Evolution of Australian estuaries and coastal waterways

Estuaries and coastal waterways are highly dynamic environments in which geomorphic changes are driven by the deposition and erosion of sediment, which may occur over a range of timescales, from almost instantaneous (e.g. river floods), to progressive change over thousands of years (Cooper, 2001). Unlike many geological processes, sedimentation in coastal waterways can occur on timescales relevant to human society. Over time, continued sedimentation leads to the progressive conversion of estuarine waterbodies into intertidal and terrestrial environments, with obvious management implications (Roy et al., 2001). Infilling of coastal waterways by sediment is not constant in time or space. Coastal waterways receive sediment from a variety of sources, including fluvial, marine and aeolian (wind) inputs as well as detritus from fringing vegetation, organic material produced within the estuary, and human induced inputs. Expansion of intertidal and supratidal environments (e.g. saltmarshes), and progradation of fluvial deltas into estuaries and coastal waterways is likely to cause even faster infilling rates (Webster et al., 2002, Pasternack et al., 2002).

Evolutionary characteristics provide an important link between the many different types of coastal waterways. Each type of wave- or tide-dominated coastal waterway is at a different stage in its evolutionary continuum, having developed to a greater or lesser extent depending on regional sea level history, and the amount of sediment supplied to it. Wave-and tide-dominated systems follow different evolutionary pathways (Figure 1). Assuming constant sea level and sediment supply, and tectonic stability, embayments (or drowned river valleys) on wave-dominated coastlines tend to develop shore-parallel sand bodies that may enclose the entrance, creating a central basin behind them and restricting marine exchange. Once the sand bodies rise above sea-level (and become 'subaerial'), they are known as barriers, and the coastal waterway becomes a wave-dominated estuary - an effective trap for terrigenous sediment. Continued infilling, mostly by terrigenous sediment, eventually results in the central basin becoming completely filled with sediment, and the river channel then establishes a more direct connection with the ocean. Once the net transport of sediment is offshore, the coastal waterway becomes a wave-dominated delta, and catchment sediment is transferred to the ocean rather than becoming trapped (Roy et al., 1980, Roy, 1984b, Heap et al., In Press). Similarly, on tide-dominated coastlines, embayments become gradually infilled by sediment until a tide-dominated estuary is created. The extensively vegetated intertidal areas trap even more terrigenous and marine sediment, until the estuary becomes totally infilled and begins to prograde (or build out) seawards, becoming a tide-dominated delta (Woodroffe et al., 1989, Mulrennan et al., 1998, Chappell, 1993, Woodroffe et al., 1993).

Evolutionary 'family tree' for Australian coastal waterways

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).

Over geologic timescales (many thousands of years), estuaries are ephemeral systems that develop during times of slowly rising or stable sea level. Rising sea levels favour the formation of estuaries, embayments, and drowned river valleys; whereas falling sea levels favour the development of deltas (Harris et al., 2002). Potential changes to the present-day sea level, human induced or otherwise, may therefore have a pronounced effect upon estuaries, and the characteristic habitats they support. For example, a rise in sea level might induce a wave-dominated estuary to revert to an embayment (e.g. move from right to left along the evolutionary pathway in the above Figure), as the barrier would be drowned or eroded, and the distribution of sedimentary environments altered (Boyd et al., 1992).

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