Estuaries & Coastal Waterways

Geomorphology (or the study of the nature and history of landforms and the processes which create them) is an easily recognisable end product of a combination of environmental factors. Under stable tectonic and sea level conditions , the gross geomorphology of coastal waterways is principally determined by the relative influence of wave, tide, and river power ( Figure 1 ; Boyd et al., 1992, Dalrymple et al., 1992), with each coastal waterway containing a distinctive suite of geomorphic and sedimentary environments.

 

Classification of Estuaries and Coastal Waterways

Much of the following is taken from a comprehensive review of Estuaries and Costal Waterways from OzEstuaries.

OzEstuaries provides comprehensive information about Australian estuaries and coastal waterways and represents the collaborative efforts of more than 100 coastal scientists from a range of government agencies and universities.

 

see also West Siberian Estuary Systems

Ternary classification of coastal systems
Figure 1. Ternary classification of coastal systems are divided into seven classes (after Dalrymple et al., 1992, Boyd et al., 1992). The position of each coastal waterway type depends on the relative influence of waves, tides, and rivers. Embayments and drowned river valleys are omitted from the diagram as they represent 'immature' coastal waterways (or coastal waterways which are not significantly filled with sediment). WDD = wave-dominated deltas, and TDD = tide-dominated deltas.

The coastal waterway classes comprise:


Characteristics of Estuary Systems

Estuaries are transgressed, drowned river valleys where fluvial, tide, and wave processes interact; they are characterized by a net landward movement of sediment in their seaward part.

• Tide-dominated estuaries contain tidal sand bars at the seaward end, separated from the fluvial zone by relatively fine-grained tidal flats (e.g., salt marshes); fluvial channel deposits exhibit heterolithic characteristics and sometimes tidal-bundle sequences.

• Wave-dominated estuaries have a coastal barrier with a tidal inlet and flood- tidal delta, separated from a bayhead delta by a central basin where fine- grained sediments (muds) accumulate.

Sediments coarsen upward from marine shales.

Sand bodies perpendicular to basin margin, narrow landward.

Formed by a mix of tidal currents and occasional storm waves.

Facies:

   

Embayments and Drowned River Valleys (from OzEstuaries)

Also known as: drowned river valleys, oceanic embayments, rias, fjords.

 

Key Features of Embayments

  1. Habitats are typically marine, with extensive subtidal environments and very narrow intertidal environments.
  2. Large entrance and efficient marine flushing, even in microtidal regions. Deep water.
  3. River flow varies, floods are buffered and do not expel marine water due to large water area.
  4. Turbidity and extent of intertidal habitats are dependant on local tidal range.
  5. Sediment (and associated contaminants) are generally not trapped. Typically floored by coarse sediment.
  6. Nutrient dynamics are generally similar to the coastal ocean.
  7. 'Immature' in terms of evolution: morphology may slowly change over time due to infilling.

Geomorphology

Embayments are the least geomorphologically complex of the seven coastal waterway classes, as they typically comprise a bedrock-lined coastal indentation (Hudson, 1991). In Australia, embayments occur along hard coasts, where they appear as topographic depressions or indentations in the country rock, that have not been significantly infilled by terrigenous or marine sediment (see Examples below).

The morphology of embayments may comprise wide and rounded bays, highly indented bays with convolute shorelines, or narrow and tapered drowned river valley systems (Albani et al., 1974, Albani et al., 1975, Perillo, 1995, Morrisey, 1995, Riggs et al., 1995). Embayments are generally bound by steep, rocky shorelines, have relatively wide, unconstricted entrances with free exchange to the ocean, and are deep relative to other coastal waterway types (Roy et al., 1980, Roy et al., 1981). The submarine topography is smooth, and typically slopes gently toward the mouth (Abell et al., 1993). River inputs (although sometimes large during peak flow conditions) are, in the long-term, small relative to the total water volume contained with the embayment and exchanged with the ocean (Hudson, 1991, Roy et al., 2001). The relative influence of waves and tides in embayments is variable, and depends on regional conditions.

In Australia, embayments are equally abundant on both wave- and tide-dominated coasts. Variations in orientation, configuration, and water depth affects the penetration of waves; strongly indented embayments support more sheltered environments, and tidal processes tend to dominate upstream (Roy et al., 1980). Due to friction, wave and tide influence are generally reduced with distance from the entrance of the embayment. Localised bedrock features such as headlands or offshore islands may also form a protective barrier and limit wave penetration into embayments. Due to a typically large exchange of water during the tidal cycle (or tidal prism), embayments are usually considered to be tide-dominated, even on microtidal coasts (Andersson et al., 1986, Roy et al., 2001, Cooper, 2001).

Image of Cascade Bay (WA)Image of Twofold Bay (NSW)Image of Broken Bay (NSW)

Examples of oceanic embayments: Cascade Bay (WA) - indented, Twofold Bay (NSW) – rounded, and Broken Bay (NSW) – narrow drowned river valley.

 

Evolution

Generally, embayments are the evolutionary precursors of modern wave-and tide-dominated estuaries and deltas (Roy et al., 2001, Fitzgerald et al., 2000). The rate of infilling by sediment depends on sediment supply from the catchment and marine sources, and the original volume of the basin. Thus, the present-day distribution of embayments is restricted to areas of complex, rocky coastal morphology, and low sediment supply. Given sufficient time, continuous sediment supply, and stable sea level , embayments ultimately ‘fill in', and have the potential to become wave- or tide-dominated estuaries , and subsequently wave- or tide-dominated deltas (Heap et al., 2001).


