Lesson 4

Minnamurra River; Geomorphic Systems

Lesson goals

Understand the geomorphic systems of the Minnamurra River.

Lesson description

The resources and supplied googleslide powerpoint presentation provided in this lesson will aid in the explanation and description of the formation of Sydney Basin and Illawarra Escarpment, river processes of erosion, transport and deposition and coastal processes such as constructive and destructive waves, long-shore drift, storm surge and subsequent dune recovery.

Estimated time

2 lessons (~100 minutes)

Supporting materials

ENS student presentation template - Geomorphic System

Outcomes

Content

Examines places, environments and natural and human phenomena, for their characteristics, spatial patterns, interactions and changes over time.
Explains geographical processes and influences, at a range of scales, that form and transform places and environments.

Geographic Skills and Tools

Analyses and synthesises relevant geographical information from a variety of sources.
Applies geographical inquiry skills and tools, including spatial technologies, fieldwork, and ethical practices, to investigate places and environments.

Focus Area - Earth's natural systems

Examples relating to geomorphic systems

- processes at tectonic boundaries

- volcanic eruptions

- soil formation

- coastal and river processes

- water storage and flows

Supporting Materials Access

Follow the link (or image below) to access the ENS student presentation template - Geomorphic System. Once opened, save to your individual google drive or download your own copy to share via google classroom etc, or print a hard copy.

Once saved, use the resources below to complete the lesson.

Geomorphic - Earths Natural Systems student presentations

Geology of Minnamurra River

The geology of Minnamurra River consists of a range of sedimentary and igneous rocks deposited 50 - 300 million years ago during the formation of the Sydney Basin, and more recently deposited marine and alluvial (land based) sediments.

The region is defined by the Illawarra Escarpment, an erosional land-form, where the inland escarpment slowly retreats due to the processes of weathering and erosion, leading to the formation of upper plateaus and cliff-lines, steep hillsides and flat, low elevation coastal plains, as shown in the image below as red, green and blue areas respectively.

Lesson activity #1

Sydney Basin

Location

The Sydney Basin lies on the central east coast of NSW and covers an area of approximately 3.5 million hectares. The basin extends from just north of Batemans Bay to Nelson Bay on the central coast, and almost as far west as Mudgee. Minnamurra River is located within the southern half of the basin, and is greatly shaped by the variety of rock formations created as the Sydney Basin formed.

Formation

The Sydney Basin was formed when the earth’s crust expanded, subsided and filled with sediment between the late Carboniferous and Triassic, approximately 300 to 50 million years ago (mya). Early stages of development occurred while the Earths continents were joined to form Pangaea and Gondwanaland. as tectonic forces folded the Australian and Antarctic continents to form the Lachlan Fold Belt. These same forces formed a large mountain range, which slowly weathered and eroded away, filling the fold belt with layers of sediment, deposited into a shallow sea. These conditions allowed extensive forests to grow, which formed layers of coal as they were buried under more sediment. At the latter stages of the Basin filling, areas of volcanic activity occurred, depositing small basalt flows, including those found at Minnamurra Rivermouth.

Over time, tectonic forces has led to the uplift of these layers to form the Great Dividing Range, which is now weathering and eroding to form the land-forms we see today, such as cliffs, valleys and gorges found across the Illawarra Escarpment.

Watch the following video to gain an understanding of the formation of the Sydney Basin's geological features

Formation of the Sydney Basin.MOV

1) Pangea/Gondwanaland

Approximately 400-300 mya the Earths Continents were joined to form a Super-continent known as Pangaea, which gradually shifted over time to form Gondwanaland. At this time Antarctica and Australia were joined along what is now the Great Australian Bight. Geologists have based these models upon a range of different evidences, including shared fossils, rock types and geological features between continents that are now many thousands of kilometers apart.

2) Lachlan Fold Belt

Tectonic forces folded the then joined continents, approximately 400 mya to form a continuous valley known as the Lachlan Fold Belt. This folding also formed a large mountain range to the west of the fold belt, which eroded away over time, and supplied sediment to the Sydney Basin.

3) Weathering and erosion of mountain range

Over time, physical and chemical processes weathered and eroded the mountains, creating large volumes of sediments. These sediments ranged in size from fine particles of silt, sand and gravel and large boulders carried by glaciers and fast flowing rivers.

