Also known as: riverine estuary, macrotidal delta
Conceptual model diagrams of tide-dominated deltas
Key features of tide-dominated deltas
- 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.
- Large entrance promotes efficient marine flushing.
- River flow typically high, and flooding may expel marine water and flush material from the delta.
- Turbidity is naturally high due to strong turbulence induced by tides.
- 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.
- 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.
- ‘Mature’ in terms of evolution. Tend to be stable in terms of morphology (given stable sea level).
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.
Figure 1.Examples of tide-dominated deltas: Burdekin River (QLD), Limmen Bight River (NT), and Norman River (QLD).
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).