Wave-dominated Estuaries

Also known as: barrier estuaries, bar-built estuaries, and intermittently closed and open lakes and lagoons ( ICOLL's).

 

 

Key features of wave-dominated estuaries

  1. A diverse range of both marine and brackish, subtidal, intertidal and supratidal estuarine habitats are supported.
  2. Narrow entrance restricts marine flushing, only a small proportion of the estuarine water volume is exchanged each tide.
  3. River flow typically high, and flooding may expel marine water and flush material from the estuary.
  4. Turbidity , in terms of suspended sediment, is naturally low except during extreme wind or fluvial runoff events.
  5. Central basin is an efficient 'trap' for terrigenous sediment and pollutants.
  6. Long residence time encourages trapping and processing ( e.g. denitrification ) of terrigenous nitrogen loads .
  7. 'Semi-mature' in terms of evolution: morphology will rapidly change over time due to infilling, resulting in shallowing of the central basin, and expansion of the fluvial delta.

Geomorphology

A wave-dominated estuary represents a coastal bedrock embayment that has been partially infilled by sediment derived from both the catchment and marine sources, in which waves are the dominant force shaping the gross geomorphology. In Australia, wave-dominated estuaries are most abundant on the south-east and south-west coasts , where they occur on exposed coastlines with a relatively small tidal influence (Roy et al., 2001, Cooper, 2001). Wave-dominated estuaries feature a supra-tidal (or sub-aerial) barrier at the mouth that encloses a broad central basin . The barrier creates a constricted entrance (which can be periodically closed) that allows the exchange of water between the central basin and the sea. Sediment in wave-dominated estuaries ranges from fine to coarse sands in the barrier and tidal inlet deposits, fine organic muds and sandy muds in the central basin, to coarse, unsorted gravels, sands and muds (mostly of terrigenous origin) in the fluvial bayhead delta (Nichol, 1991). Depending on the degree of sediment infilling, the central basin of wave-dominated estuaries may be irregularly-shaped, following the outline of the drowned bedrock valley (Riggs et al., 1995).

In the case of wave-dominated estuaries formed in unconsolidated coastal deposits the central basin may be oval-shaped and oriented parallel to the coast (Chapman et al., 1982, Morrisey, 1995). At the head of a wave-dominated estuary is a fluvial bayhead delta that extends into the central basin and is comprised of vegetated and unvegetated levees, channels, and intertidal areas. The fluvial bayhead delta is constructed from terrigenous material from the catchment being deposited and the mouth of the river (Webster et al., 2002, Pasternack et al., 2002).

Taggerah Lakes (NSW)Broke Inlet (WA)Angurugubira Lake (NT)

Examples of wave-dominated estuaries: Tuggerah Lakes (NSW), Broke Inlet (WA), Angurugubira Lake (NT).

 

Evolution

The evolution of wave-dominated estuaries is characterised by infilling (sedimentation rates) of the valley, principally the central basin (Roy et al., 1980). As such, wave-dominated estuaries evolve or mature by the simultaneous seaward progradation of the fluvial bayhead delta , and the landward progradation of the flood tidal delta , and also by the expansion of fringing intertidal flats (Roy, 1984a). Recent studies quantifying the areas of geomorphic and sedimentary environments in Australia's wave-dominated estuaries (e.g., Roy et al., 2001, Heap et al., In Press) have demonstrated that infilling is dominated by the expansion of intertidal environments around the central basin and progradation of the fluvial bayhead delta and alluvial plain, rather than from progradation of the flood tide delta. Given sufficient time and constant sediment supply, wave-dominated estuaries have the potential to evolve into wave-dominated deltas when the central basin is completely infilled (or is bypassed by the river channel), and terrigenous sediment is exported directly to the ocean rather than being trapped (Heap et al., In Press).

 

Habitats and ecology

Wave-dominated estuaries generally contain "true estuarine" (or euryhaline) species, and transient visitors from full marine environments (Paterson et al., 2000, Potter et al., 1994, Rainer et al., 1981b). This is because wave-dominated estuaries provide a diverse range of habitats, such as high-energy sandy beaches and channel sands, sheltered deep muddy basins , shallow water habitats, mangroves , saltmarshes , and intertidal flats (Roy et al. 2001). Depending upon entrance conditions, and latitude, saltmarshes and mangroves can occur around the edges of the central basin, and the high-energy conditions of the inlet produce a sandy substrate and relatively clear shallow waters, that generally support various seagrasses (Rainer et al., 1981a, Abal et al., 1996, Hannan et al., 1998, Humphries et al., 1992). Central basin muds often support benthic micro- and macroalgae (Cahoon et al., 1999), and various invertebrates . Wave-dominated estuaries that have undergone slow infilling can contain large areas of rocky shore and reef habitats that support a variety of biota (Griffiths, 2001).


