|Mark's wonderful picture of the delta on Watson Taylor's Lake|
A delta is formed when sediment suspended in flowing water settles out as it reaches a large water body. Probably the most well known deltas in the world are the Mississippi River Delta, the Ganges River and the Nile River. However, it also creates a question, why don’t we see deltas up and down the Northern Rivers and North Coast areas?
Several studies of off-shore sedimentation have been done along the coast, the earliest studies tended to be looking mainly for heavy mineral deposits such as ilmanite, rutile, zircon and even gold or for military/oceanographic purposes. However, both these studies and others specifically to understand the off-shore environment have demonstrated some interesting facts including why we don’t have river deltas.
The first part of understanding the off-shore sedimentary environment is to understand that currently the sea level is at a very high level in historic terms. It reflects the current warm interglacial period that has arisen. The lowest sea levels that most 'recently' occurred was following the beginning of the Pleistocene which was the period since the the last 130 000 years or so (Roy & Thom 1981 & Drury 1982). According to Drury (1983) and many other authors, sea levels much lower early in the Pleistocene including instances of maybe 100 metres or more (Den Dexter 1974 suggested around 200metres lower at the beginning of the Pleistocene . This caused erosion of most pre-existing soft sediments along what is now the submerged the continental shelf. But it was not a simple transition from glacial to interglacial with many cycles during the Pleistocene and corresponding to alternating periods of coastal sedimentary deposition followed by erosion of those new sediments, so it was a fairly complicated period.
Since the beginning of the Pleistocene Roy & Thom (1981) thought that it was likely that there were only two major causes of movement of sediments along the coast, the first was the effect of sea level fluctuations during interglacial and glacial periods and the second wave and wind action which had the effect of transporting sediment northward. These forces were probably enough to create sand barriers such as those preserved on the Northern Rivers inland from the active Holocene sand barriers and beach systems we enjoy today (more about the Pleistocene sand barriers in a future post). But, Roberts and Boyd (2004) indicated that Roy & Thom (1981) might not be totally correct in thinking there were only two major causes because in some areas the Eastern Australian current also seems to be a significant driver of sediment. In fact they noted that off the coast of Byron Bay in as little as 30metres of water the Eastern Australian Current was present and could scour away any sediments that might have been deposited or stopping sediments from being deposited.
This means that when the rivers, be they the Tweed, Clarence, Richmond, Bellinger, Nambucca, Macleay, Hastings or any others drop their sediment load, the presence of currents then sweeps the finest sediments away, mainly further out to sea, maybe to the edge of the continental shelf. The heavier sediments which drop closest to the coast are affected by waves and storms which drive the sandy sediments northward along the coast which contribute to the barrier beach systems we have in abundance.
This is probably a simplistic way of explaining and I've missed a few complicating factors such as continental shelf slopes but it seems that because of the combination wave, storm and sea current process we don’t get any river deltas in our region, unless they are protected by sand barriers such as the one protecting Watsons Talylors Lake on the Camden Haven River.
*Den Exter, P. 1974. The coastal morphology and late Quaternary evolution of the Camden Haven District. University of New England, PhD thesis.
*Drury, L.W. 1982. Hydrogeology and Quaternary Stratigraphy of the Richmond River Valley, New South Wales. University of New South Wales, PhD thesis.
*Roberts, J.J. & Boyd, R. 2004. Late Quaternary core stratigraphy of the northern New South Wales continental shelf. Australian Journal of Earth Sciences v51.
*Roy, P.S. & Thom, B.G. 1981. Late Quaternary marine deposition in New South Wales and southern Queensland – an evolutionary model. Journal of the Geological Society of Australia v28.