How are estuaries formed?

The estuaries of the southern coast of Western Australia are of recent origin: about 6000 years old. Nevertheless, the history of the South Coast estuaries is part of the long geological story about the coastal belt where rivers flow into the Southern Ocean.

The basement rocks of the region, the hard Pre-Cambrian granite rocks, were eroded into hills and valleys long before Australia separated from Antarctica some 60 million years ago. Then in the late Eocene age (40 million years ago), the sea inundated the coast as far inland as the Stirling Range and covered the drowned landscape with marine sediment. This sediment is now the Plantagenet Group, which includes the spongolite rock (called Pallinup Siltstone) of the 100 kilometres wide plain, found at about 200 metres above sea level. Later, when the Darling Plateau rose, the coastal belt - called the Ravensthorpe Ramp by Cope in 1975 - tilted from the Jarrahwood Axis, sloping gently to the coast and beyond to the edge of the continental shelf. The figure below shows how this occurred. Rivers carved valleys through the spongolite to the coast, where the Pre-Cambrian granite rocks survive as coastal hills and headlands, such as the Albany mount and Cape Riche.

Some of these peaks remained as islands, but during the interglacial periods of the Pleistocene (the last two million years) when the sea level was about the same as at present, they were linked by coastal dunes which enclosed bays such as the Albany harbour and Wilson Inlet. The dune sand became cemented to limestone - called Tamala Limestone - the cliffs of which now form much of the southern coastline. In the last ice age sea level was more than 100 metres lower than it is now, the coastline was 30-40 kilometres further south near the edge of the continental shelf, and there were valleys and perhaps lakes where the estuaries are now. When the polar ice began to melt 20 000 years ago, the sea level rose rapidly and by about 6000 years ago had reached its present level. Sea water flooded the valleys and they became the estuaries and coastal lagoons of today.

At first, the estuaries were always open to the sea and sea water mixed freely with fresh water from the rivers, but subsequently there have been great changes. The ocean beaches formed again from sand washed up from the sea bed and eroded from the dunes. Sand was added to the old dunes and built new dunes and spits (like the Middleton Beach - Emu Point peninsula) which narrowed the mouths of most estuaries. Beach sand built flood tide deltas into the estuaries and sand bars that have blocked the mouths of many. Now exchange with the sea is restricted to periods when the bars are open. They are poorly flushed and the salinity regime is totally different from that of 6000 years ago. Some such as Lake Gardner near Albany and the Jedacuttup Lakes (in the Shire of Ravensthorpe) are now coastal lakes.

When first flooded, the estuaries were deeper than they are now, some probably with valleys down to hard rock. For a time the sea level may have been up to two metres higher than it is now. Sediment eroded from the catchments and sand from the sea beaches has progressively filled the Pleistocene valleys, some now almost to sea level. Coarse sediment brought by floods has built river deltas and spits. Waves have smoothed sandy shorelines and built beach ridges which the vegetation has stabilised. Samphire and other salt-tolerant plants have trapped sediment and formed salt marshes that have encroached on the open water. These same processes continually reshape the estuaries of today and reduce their volume.

Figure 5-2:
(A) Jarrahwood Axis and Ravensthorpe Ramp
(B) Perpendicular profile of the coast (stippled area is Pallinup Siltstone)
Adapted from Cope 1975.