Wave-Dominated Deltas

Also known as: riverine estuary (See Also: Deltas )

 

 

Key features of wave-dominated deltas

  1. Habitats supported are variable, generally mostly brackish subtidal, intertidal and supratidal habitats.
  2. Narrow entrance restricts marine flushing; only a small proportion of the water volume is exchanged each tide.
  3. River flow typically high, and flooding commonly expels marine water and flushes material from the delta.
  4. Turbidity , in terms of suspended sediment, is highly dependant on catchment inflow, however is naturally low except during extreme fluvial runoff events.
  5. Sediment (and associated contaminants) are mostly expelled into the coastal ocean.
  6. Short residence time ( e.g. efficient flushing) results in little processing or trapping of nutrients .
  7. 'Mature' in terms of evolution. Tend to be stable in terms of morphology (given stable sea level ).

Geomorphology

Wave-dominated deltas are comprised of a river that is directly connected to the sea via a channel(s) that is usually flanked by low-lying vegetated floodplain and swampy areas. Entrances of wave-dominated deltas are relatively narrow due to constriction by a barrier (or sandbar) and, due to the relatively high river influence throughout the system, are rarely closed off from the ocean.

In Australia, wave-dominated deltas are most abundant on the north-east, south-east, and south-west coasts , and represent 'mature' forms of wave-dominated estuaries , having been largely infilled by sediment from terrigenous and marine sources (Roy et al., 2001, Roy, 1993). Therefore, they do not necessarily display the morphology of the ancestral bedrock valley in which they have developed (Boyd et al., 1992, Dalrymple et al., 1992, Hinwood et al., 1999).

Australia's high relative aridity, low relief, and geological antiquity has resulted in a distinct lack of large deltas (by world standards) and associated continental river systems. Total discharge of terrigenous material is small by world standards (Fryirs et al., 2001). Many wave-dominated deltas in Australia do not contain large coastal protuberances, due to this lack of sediment supply, in combination with shoreline erosion and sediment redistribution by wave energy (Heap et al., In Press).

Robinson River (NT)Richmond River (NSW)Gascoyne River (WA)

Examples of wave-dominated deltas: Robinson River (NT), Richmond River (NSW), Gascoyne River (WA).

 

Evolution

During the later stages of deltaic evolution (or sediment infilling ), the connectivity between the river channel and tidal inlet increases. This results in more efficient delivery of fluvial sediment to the ocean, and the bypassing of remnant central basin features (i.e. 'cut-off embayments'), which slowly infill and become swamp areas. As a consequence, the gross morphology (and thus habitat distribution and abundance) of wave-dominated deltas is relatively stable, and may persist over long periods of time with little change (Heap et al., In Press). In areas with increased sediment supply, such bypassing to the coast can allow the barrier to prograde, which may result in the formation of a coastal protuberance adjacent to the mouth of the river (Roy et al., 1980). Large magnitude river floods may also cause temporary delta-front progradation, however wave erosion and currents tend to disperse this sediment (Cooper, 1993, Hume et al., 1993).

 

Habitats and ecology

Wave-dominated deltas typically support 'euryhaline' estuarine species, as well as transient visitors from full marine environments, depending on river flow conditions (Roy et al., 2001). Intertidal habitats are limited in extent, and are restricted mainly to the barrier and flood tidal delta (Cooper 1993). Wave-dominated deltas typically support high-energy sandy beaches and channels , intertidal mudflats , saltmarshes , and mangroves (Alongi et al., 1999). Typically, sandy channel margins support various macrophytes such as seagrasses (Abal et al., 1996).


Coastal Lagoons and strandplain associated creeks

Also known as: ICOLL's, closed or blind estuary, interbarrier estuary, dune-swale creek.

 

Key features of coastal lagoons and strandplain-associated creeks

  1. Habitats supported are limited by chemical conditions induced by poor exchange with the marine environment, and highly variable salinity.
  2. Intermittent entrance isolates the coastal waterway from the ocean for long periods.
  3. River flow is intermittent to non-existent. Flooding is therefore uncommon, however can cause large impacts such as entrance breaching and scouring of the central basin.
  4. Turbidity is naturally low, however shallow basins are susceptible to wind-wave resuspension, particularly if seagrasses are not present.
  5. Central basin (where present) is an efficient 'trap' for terrigenous sediment and pollutants.
  6. Long residence time encourages trapping and processing (e.g . denitrification ) of terrigenous nutrient loads, however the system may be susceptible to overloading due to small size.
  7. Evolution, in terms of infilling, is very slow due to the lack of significant sediment input.

Geomorphology

Coastal lagoons and strandplain-associated coastal creeks are small, shallow basins that have very low (or negligible) freshwater input (see Examples below). In Australia, they are most abundant on the south-east coast, south-west coast, and in the Gulf of Carpentaria . The catchment for these systems is limited to the immediate hinterland. Due to the lack of significant freshwater input (and associated terrigenous sediment) and strong tidal currents, the entrances to these coastal waterways are often intermittently or permanently closed, resulting in isolation from marine influence for long periods (Ranasinghe et al., 1999). The geomorphology of coastal lagoons is similar to wave-dominated estuaries , however they lack a distinct fluvial bay-head delta (Roy et al., 2001). strandplain-associated coastal creeks are narrow, generally shallow water bodies that occur on wave-dominated coasts (Otvos, 2000). They are generally oriented parallel to the coast, and develop on prograding coastal sequences formed from beach ridges, dunes, and barriers . Coastal lagoons, and other small waterways associated with wave-dominated coastlines, tend to experience very low wave and tide energy within, as tidal waters are often unable to penetrate the closed (or very narrow and intermittent) entrances (Harris et al., In Press). Additionally, low or non-existent river flow is conducive to very low energy conditions, except during extreme flood conditions. The most significant physical energy source in many systems is internally generated wind-induced waves, however these usually remain quite small due to the limited size of the waterway (Morrisey, 1995).

