What is the maturity index of a coastal waterway?
The “maturity index” of an estuary is the ratio of the present water surface area (Ap), to its palaeo-water area (Ao) referenced to around 6,500 years before present, when sea level achieved its present position. The maturity index represents the degree to which the coastal waterway has been infilled by marine and terrigenous sediment. The maturity index is expressed as:
Maturity Index = Ap/Ao
The palaeo-water area of an estuary (Ao) can be estimated from the present open water area, plus the area that has been infilled since the last sea level rise (e.g. the alluvial floodplain, and other intertidal to supratidal estuarine sediments). Ao can be derived as follows:
Figure 1. Evolutionary ‘family tree’ for Australian coastal waterways showing different infilling pathways for wave-dominated and tide-dominated systems. Coastal lagoons, strandplain associated creeks and tidal creeks have been omitted as they do not receive significant amounts of fluvial sediment. From Ryan et al., 2003((Ryan, D.A., Heap, A.D., Radke, L.C. and Heggie, D.T. 2003. Conceptual models of Australia’s estuaries and coastal waterways: applications for resource management.)).
What is the lifespan of a coastal waterway?
The Lifespan of a coastal waterway is the length of time available before estuarine habitats are lost (e.g. central basin and intertidal areas) due to infilling, and only deltaic habitats remain (mainly channels and swamp areas). The lifespan is based on the volume of the coastal waterway (known also as accommodation space) divided by the sediment accumulation rate (Rs):
Lifespan = Volume/Rs
The sediment accumulation rate is a function of the volume fluvial sediment input (Qf), plus the marine sediment input (Qm), minus the volume of sediment exported from the estuary to the adjacent continental shelf or coastal littoral drift system (Ql). Hence:
Lifespan = Volume/(Qf + Qm – Ql)
What causes the maturity index to change?
The maturity and lifespan of coastal waterways is influenced by both sedimentation and erosion. Some natural controls on the sedimentation rates experienced by coastal waterways include climate (rainfall, seasonality), geology, slope (or topography), vegetation and the size of the catchment.
Human activities and uses that give rise to excessive loads of fine sediment include:
- agriculture occurs on steep slopes and;
- in cases where riparian vegetation has been removed (see river through forests indicator).
Coastal waterways to which this indicator applies
Transgressive coastlines, characterised by wave-dominated estuaries, wave-dominated deltas and lagoons, have a high sediment trapping efficiency, and are susceptible to increases in the magnitude of sediment loads carried by rivers. They are also more susceptible to the accumulation of particle-associated contaminants such as heavy metals and other toxicants, as these are retained in the coastal waterway, rather than being exported to the ocean.
In comparison, prograding coastlines characterised by tide-dominated deltas, export most of their sediment loads to the sea, and have a generally low sediment trapping efficiency. These coastal waterways are effectively ‘fully mature’, and further sedimentation does not greatly influence their morphology. As such, they contain a suite of habitats that will not be significantly affected by sedimentation.
Significance of maturity index and lifespan
As a wave-dominated estuary matures, the configuration of habitats will alter, as the central basin infills and is replaced by a channel system linking the river directly to the coast. The net result is an increase in turbidity levels (due to shoaling of the estuarine bed and increased wind-wave resuspension)((Roy, P. 2000. Structure and function of south-east Australian estuaries: A framework for addressing management issues.)), the loss of some habitats and a reduction in the overall species diversity((Roy, P.S., Williams, R.J., Jones, A.R., Yassini, I., Gibbs, P.J., Coates, B., West, R.J., Scanes, P.R., Hudson, J.P. and Nichol, S. (2001). Structure and function of southeast Australian estuaries. Estuarine, Coastal and Shelf Science, 53: 351-384.)). Knowing that immature estuaries are at risk of experiencing changes in habitat configuration, particularly if catchment sediment inputs are high, provides managers with crucial information for planning and decision making.
