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 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. These habitats may also intermittently support birds, turtles and seals.
Further information about barriers can be gained from the beach and dune area indicator fact sheet.
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.
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 their associated fish communities.
Saltmarsh environments generally 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 generally consist of poorly-sorted anoxic sandy silts and clays. Carbonate concentrations are generally low, and concentrations of organic material are generally high. Saltmarshes are generally 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).
Further information about saltmarshes can be gained from the saltmarsh area indicator fact sheet.
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 concentrations of organic material is 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.