Nadgee Lake (NSW)Little Lagoon (NT)Jerusalem Creek (NSW)

Examples of coastal lagoons: Nadgee Lake (NSW), Little Lagoon (NT), and a strandplain-associated coastal creek: Jerusalem Creek (NSW). Oblique photos courtesy of the NSW Department of Land and Water Conservation.

 

Evolution

Coastal lagoons and strandplain-associated creeks evolve on wave-dominated coastlines by the partial or total closure of small coastal embayments , by a sub-aerial sand barrier , or by the flooding of beach ridges (Harris et al., In Press). Coastal lagoons represent an end member of the spectrum of wave-dominated coastal waterways where fluvial input is negligible, and experience the same or similar evolutionary trends as wave-dominated estuaries except for the lack of significant infilling by terrigenous material (Boyd et al., 1992). Infilling is slow, and is dominated by marine-derived sediment when the entrance is open (Boyd et al., 1992).

 

Habitats and ecology

Despite intermittently experiencing significant variations in salinity , coastal lagoons and strandplain-associated coastal creeks are usually colonised by estuarine invertebrates and other 'euryhaline' aquatic organisms that can tolerate a wide range of salinity conditions (Rainer et al., 1981b). There is usually a high mortality of marine species during periods of closure that provides an opportunity for recruitment and development of estuarine and/or low salinity species. Even very small (<100 m2) coastal lagoons and strandplain-associated coastal creeks are important habitats for a diverse assemblage of juvenile (and small-sized) fishes, some of which are economically important (Griffiths, 2001, Hannan et al., 1998, Norris et al., 1993, Pollard, 1994). The duration of water exchange between the ocean and the coastal lagoon is probably the most important factor influencing the recruitment of marine organisms (Potter et al., 1994). Many coastal lagoons and strandplain-associated coastal creeks support macroalgae , limited seagrass beds , saltmarshes , and floodplain species. Mangroves typically do not occur, due to the lack of connection with the open ocean (Roy et al., 2001).

 


Tide-dominated Estuaries

Also known as: macrotidal estuaries, open estuaries.

 

Key features of tide-dominated estuaries

  1. A diverse range of both marine and brackish, subtidal, intertidal and supratidal estuarine habitats are supported. Intertidal and supratidal areas are often extensive, whereas turbidity may preclude seagrasses in some areas.
  2. Large entrance promotes efficient marine flushing.
  3. River flow is typically high, however the effects of floods are buffered by large water area, and large tidal exchange.
  4. Turbidity is naturally high due to strong turbulence induced by tides.
  5. Flanking environments such as intertidal flats, mangroves, saltmarshes and saltflats tend to trap terrigenous sediment and pollutants. Marine flushing results in loss of some material to the coastal ocean.
  6. Tidal movement over flanking environments encourages the trapping and processing ( e.g . denitrification ) of terrigenous nutrient loads. Marine flushing results in loss of some material to the coastal ocean.
  7. 'Semi-mature' in terms of evolution: infilling by marine and terrigenous sediment will result in expansion of flanking environments, narrowing of channels, and seaward progradation.

A tide-dominated estuary represents a bedrock coastal embayment that has been partially infilled by sediment derived from both the catchment and marine sources, in which tidal currents, rather than waves, are the dominant force shaping the gross geomorphology. In Australia, tide-dominated estuaries are generally found on the northern, north-eastern and north-western coasts , and are most abundant on low-gradient coasts characterised by meso- to macro-tidal ranges (Harris et al., 2002, Dalrymple, 1992).

Geomorphology

Tide-dominated estuaries generally consist of a landward-tapering funnel shaped valley, bounded by various intertidal sedimentary environments such as intertidal flats , mangroves , saltmarshes , and saltflats . Depending on the degree of sediment infilling, the boundaries of tide-dominated estuaries may follow the irregular outline of the drowned valley (Riggs et al., 1995), or, in more mature cases are smooth and intersected by small tidal creek dendritic drainage networks (Wells, 1995, Wolanski et al., 1992).

Major structural elements inside the estuary include elongate tidal sand banks , which occur in the wide entrance, oriented perpendicular to the coast and parallel to the direction of dominant tidal currents (Fitzgerald et al., 2000, Green et al., 2001). The tidal sand banks are usually dissected by deep channels containing strong tidal currents. Landward of the estuarine channels, the source river that feeds into tide-dominated estuaries often features a straight–meandering–straight river channel profile (see East Alligator River, below). This represents the point at which the convergence of seaward-directed water and sediment transport by the river, and landward-directed water and sediment transport by tides occurs (Dalrymple et al., 1992, Bryce et al., 1998). Due to strong tidal currents generated by large tidal ranges, tide-dominated estuaries are usually highly turbid .

Fitzroy River (QLD)Victoria River (NT)East Alligator River (NT)

Examples of tide-dominated estuaries: the Fitzroy River (QLD), the Victoria River (NT), and the East Alligator River (NT).