6) Transport and deposition of sediments

These sediments were transported by streams and rivers downstream, and were deposited over time into a shallow marine environment. Evidence of this marine environment can now be found through the presence of fossil shells and other marine organisms in the sedimentary rocks throughout the Sydney Basin.

7) Formation of Coal

The swampy conditions in the Sydney Basin at this time led to the growth of extensive forests during the Carboniferous period approximately 300 mya. Over time, the vegetation fell into shallow water in swamps and slow moving streams. This water formed a low oxygen (anoxic) environment which prevented the decomposition of the plant matter. Instead, it was compressed to form peat, and under further pressure turned to lignite and eventually coal.

8) Volcanic activity

A number of volcanic eruptions occurred throughout the formation of the Sydney Basin, as magma from below the crust was forced upwards through cracks and faults to form small volcanoes. These volcanoes erupted into shallow seas, quickly cooling the lava to form basalt and latitie, two hard igneous rocks which are resistant to weathering and erosion. These rocks form the eastern areas of Minnamurra River, including Minnamurra Headland and Stack Island.

9) Uplift of basin

Over time, the rock layers which formed Sydney Basin were pushed upwards by tectonic pressure. This led to the formation of mountain ranges that are still present today, such as the Blue Mountains and Illawarra Escarpment.

10) Weathering and erosion of basin

The Sydney Basin is now an erosional land-form, as the processes of weathering and erosion slowly remove the material over time. The Illawarra Escarpment is an excellent example of an erosional land-form, and has led to the formation of the landscape surrounding Minnamurra River.

Lesson activity #2

Geological Mapping

Southern Coalfields

Geology of the Wollongong - Kiama Region

Kiama Coastal Sediments

Use the interactive 'Spatial Map Viewer' to examine the topography of the Minnamurra River catchment

Use the Geological maps provided to estimate and draw the geological stratigraphy (layers) of the Illawarra Escarpment, between Knights Hill and Minnamurra Rivermouth

Escarpment Stratigraphy

Lesson activity #3

River Processes

Weathering and Erosion

Movement of water causes the breakup and removal of sediments.

Transport

Flowing water transports eroded sediments downstream

Deposition

As energy levels decrease as faster flow reduces, sediments settle out of the current and are are deposited

Erosional Processes

Rivers erode their banks and bed as they flow, slowly breaking down sediment, before it may be removed and transported downstream. This process wears away the beds and banks of rivers, and can even change the shape of a river's channel and broader landscape over many thousands of years.

Hydraulic Action

Hydraulic action occurs is when the high power of water pressure from the river forces water into small air pockets on the river bank and bed, causing them to break apart over time. As the water pressure builds against the river banks, it slowly breaks them down over time.

Abrasion

Small rocks and pebbles are pushed along the bed of a river by the flowing current. Over time, this has a sandpapering or 'abrasive' effect on the river bed, and the smaller rocks grinding over the surface eventually wear it down into features such as 'potholes'.

Attrition

The rocks carried by rivers in the current are knocked against one-another in the current, rounding off sharp edges and corners. The rocks and pebbles continue to be carried along by the river, eventually becoming smaller and more rounded.

Solution

Some kinds of rocks , such as limestone, can be slowly dissolved by water over time. The dissolved rock is carried in the water as a solution, and may eventually settle out as 'precipitation', forming features such as those found within limestone caves.

Impacts of erosion

Erosional processes lead to three changes to a riverbed.

Headward erosion increases the length of a river, as the headwaters are eroded further inland.

Lateral erosion increases the width of the river channel, as the banks are eroded further apart.

Vertical erosion increases the depth of a river channel, forming features such as the Minnamurra Canyon.

Headward erosion

Increases length of river channel

Vertical erosion

Increases depth of river channel

Lateral erosion

Increases width of river channel

Channel Profile

As a river flows from the source, transition and floodplain zones, vertical and headward erosion is reduced, and lateral erosion is increased. This results in the river channel profile broadening from a narrow valley to a wide, meandering river.

Source zone bed profile

Location - Minnamurra Canyon

Transition zone bed profile

Location - Minnamurra Rainforest

Floodplain zone bed profile

Location - Minnamurra Estuary

Transport Processes

The force and energy of flowing water allows rivers to transport large volumes of sediment to be downstream. This transport carries weathered and eroded material from the headwaters in the source zone downstream towards and into the ocean.

