Geomorphic Systems

The Hawkesbury-Nepean River lies in a large geological structure in eastern NSW known as the Sydney Basin. The Hawkesbury drains most most of this depression. The Hunter, Hacking, George and Parramatta Rivers drain smaller catchments within the Sydney Basin. 

The formation of the Sydney Basin

Approx. 300 million years ago

Australia was part of the ancient supercontinent Pangea.

300 – 250 million years ago 

Gondwana split from Pangea. During the Permian period, Gondwana began to break apart and sea levels rose submerging Australia.

The eastern part of Australia, including the Sydney Basin, experienced a period of great folding and faulting of tectonic plates, which led to the formation of the New England Fold Belt to the north-east and the Lachlan Fold Belt to the west. The Sydney Basin formed as a lowland between these two belts.

250 to 66 million years ago

The Triassic, Jurassic and Cretaceous periods saw significant sedimentation in the Sydney Basin. The basin started to fill with marine deposits from the ocean, and rivers transported eroded terrestrial material from the New England Fold Belt into the basin. This contributed to the formation of rock layers crucial to the formation of the Hawkesbury River, including sandstone and shale.

The final-break up of Gondwana during this period resulted in the creation of the basin’s seaward boundary to the east.

The formation of the Hawkesbury River

66 million years ago

Tectonic uplift and erosion processes during the Paleogene and Neogene eras led to the formation of the Great Dividing Range, a series of plateaus and low mountain ranges in eastern Australia. This formed a watershed that caused a number of river systems to develop, which flowed to the east and discharged into the ocean. The Hawkesbury River originates from these river systems.

During this time, widespread volcanic activity across eastern Australia caused extensive deposits of basalt that covered the Basin landscape. 

16 000 years ago

The Hawkesbury River, being a tide-dominated drowned river valley, experienced significant changes due to post-glacial sea-level rise. The rise in sea levels drowned existing valleys and gorges which had been carved into the lower reaches of the River.

11 000 years ago

Changes in climate around 11 000 years ago caused a reduction in freshwater inflows. This contributed to the formation of sand bars in areas like Brisbane Water and Pittwater. Processes of weathering, erosion and deposition during this period formed the river that we know today.

There are three main geological formations in the Hawkesbury-Nepean Catchment. The youngest geological stratum in the Sydney Basin (except for the mostly eroded basalt deposited during extensive volcanic activity 15 - 18 million years ago) is the Wianamatta Group. The Hawkesbury Sandstone lies beneath this layer and then the Narrabeen Group of sandstones and shales. 

The Wianamatta Group contains several different forms of shale which have been critical to the European history of the area. Shale is a great deal more fertile than the sandstone that dominates the areas around Botany Bay, the first European settlement. It is these extensive deposits of Wianamatta shale along the Cumberland Plains that led to the establishment of the Hawkesbury region as Sydney's 'food bowl' during the early settlement period, and the pattern of settlement that followed thereafter. 

The steep and rugged sandstone cliffs of Hawkesbury Sandstone and Narrabeen Group have resulted in a lack of expansion into areas north east and north west of the Hawkesbury River. As a result, large areas of native vegetation have been retained, and today, much of these areas are protected. 

Most river valleys tend to widen and become more shallow as they approach the sea. This is due to a combination of geological, hydrological, and geomorphological processes some of which include:

• Decreased gradient: The gradient or slope of the land tends to decrease as rivers move toward the sea. Lower gradients result in reduced energy for erosion and transportation of sediment, leading to the deposition of sediment along the riverbed and banks.

• Increased sediment load: Rivers carry sediment eroded from the land they traverse. As a river nears the sea, the sediment load tends to increase due to the cumulative effects of erosion along the course. 

• Reduced energy and velocity: With the decrease in slope and the proximity to the sea, the river's energy and velocity diminish. Lower energy levels mean that the river has less power to carry and transport sediment, resulting in the settling of particles on the riverbed and the widening of the channel.

This typical process of widening and becoming more shallow is not the case for the Hawkesbury-Nepean River. Along the upper and middle course of the River, there are narrow sandstone gorges at Castlereagh and Sackville. During flooding, these gorges create natural chokepoints and in the case of Sackville Reach, act as hydrological bottlenecks. As a result, floodwaters from the River's tributaries back up and rise rapidly, causing deep and widespread flooding across the floodplain. This is known as the 'bathtub effect' because much like a bathtub with more water coming out of a tap that can be drained, the volume of water will continue to increase. 

As a result, extensive flood plains have been created in three major areas of the catchment: (1) between Penrith and Castlereagh; (2) between North Richmond and Wilberforce; (3) downstream of Sackville to Spencer. The deposition of eroded material around these flood plain areas mean that some parts of the upper and middle course of the River are more shallow and wider than sections closer to the lower course of the River between Sackville and Brooklyn. 

Watch the following video to gain a better understanding of the 'bathtub' effect and its impact on residents in the Hawkesbury and Penrith areas.