 

Evolution

Tide-dominated estuaries evolve by infilling of the valley with terrigenous and marine-derived sediment, and a gradual translation of geomorphic and sedimentary environments in a seaward direction (Dalrymple et al., 1992). Quantification of the area of geomorphic and sedimentary facies in Australia's tide-dominated estuaries (Heap et al., In Press) has demonstrated that there is no (or very little) significant change in the distribution of sedimentary environments during evolution. However, infilling is characterised by the expansion of intertidal flats and saltflats around the margins of the estuary, and expansion and merging of tidal sand banks in the tidal channels (Harris, 1988), reflecting the deposition of sediment trapped within the estuary. Vegetation associated with mangrove , saltmarsh , and saltflat environments plays a major role in determining the form of the estuary during early stages of evolution, because of its capacity to trap fine sediment (Woodroffe, 1992, Woodroffe et al., 1993). As such, tide-dominated estuaries have undergone a 'Big Swamp' evolutionary phase, that was characterised by the rapid expansion of marginal marine environments. This was followed by relatively slow accumulation that enabled freshwater wetlands to gradually replace mangroves and saltmarshes, and slow seaward progradation (Woodroffe et al., 1989, Mulrennan et al., 1998, Chappell, 1993, Woodroffe et al., 1993).

 

Habitats and ecology

Tide-dominated estuaries contain habitats such as channels , intertidal mudflats , mangroves , saltmarshes , saltflats , and rocky shores and rocky reefs (Semeniuk, 1982). These habitats typically support marine species, including transient visitors and permanent residents (Cyrus et al., 1992, Pusey et al., 1993), however the biota of these estuaries is less well documented than their wave-dominated counterparts (Boyd et al., 1992). Plant productivity seems to increase with increasing tidal range, due to greater rates of flushing and the consequent renewal of nutrients (Morrisey, 1995). Littoral mangrove forests dominate many of Australia's tide-dominated estuaries, and plains vegetated with grasses, sedges and herbs, as well as freshwater wetlands and floodplain vegetation (such as Melaleuca spp.) lie above the influence of most (but not all) tides (Woodroffe et al., 1989). Turbid water within the estuary largely precludes the growth of subaquatic benthic macrophytes (such as seagrasses), and also limits the distribution and depth range available as habitat for phytoplankton. These species are able to survive further seaward due to lower turbidity (Semeniuk, 1996).

 

See also: Tidal Environments


Tide-dominated Deltas

Also known as: riverine estuary, macrotidal delta

 

Key features of tide-dominated deltas

  1. A diverse range of both marine and brackish, subtidal, intertidal and supratidal estuarine habitats are supported. Intertidal and supratidal areas are often extensive, whereas turbidity may preclude seagrasses in some areas.
  2. Large entrance promotes efficient marine flushing.
  3. River flow typically high, and flooding may expel marine water and flush material from the delta.
  4. Turbidity is naturally high due to strong turbulence induced by tides.
  5. Flanking environments such as intertidal flats, mangroves, saltmarshes and saltflats may trap terrigenous sediment and pollutants. River flow and marine flushing result in the loss of some material to the coastal ocean.
  6. Tidal movement over flanking environments encourages the trapping and processing ( e.g. denitrification ) of terrigenous nutrient loads. River flow and marine flushing result in the loss of some material to the coastal ocean.
  7. 'Mature' in terms of evolution. Tend to be stable in terms of morphology (given stable sea level ).

Geomorphology

Tide-dominated deltas are comprised of a river that is directly connected to the sea via channels that are typically flanked by low-lying vegetated floodplains and swamp areas. Because of the dominance of tidal processes, the geomorphology of tide-dominated deltas features a landward tapering funnel-shaped valley, and the river is connected to the sea via a series of distributary channels. Channels may be separated by large expanses of low-gradient vegetated swamps (Bhattacharya et al., 1992, Woolfe et al., 1996).

In Australia, tide-dominated deltas are most abundant on the north-east coast , and represent the 'mature' form of tide-dominated estuaries , which have largely been infilled by sediment from terrigenous and marine sources (Heap et al., In Press). Because net bedload transport is offshore, tide-dominated deltas do not exhibit the 'straight-meandering-straight' channel morphology seen in many tide-dominated estuaries (Dalrymple et al., 1992). Due to the degree of sediment infilling, the gross geomorphology of tide-dominated deltas may not exhibit the morphology of the antecedent valley (if present).

Tidal sand banks are a major structural element within the entrances of tide-dominated deltas, and are oriented perpendicular to the coast, and parallel to the direction of dominant tidal currents. The tidal sand banks are usually dissected by deep channels containing strong tidal currents (Jones et al., 1993).

Australia's high relative aridity, low relief, and geological antiquity has resulted in a distinct lack of large deltas (by world standards). Associated continental river systems and total discharge of terrigenous material are small by world standards (Fryirs et al., 2001). The dominance of offshore sediment transport and generally low wave-energy at the coast means that tide-dominated deltas usually construct lobate shoreline 'protuberance', which extends onto the inner continental shelf. Due to strong tidal currents generated by large tidal ranges, tide-dominated deltas are usually highly turbid .

Burdekin River (QLD)Limmen Bight River (NT)Norman River (QLD)

Examples of tide-dominated deltas: Burdekin River (QLD), Limmen Bight River (NT), and Norman River (QLD).

