits.

Case studies of the gross stratigraphy of fluvial-dominated, wave-dominated, or tide-dominated intra-estuarine deltas from southwestern Australia demonstrating generalized lithology in terms of sand, muddy sand, and mud are illustrated in Figure 7. The fluvial-dominated Harvey River delta shows a finger of sand (the prograded fans of sand at the delta front) overlying prodelta muddy sand and levee deposits of mud. The fluvial-dominated Collie River delta shows a sheet of riverine sand overlying prodelta mud and a capping of finer sand that has developed by construction of cheniers. The wave-dominated Deep River delta shows a stratigraphy of delta front and prodelta subtidal muddy sand overlain by the sand of beaches and beach ridges, with muddy sand filling inter-beach ridge swales. The wave-dominated Murray River delta shows a stratigraphy of delta front and prodelta subtidal sand overlain by sand of beaches and beach ridges and bars, with muddy sand filling inter-beach ridge swales and lagoons in a prograded bar-and-lagoon sequence. The tide-dominated Preston River delta shows a stratigraphy of subtidal sand that has shoaled through muddy sand with a capping of low-tidal to high-tidal mud.

Discussion and conclusions

Intra-estuarine deltas (also termed “bayhead” deltas) are the fluvial deposits that accumulate where one or more rivers enter an estuary. As with deltas formed in open marine coastal environments, these deltas can be classified as to morphology based on the response of the riverine sedimentary deposits to hydrodynamic setting. As such, deltas developed by fluvial-dominated, wave-dominated, or tide-dominated conditions can be identified. However, unlike the open marine coastal environment where the hydrodynamic conditions are regionally more uniform, deltas in estuaries experience a diversity of hydrodynamic conditions in the one deltaic setting and across the estuary. This is particularly the case where the estuary is large and complex in shape and where there is a strong component of wind that directs surface currents and wind waves. The main factors determining the morphology and landforms of deltas within estuaries are (1) the seasonality and strength of river flow which determines hydrodynamic conditions and the supply of sediment; (2) the salinity of the estuarine-receiving basin which determines the style of interchange of the river water with estuarine water (hypopycnal flow versus homopycnal flow versus hyperpycnal flow), the style of sediment delivery into the estuary, and, to some extent, the shape of the delta; (3) the magnitude of tides and wind waves which, in concert with the magnitude of river flow, will determine whether the hydrodynamic conditions will be dominated by fluvial, wave, or tidal processes; (4) the shape of the estuary; and (5) where the river(s) is/are located. Relatively simple estuaries, e.g., narrow linear valley tracts with a single river mouth, are subject to interactions of river flow, tidal flux, and wind waves, with the delta morphology reflecting the locally dominant hydrodynamic condition. Complex estuaries and estuaries with large fetch create conditions where complicated hydrodynamics of waves and wind-induced currents interact with the shores of the estuaries and act on the deposits of the river or rivers entering the estuary, each river potentially being subject to differing hydrodynamics.

The morphology and landforms of intra-estuarine deltas respond to fluvial, wave, and tidal conditions. Fluvial-dominated deltas can be lobate, fan-shaped, elongate, or digitate. Wave-dominated deltas can be a lobate complex of prograded beach ridges, a series of beach ridges and/or recurved spits with intervening swales and/or linear lagoons, or a prograded series of bars and linear, oval to circular lagoons. The delta front of tide-dominated deltas usually is a crenulate to palmate complex of prograded subaqueous to tidal shoals, and the landward part of the delta is a floodplain. Complexity of local hydrodynamics may result in an intra-estuarine delta with different morphologies in different parts of the delta or contrasting landforms reflective of hydrodynamic conditions in a specific part of that delta.

Stratigraphy of intra-estuarine deltas is variable from delta to delta within the one estuary and variable between deltas in different estuaries because of the sediment types available and the hydrodynamic setting of the delta. The richness and variety of stratigraphic types in intra-estuarine deltas are due to (1) the provenance of the contributing rivers and types of sediment entering the estuary, (2) seasonality and strength of river flow, (3) tides, (4) wind and wind waves, and (5) climate. Climate plays a part in the development of delta stratigraphy in that rainfall and evaporation determine the nature of the high-tidal and supratidal lithologies and can determine if organic matter-enriched sediment and/or peat forms the upper part of the stratigraphy. Climate also determines the nature of biota that contributes shelly material as gravel and sand.

Depending on whether deltaic sediments are dominated by sand, or mud, or mixtures of sand and mud, or contain gravel, and whether the delta is fluvial dominated, wave dominated, or tide dominated, intra-estuarine deltas exhibit a variety of stratigraphic types. Sand-dominated deltas in fluvial-dominated conditions comprise a wedge of sand, while under wave-dominated conditions comprise a wedge of sand of beach-to-beach ridges stacked and prograded into the estuary, and those formed under tide-dominated conditions show a sequence of tide-aligned sand shoals, bars, and lenses aggraded to the level of high tide and that have been covered by floodplain sand deposits. The sand-and-mud-dominated deltas in fluvial-dominated settings comprise a wedge of sand overlain by muddy sand and in turn overlain by mud, while in wave-dominated settings are a sheet of mud and muddy sand deposited below the prevailing wave base with a capping of sand of beach-to-beach ridges stacked and prograded into the estuary. Those developed in tide-dominated settings show a sequence of tide-aligned sand shoals, bars, and lenses shoaled to the level of high tide through lithologies of muddy sand and mud and covered by floodplain mud deposits. Mud-dominated deltas in fluvial-dominated conditions are a wedge of mud prograded into the estuary, while those developed in wave-dominated settings comprise a wedge of mud but with local sand and gravel in sheets of muddy sand, lenses of sand, and cheniers with mud accumulating below the prevailing wave base. Mud-dominated deltas in tide-dominated settings comprise a sequence of tide-aligned mud shoals that aggrade to the level of high tide as mud sheets and are covered by floodplain mud.

Cross-references

• Delta Plain

• Estuarine Deltaic Wetlands

• Estuarine Geomorphology

• Sediment Erosion

• Shoreline Changes

• Species Zonation

• Stratigraphy of Estuaries

• Tidal Hydrodynamics