The lifespan and Maturity index are useful for detecting the severity of changes to estuaries since European settlement of Australia. For example, a coastal waterway maturity index of 0.5 (50%) indicates that it has taken about 6500 years to become half-filled. If the lifespan calculated for this same coastal waterway (using present-day measured sedimentation rates) is small compared to 6500 years, the difference may be attributed to anthropogenic activities (e.g. a post-European increase in the rate of sediment infilling). Studies have shown, for example, that many Australian fluvial systems have increased their sediment loads twofold since European settlement((Wasson, R.J., Olive, L.J. and Rosewell, C.J. (1996). Rates of erosion and sediment transport in Australia. In: D.E. Walling and B.W. Webb (Editors), Erosion and Sediment Yield: Global and Regional Perspectives. IAHS Special Publication, pp. 1-11.)). The measure of lifespan is also useful as a management tool since coastal waterways with a short lifespan are likely to be more severely affected by land clearance or other disturbances in their catchments than those with a longer lifespan.
Considerations for measurement and interpretation
- Accurate measurements of the palaeo-water area (circa 6500 years ago) are sometimes difficult to obtain, due to lack of knowledge regarding the pre-Holocene infilling history of the coastal waterway((Nichol, S. L. and Murray-Wallace, C. V. (1992) A partially preserved last interglacial estuarine fill; Narrawallee Inlet, New South Wales. Australian Journal of Earth Sciences 39(4):545-553)). For example, older Last Interglacial sediments may be erroneously interpreted as being younger than 6500 years, thereby overestimating the palaeo-water area.
- Sediment accumulation rates, if inaccurate, will yield large differences in Lifespan estimations. Incorrect estimations of Lifespan may result from inaccuracies in terrigenous sediment input, compaction of sediment, accumulation of organic material in the central basin, and loss of sediment to the coastal ocean due to large floods.
- Relative contribution of marine sediments is often high, much sediment was emplaced earlier in the history of the coastal waterway.
Existing information and data
A database containing physical information for 780 Australian coastal waterways was created by Bucher & Saenger((Bucher, D. and Saenger, P. (1994). A classification of tropical and subtropical Australian estuaries. Aquatic Conservation: Marine and Freshwater Ecosystems, 4: 1-19.)) and later updated and modified by Digby et al.((Digby, M.J., Saenger, P., Whelan, M.B., McConchie, D., Eyre, B., Holmes, N. & Bucher, D. (1998). A physical classification of Australian Estuaries (Report Prepared for the Urban Water Research Association of Australia No. 4178). Southern Cross University, Centre of Coastal Management, Lismore, NSW, 47pp.)). The most recent version of this database (now known as the Australian Estuarine Database; AED) was produced by Geoscience Australia and includes information on over 1,000 coastal waterways that may be accessed via OzCoast and OzEstuaries.
The AED includes an independent assessment of the geomorphology of 780 coastal depositional environments via a visual inspection of aerial photographs, LANDSAT TM images, maps, and nautical charts (see acknowledgements)((Heap, A., Bryce, S., Ryan, D., Radke, L., Smith, C., Smith, R., Harris, P. and Heggie, D. 2001. https://data.gov.au/dataset/australian-estuaries-and-coastal-waterways-a-geoscience-perspective-for-improved-and-integrated. AGSO Record 2001/07, pp. 118.)). Each waterway was classified, using established frameworks, as wave or tide-dominated estuaries, tide-dominated deltas and wave-dominated deltas, lagoons and strandplains and tidal flats((Boyd, R., Dalrymple, R.W. & Zaitlin, B.A. (1992). Classification of clastic coastal depositional environments. Sedimentary Geology 80, 139-150.))((Dalrymple, R.W., Zaitlin, B.A. and Boyd, R. (1992). Estuarine facies models: conceptual basis and stratigraphic implications. Journal of Sedimentary Petrology 62(6): 1130-1146.)). Only the visible geomorphology was used to determine the classification((Heap, A., Bryce, S., Ryan, D., Radke, L., Smith, C., Smith, R., Harris, P. and Heggie, D. 2001. https://data.gov.au/dataset/australian-estuaries-and-coastal-waterways-a-geoscience-perspective-for-improved-and-integrated. AGSO Record 2001/07, pp. 118.)). Open water areas, mangrove areas and saltmarsh and saltflat areas are also provided for use in maturity index calculations.
More information on aquatic sediments (changed from natural).
Brendan Brooke, Geoscience Australia
Peter Harris, Geoscience Australia
David Ryan, Geoscience Australia