 

Evolution

During the latter stages of deltaic evolution (or sediment infilling ), the connectivity between the river channels and tidal inlet increases. This results in more efficient transmission of fluvial sediment directly to the ocean, as much of the system is comprised of a floodplain area that is above the influence of most tides (Evans et al., 1992). The distribution of environments such as intertidal flats , mangroves and saltmarshes is not significantly different from tide-dominated estuaries , except for the formation of tidal sand banks seaward of the mouth due to the net offshore bedload transport. Tide-dominated deltas have reached a point in their development where further evolution involves progradation of the coastline onto the inner continental shelf , although this process can be limited by sediment supply and the effects of sediment redistribution by tidal (and other) currents (Heap et al., In Press).

 

Habitats and ecology

Tide-dominated deltas provide habitats such as channels, intertidal mudflats , mangroves , saltmarshes , and saltflats (Semeniuk, 1982). These habitats typically support marine species, however the biota of these systems is less well documented than their wave-dominated counterparts (Dalrymple et al., 1992). Plant productivity seems to increase with increasing tidal range, due to greater rates of flushing and the consequent renewal of nutrients (Morrisey, 1995). Littoral mangrove forests are common in many of Australia's tide-dominated deltas, however tide dominated deltas have far less mangrove and saltmarsh area relative to estuaries (Woodroffe et al., 1989). Plains vegetated with grasses, sedges and herbs, as well as freshwater wetlands and floodplain vegetation (such as Melaleuca spp.) lie above the influence of most tides. Turbid water within the delta largely precludes the growth of subaquatic benthic macrophytes (such as seagrasses), and also limits the distribution and depth range available as habitat for phytoplankton. These organisms are able to survive further seaward due to lower turbidity (Semeniuk, 1996).

 


Tidal Creeks

Also known as: tidal channels, mangrove creeks, macrotidal mud flats.

 

Key features of tidal creeks

  1. A diverse range of both marine and brackish, subtidal, intertidal and supratidal estuarine habitats are supported. Intertidal and supratidal areas are often extensive, whereas turbidity may preclude seagrasses in some areas.
  2. Large entrance promotes efficient marine flushing.
  3. River flow is intermittent to non-existent. Flooding is therefore uncommon, however, the effects of any floods are buffered by the large water area and high tidal exchange.
  4. Turbidity is naturally high due to strong turbulence induced by tides.
  5. Flanking environments such as intertidal flats, mangroves, saltmarshes and saltflats tend to trap sediment and pollutants. Marine flushing results in loss of some material to the coastal ocean.
  6. Tidal movement over flanking environments encourages the trapping and processing (e.g. denitrification) of nutrient loads . Marine flushing results in loss of some material to the coastal ocean.
  7. Evolution, in terms of infilling, is driven by trapping of marine sediment, which results in the gradual expansion of flanking environments and seaward progradation.

Tidal creeks are wide, funnel-shaped coastal waterways, that have very low (or negligible) freshwater input, and typically develop in low-gradient, seaward-sloping coastal flats (Dalrymple, 1992, Semeniuk, 1996, Semeniuk, 1982, Semeniuk et al., 1982). Tidal creeks are the most common type of coastal waterway in Australia, and are most abundant in north-western Australia and the Gulf of Carpentaria, where they occur along mostly macrotidal, low-gradient coastal plains, due to the overwhelming influence of tidal currents in these systems

Geomorphology

The geomorphology of tidal creeks is usually comprised of a straight, sinuous, or dendritic tidal channel(s) that taper (in a negative-exponential fashion upstream) and shoal to landward (Wolanski et al., 1992). The coastal mudflats that generally surround tidal creeks tend to be at or above the limit of high tide, and seawater is mainly confined to the tidal channel , except during spring tides. Because of their relatively small size, and low freshwater input, they lack the major structural elements such as tidal sand banks that are characteristic of tide-dominated estuaries and deltas . Tidal channels are frequently interconnected, and flanked by large areas of low-gradient intertidal flats , mangroves , saltmarsh , and saltflat environments (Wells, 1995).

The outline of tidal creeks may partially follow the irregular outline of the drowned bedrock embayments in which they have developed (Riggs et al., 1995), or may be smooth and funnel-shaped, sometimes intersected by smaller creek networks (Wells, 1995). Tidal creeks are highly variable in size (Heap et al., 2001) and, due to strong tidal currents generated by large tidal ranges, are usually highly turbid . Tidal creeks are generally distinguished by relatively high tidal energy throughout the system. However, frictional forces reduce tidal energy to landward, and in some of the more tapered systems, amplification of the tidal wave occurs to locally to elevate water levels inside the system and on the surrounding intertidal flats (Dalrymple et al., 1992).

Ilamaryi River (NT)Morning Inlet (QLD)Barker Inlet (SA)

Examples of tidal creeks: Ilamaryi River (NT), Morning Inlet (QLD), and Barker Inlet (SA).

 

Evolution

The evolution of tidal creeks is characterised by slow progradation and seaward translation of the tidal channel and surrounding intertidal flats, which is driven mostly by landward transport of marine sediment (Belperio, 1993, Woodroffe, 1992). Because of the low freshwater input, the system does not usually contain an alluvial floodplain, and remains bounded by intertidal environments, even in the most mature cases (Boyd et al ., 1992, Green et al ., 2001, Fitzgerald et al ., 2000). Marine sediment moves into tidal creeks by shoreward-directed bedload transport, and infilling occurs as the channel shoals, and the intertidal flats merge (Harris, 1988, Knighton et al., 1992).