The distance that sediments may be transported is determined by both the particle size and the water velocity, with smaller particles transported more readily than large, and higher velocities leading to greater rates of transport than slower moving water.

Suspension

Fine particles of sediment, such as silt and mud, are suspended in the water column, and prevented from settling out trough water flow and turbulence.

Traction

Larger sediments such as rocks and boulders may be moved during high flow conditions through traction, where the water pressure building up behind the rocks pushes and rolls the sediment downstream.

Saltation

Salutation, occurs when medium sized particles such as gravel and pebbles are lifted and carried by the water flow for short periods, before settling the the riverbed once again. This causes them to effectively bounce along the streambed downstream.

Solution

Dissolved minerals are transported in the water column, and may react with other dissolved chemicals such as salt to form precipitates and eventually settle out.

Depositional Processes

Deposition occurs when water velocity slows to the point that sediment particles can no longer be transported. Larger sediment such as gravel is more readily deposited than fine sediment such as silt, as larger sediments require higher water velocity to be moved, while fine particles may remain in suspension for long periods.

A - Riverbank & Riverbed Deposition

Water flowing closest to the riverbed is slowed down by friction with the substrate below. Sediment settles out of this slower moving water, leading to deposition.

B - River-bend Deposition

Water flowing on the inside of a river-bend is slowed down relative to the outside of the bend due to friction with the riverbank. This leads to sediment deposition upon the inside bank of river-bends.

C - Rivermouth Deposition

As water flows from the river into a larger water body, such as a lake or ocean, the energy is dissipated and the flow is slowed. This drop in velocity leads to sediment being deposited to form sandbars and deltas.

Lesson activity #4

Coastal Processes

Natural cycles of erosion and deposition

The main process that forms coastal dune systems is called the accretion cycle - where sand is deposited, stored and removed in a constant cycle. At any one time on a beach it is either undergoing erosion or deposition, depending on the conditions present.

Erosion

Erosion is the process in which weathered material such as sand, is transported by wind or water.

Deposition

Deposition is the process where eroded material, such as sand, is laid down.

Waves both build and erode the dunes

When the backwash is stronger than the swash it causes EROSION

When the swash is stronger than the backwash it causes DEPOSITION

Destructive waves

Destructive waves are large and contain high energy and are more frequent. The backwash has less time to soak into the sand, creating more running water to transport the sand out to sea.

Constructive waves

Constructive waves are smaller and contain less energy. As the wave breaks it carries sand up the beach which is deposited as the backwash slowly drains away.

Source: Geoactive 2, Chapter 9 'Coastal Management', (2010)

Constructive waves deposit sand on the beach which then dries out and is blown to the back of the beach by the wind. Grasses such as spinifex trap the sand. Over time, succession occurs and back dunes form with shrubs such as wattle, tea tree and banksia. This vegetation is crucial in stabilising the dunes.

Longshore drift moves sand along the beach

Source: Geoactive 2, Chapter 9 'Coastal Management', (2010)

Longshore drift is the action of sand moving from one end of the beach to the other. This happens in a zig-zag pattern due to the sand depositing on the beach at the angle of the wave, but washing back into the water at a right angle due to gravity.
On the east coast of Australia, sand moves northward in the swash along the beach due to the angle of the waves caused by direction of the prevailing southerly winds, being particularly strong in winter.
However, in summer prevailing north-east winds cause longshore drift to the south.

Storms surges and high tides

During storms the energy of waves increases. The waves become destructive and erosion occurs to the dunes.

Storm conditions such as onshore winds and atmospheric pressure changes in the ocean's surface can cause a storm surge which is a sudden rise of water (between 2-5 metres higher than normal). If the storm surge occurs simultaneously with a very high tide the height of the water can increase even higher, with the potential for erosion and flooding to be catastrophic.

Shoreline recovery following a storm

Although storms cause erosion to beaches, the sand eventually deposits back onto the beach.

The adjacent diagram shows in the storm profile how sand is transported offshore to form a 'storm bar'. This sand bar functions to slow the energy of the storm waves and eventually the sand returns to the beach as the lower energy waves become constructive.

East Corrimal Beach, August 2020

Evidence of severe foredune erosion following an East Coast Low in July 2020

East Corrimal Beach, five months later

Evidence of sand deposition, building back the beach and reforming the foredune

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