Where the river joins the sea

In previous posts I've discussed a few peculiarities with the way some of our rivers flow, in particular the Clarence River which once ran backwards and the Wilsons River which flows away from the sea. This post is about another strange feature of the Northern Rivers which is the way many of them discharge into the sea.

Many people in the region will be aware of various issues with regard to erosion of sand our beaches or even deposition of sand choking river and creek mouths. Many people may be aware of Byron Shire Council having a policy of planned retreat from the areas along Belongil Beach at Byron Bay. Others may have heard of the silting up of Nambucca Harbour. But even less will realise that the biggest cause of these different problems is actually the same.

Richmond River mouth at Ballina. Note the white water of the Bar.
 Because of longshore drift the Ballina Bar is often treacheous.

But, let me back up for a moment. Have a look at Google maps or (even better) a paper map of the north coast of the New England / New South Wales area. Look at most of the major rivers. The Nambucca River, Clarence River, Richmond River, Tweed River. Look too at some of the smaller streams such as Tyagarah Creek, Cudgen Creek and others. What you might notice about all these streams is that they seem to flow north and roughly parallel to the coast only a short distance inland. They also join the sea on the southern side of headlands and on the northern side of long sandy beach systems. And therein lies the cause.

Along the coast of Eastern Australia are currents, the most well known is the Eastern Australian Current that flows south. However, the prevailing wind conditions which blow from the south to the north means that the direction of small currents and wave action is directed northward, these are called longshore currents. This has been the case during the Holocene (for many thousands of years) and has resulted in enormous amounts of sand being transported slowly up the coast line, where much of it ends up in southern Queensland forming Fraser Island.

Where the most direct route for the regions rivers would be to join the sea at right angles, longshore drift has caused sand dunes to build up sometimes even to the extent that it sometimes closes the mouths of the rivers. The movement of the sand has slowly pushed the river mouths further and further to the north until the come to an outcrop of rock which blocks the way. At this point the river mouth will cease to migrate along the coast and remain relatively stable until some storm, flood or man-made change occurs. A great example of a man-made change is Coffs Harbour, but more on that another time.

But why does the beach erode in many other places? Well, simply it is the impact of the headlands. On the northern side of the headlands along our coast there is only a little supply of sand (since the headland directs the sand away). Instead this is were sand is sourced to be transported north along the beaches. Places like Belongil Beach at Byron Bay are excellent examples where sand is naturally carried away northward along the edge Byron Marine Park, leaving houses built next to the sea at risk of being destroyed by the erosive processes.

As an aside, longshore currents are also partly responsible for the creation of some mineral deposits which have historically been mined. But more on that in a future post. 

Since I wrote the above, an anonymous comment raised an interesting point which quite reasonably raises questions my statements about the sand stability north of Byron Bay headland. I have reproduced the comment in red below:

Despite the position of rock headland anchor points and the change in coastal alignment along Northern NSW, any differential in longshore drift rates (sand losses from the sediment budget)should have equilbrated during the Holocene period, including sand losses into the deepwater sand lobe off Cape Byron. Erosion at Belongil Spit is more likely due to the interrupted supply caused by the Richmond River breakwaters at Ballina.

Bibliography/references:

White, M. E., 2000. Running Down, Water in a Changing Land. Kangaroo Press.