 


Estuarine Environments (from OzEstuaries)

Back barrier | Central basins | Channels | Flood & ebb tidal deltas | Fluvial deltas | Inner Continental Shelf
Intertidal flats | Mangroves | Rocky reefs | Saltmarshes | Saltflats | Tidal sand banks

 

1. Barrier/back-barrier (also known as bar, sand spit, barrier island, strand)

Barrier environments are a distinctive component of wave-dominated estuaries and are common shoreface features on any coastline subjected to high wave energy. Barriers often consist of an intertidal to supratidal beach-face, cusps, shallow channels, a berm, and dunes interspersed by blow-outs. Back-barrier regions may contain wash-overs (sediment washed into the estuary during major storms). Sediments comprise well-sorted fine to coarse, quartz-rich sands. Heavy minerals (monzanite, illmenite, cassiterite etc.) may occur in low concentrations. Carbonate concentrations are generally high (particularly in tropical estuaries), except in the supra-tidal dunes, and concentrations of organic material are generally low. The porous nature of the sandy sediments generally results in well-oxygenated sub-surface sediments. On prograding, wave-dominated coastlines, ancient barriers may be landlocked as younger barriers form. Except for the beach-face, surfaces are generally vegetated. Infauna and epifauna (eg. interstitial microfauna, crustaceans, worms and molluscs) occur at supra-tidal to sub-tidal elevations. The stability of biological communities is variable, and is generally associated with dune-stabilising vegetation above supra-tidal elevations. In modern settings, these habitats may also intermittently support birds, turtles and seals.

 

2. Central basins

Central basins are uniform, lower energy environments in the deeper and quieter parts of estuaries, and are often formed landward of barrier bar deposits in wave-dominated estuaries. Sedimentologically, central basins typically comprise poorly-sorted, organic-rich sub-tidal mud and sandy mud. The shallower margins of central basins often feature coarser sediments (sands), which result from the action of wind waves and fluctuating water level in some estuaries. Carbonate concentrations are generally low, however, localised shell bioherms made up of gravel-sized estuarine bivalve shells may develop. Concentrations of organic material are generally very high, causing a black to dark grey appearance in the sediments. Surfaces are generally planar and not vegetated, however some seagrass growth in the shallower parts of the central basin usually occurs. Sub-surface sediments may be anoxic, but is generally heavily bioturbated due to an abundance of infauna and epifauna.

 

3. Channels (also known as tidal channels or river channels)

Channels are environments of frequently high energy, in terms of tidal movement (e.g. tidal channels) or fluvial flow (e.g. river channels). Thus, salinity, water quality and sediment types are variable, however, coarser grained sand to gravel (lag) deposits are common on the channel floor. Channels are often found in association with fluvial (bayhead) deltas, flood and ebb tidal deltas, tidal sand banks, and intersecting Intertidal Flats and Mangroves in macrotidal environments. Channels may be intermittent, and may also be abandoned as river or tidal flows change course. Concentrations of carbonate and organic material vary, and are typically higher in tropical estuaries. Channels are often non-depositional environments and are sometimes erosional. Channels are typically subtidal, however in macrotidal regions entire channel networks may be exposed at low tide. Channels are important environments for a wide range of marine and estuarine organisms (depending on salinity and turbidity), and provide shelter and access for larger estuarine predators, as well as potential seagrass habitat.

 

4. Flood and ebb tidal deltas

Flood and ebb tidal deltas are subtidal to supratidal dunes and channels, typically found in the entrances of wave-dominated estuaries and deltas (adjacent to the barrier), and are formed by redistribution of sediment by tidal movement in and out of the entrance. Sediments comprise moderately- to well-sorted, quartz-rich sand. Gravels often occur as a lag in the main tidal channels, where tidal currents are strong. Heavy minerals may occur in low concentrations. Carbonate concentrations are generally high, and concentrations of organic material are generally low. Flood oriented bedforms can occur on the shoals (eg. straight crested, full-bedded small dunes) and ebb-oriented bedforms (eg. sinuous crested, full-bedded small to medium dunes) can occur in the channels. Seagrasses and associated communities often occur where tidal currents are weak. Infauna and epifauna (eg. interstitial microfauna, crustaceans, worms and molluscs) occur at supra-tidal to sub-tidal elevations.

 

5. Fluvial (bayhead) deltas

Fluvial deltas are complex associations of geomorphological settings, sediment types and ecological habitats, at the point where a freshwater source enters an estuarine water body. Environments range from subtidal channels through intertidal to terrestrial levees, shoals and mouth bars. At the mouth of the channel, the flow velocity is abruptly reduced as the river water enters the standing water of the lake or sea (often into a central basin). The delta front immediately forward of the channel mouth is the site of deposition of bedload material. Sediment types range from clean fluvially-derived sands and gravels, to poorly sorted sands, muds and terrestrial organic material. Deposition of sediments and associated organic materials follows a cyclic pattern, driven by episodic floods. Carbonate concentrations are generally low, whereas concentrations of organic material are generally very high. Bedforms in the channel and inter-distributary bays are poorly developed due to large fluctuations in river energy and generally low tidal energy. Supra-tidal regions are usually well vegetated with saltmarsh, mangrove or terrestrial woodland ecosystems. Due to large salinity variation, the diversity of fish and crustacean species is often limited.

