2. Lake Formation

• Geology
• Sea Level Changes
• Formation of Beach Dune Barriers
• Lake infilling

Geology

There are hills to the north, west and south of the lake and sand dunes to the east. Geologists estimate that the hills began forming some 200 million years ago and that volcanic and uplift activity continued until around 20 million years ago. The sand dunes which form the barrier between the lake and the ocean are the result of much more recent sea level changes.

Sea Level Changes

For millions of years the sea level has been changing as a result of ice ages coming and going. The graph below shows an estimation of world sea levels through time, relative to the present level.

You can see from the graph that there have been twelve ice ages of varying intensity over the last 250,000 years and that the sea level has fallen to more than 100 metres below the present level. The graph also shows that the last ice age about 20,000 years ago was very severe and that the thaw was very long, producing today's relatively high sea level (we have to go back over 120,000 years to find a higher level).

Figure 1: Changes in sea level over the past 250 000 years

Because the sea level is now higher than it has been for a very long time, old coastline features such as beaches and cliffs are now underwater. The present day coastline features are therefore relatively recent. It has only been since the sea level stabilised in about the last 6000 years, that the beach dune system along the NSW Coast has been able to develop.

Formation of Beach Dune Barriers

As the last ice age began to thaw, the rising waters and waves pushed before them sands from ancient beaches. These sands were reworked by the waves and wind to form the present day beach dune barrier between the lake and the ocean.

As they formed, the shape of the beaches was influenced by the direction of the prevailing winds and waves (the south east) and by the location of high rocky ground, which became the beach headlands. Sand moving into the coastal bays between the rocky headlands closed off the bays and, in doing so, formed coastal lakes (see Figure 2 below).

Lake Illawarra is relatively large - 35km² and the dunes formed before the entire basin was filled. The dunes only formed a barrier along the coast leaving the lake area behind the dunes empty. As a result, the entrance through the dunes is shallow (around 1 metre) with the enclosed basin also being relatively shallow (maximum depth of 3.5m)

We can now see that:

• one of the factors determining lake depth is the shape of the coastal bays and inlets before the last sea level rise (there are also other factors such as how easily the catchment soils erode);
• the sand in the Lake Illawarra entrance channel has been pushed up from the sea, not washed down from the catchment.

Lake infilling

The lake is in an advanced state of infilling. The average depth to bed rock is of the order of 10 - 20m. The average lake depth is now only 1.9m and is infilling at a rate of 2-3mm/year.

Figure 2: Formation process of Coastal Lakes

Lake Illawarra is nearing the end of Stage B in geological time.

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