 

6. Inner continental shelf

The inner continental shelf environment represents the shallow marine environment directly seaward of the entrance of the estuary/coastal waterway. Seabed morphology and sediment types are variable, as this environment occurs throughout wave- and tide-dominated coastlines, and in any climatic zone. Biota existing in this environment are typically marine or ocean-dwelling organisms only, as this environment is influenced by freshwater during extreme flood events only (depending on local conditions).

 

7. Intertidal flats

Intertidal mud flats are unvegetated, generally low gradient, and low energy environments, consisting of poorly- to moderately-sorted sandy mud and muddy sand. Gravel may be present in moderate concentrations at the base of shallow drainage channels, and coarser sediments typically occur closer to the low tide mark. Carbonate concentrations are moderate (reflecting shelly material in the sediments) and the concentration of organic material is variable, but generally high. Intertidal Flats are wider and more extensive in macrotidal systems. Surfaces tend to occur from mean low water spring to mean high water spring elevations and are usually flat and not vegetated, but may be dissected by shallow (and often vegetated by saltmarsh species) drainage channels. Biological activity consists of both high and low tide visitors, as well as permanent inhabitants. Burrowing infauna , crustaceans, molluscs, fish and birds are generally abundant.

 

8. Mangroves

Mangrove environments generally consist of sediments associated with stands of salt-tolerant mangrove forest (comprised of various species of mangrove trees and shrubs). In some ways, mangroves can be considered the tropical equivalent of saltmarsh communities (although the two often co-exist). Surfaces beneath the mangrove forests generally occur from mean sea level to mean high water spring elevations, and are often associated with tidal creek drainage networks. Mangrove forests are generally more common and extensive in tropical regions. Sediment that accumulates (due to trapping and baffling by vegetation) beneath the mangrove forests generally comprises strongly-reduced, poorly- to moderately-sorted silts and clays. Carbonate concentrations are generally low. Concentrations of organic material are generally high. Mangroves typically support a diverse and productive community of flora and fauna. Burrowing infauna, epifaunal invertebrates (such as sessile organisms and crustaceans), molluscs, and low-tide and high-tide visitors (such as fish and water birds) are common inhabitants of mangrove forests.

 

9. Rocky reefs

Rocky reefs feature a hard substrate that may occur at supra-tidal to sub-tidal elevations. Surfaces are generally non-depositional and sometimes erosional, and are usually dominated by epifaunal and algal communities. Bedrock is often a major control on waterway shape (width, length and depth). Below the waterline, common habitats include intertidal rocky shorelines to subtidal reefs. Bedrock/rocky reefs limit the available habitat for burrowing organisms, but are important habitats for sessile organisms, organisms requiring sheltered conditions, and associated fish communities.

 

10. Saltmarshes

Saltmarsh environments consist of high-intertidal to supratidal halophytic vegetation such as salt-tolerant grasses, reeds, sedges and small shrubs, which stabilise fine sediments that have been transported by water. Sediments consist of poorly-sorted anoxic sandy silts and clays. Carbonate concentrations are low, and concentrations of organic material are generally high. Saltmarshes are more common in temperate regions, often in environments that would typically be colonised by mangroves in tropical regions. Saltmarshes have low gradients and may be dissected by shallow brackish pools. Saltmarshes and associated vegetation are habitats for a wide range of bioturbating infaunal and epifaunal invertebrates, as well as low-tide and high-tide visitors, such as fish and water birds.

 

11. Saltflats (also known as saltpans, sabkhas)

Saltflats, or saline supratidal mudflat facies, occur in dry evaporative environments, often in the tropics, that undergo infrequent tidal inundation. Sediments comprise poorly-sorted sandy silts and clays, including mineral deposits such as gypsum and halite, and desiccation cracks. Carbonate concentrations are generally high, and concentrations of organic material are generally low. Saltflats tend to be low gradient, and mostly featureless, with a varying degree of algal colonisation, and often with vertically accreting algal mats. saltflats generally occur above mean high water spring, and infrequent inundation by king tides creates a highly evaporative environment in which algal mats and salt tolerant grasses may be present. Very high levels of surface and groundwater salinity often precludes the growth of higher vegetation and biota (some infauna and epifauna may occur at lower elevations). Saltflats are habitats for birds, particularly during the wet season.

 

12. Tidal sand banks

Tidal sand banks are sedimentary features commonly found within tide-dominated estuaries, deltas and tidal creeks. Tidal sand banks are typically subtidal to intertidal in elevation, and consist of elongate linear to sinuous sand bars comprised of moderate- to well-sorted fine muds to sands. Channels dissecting tidal sand banks are scoured by strong currents, exposing the underlying bedrock or leaving a lag gravel, composed of shell debris and rock fragments. The banks and channels are often approximately aligned with the main tidal currents typically perpendicular to the shoreline, and sediments may fine towards the head of the estuary. Concentrations of carbonate material are generally high, whereas organic material concentrations are generally low, these tend to be higher in tropical estuaries. Strong tidal shear stresses and highly variable bottom morphology result in turbulent, well oxygenated, and turbid waters. Tidal sand banks may be vegetated, however high turbidity often limits primary productivity.


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