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Geology
Geology
The Longneck Lagoon Catchment is a small sub-catchment of the Hawkesbury Nepean Catchment. Both catchments fall within the Sydney Basin which was laid down during the Permian period approximately 250 - 300 million years ago.
The most common geological formations found within the Longneck Catchment are mainly Hawkesbury sandstones and Wianamatta shales. They were deposited during the Triassic period (66 - 250 million years ago). During this time, much of the Sydney basin was submerged by the ocean, and rivers transported eroded terrestrial material from the New England Fold Belt into the basin. Longneck Lagoon Catchment is a low point within the Sydney Basin. The upper areas of the Longneck Catchment consist mainly of Wianamatta shales.
Tertiary sediments are also found in the Longneck Catchment. The Tertiary period was approximately 2.6 - 66 million years ago. The upper sequence of tertiary alluvium is dominated by Lononderry clay. Underneath this is Rickaby's Creek gravels. These layers dominate the lower areas of the Longneck Catchment and are probably a result of deposition from previous courses of the Hawkesbury River and major creeks.
During the Quarternary period which began about 2.6 million years ago, sediments were formed by recent depositional events in the Hawkesbury floodplain. These deposits are silts and sandy material which have resulted from weathering. These silts and sands also dominate the lower areas of the Longneck Catchment.
Source 1:A map showing the different soil types and underlying geology of Longneck Lagoon.
Human impacts on soils
Human impacts on soils
Source 2:A diagram showing the causes and impacts of dryland salinity.
Sources 3 and 4:Photographs showing evidence of salt scalds at Longneck Lagoon. Credits: Longneck Lagoon EEC.
Salt scalds
Salt scalds
Salt scalding is the development of a hard impermeable surface on soils as a result of wind or sheet erosion (dry scald) or through deposition of salts and clays following evaporation of surface water (wet scald). Wet salt scalds are present in the lagoon foreshores.
These wet scalds have resulted from the removal of native vegetation in the catchment, especially during early European settlement until 1971, when Longneck Lagoon and the surrounding woodland were classified as a reserve. Native vegetation is effective at absorbing and transpiring most of the water entering the soil profile from rainfall. As a result, only a small proportion of rainfall can reach the groundwater system. Water entering the groundwater system is known as groundwater recharge. When native vegetation is removed, unused rainwater leaks into the ground causing groundwater to rise. Groundwater carries dissolved salts from the underlying soil and bedrock material through which it travels. As saline groundwater comes close (within two metres) to the soil surface, salt enters the plant root zone. Even where the groundwater does not bring much salt with it, the ‘water-logging’ of the root zone can damage or kill vegetation.
Salt scalds are a concern for the health of the lagoon because swamp vegetation cannot survive in soils with high salt levels. Salinity in surface soils can also make the land more susceptible to erosion. Local runoff can erode surface soils leading to the enlargement of salt scald areas leading to larger areas of the wetland that are unlikely to produce any regeneration.
Erosion
Erosion
Water erosion has become a problem around the Scheyville area. Erosion occurs as a result of the clearing of native vegetation including trees, shrubs and native grasses. When it rains, plant leaves and root systems help slow rainwater down which leads to higher rates of infiltration. The removal of this vegetation can lead to increased runoff. When there are higher rates of runoff, soil is washed into the creeks and lagoon leading to sedimentation. This sedimentation can reduce water depth, impact on water quality and smother wetland habitats.
The Hawkesbury River 'bathtub effect'
The Hawkesbury River 'bathtub effect'
Longneck Lagoon's close proximity to the Hawkesbury River makes it susceptible to flooding. Large scale flooding events along the Hawkesbury are caused by a combination of atmospheric, hydrological and geomorphic factors.
The Hawkesbury-Nepean Catchment area is approximately 21,400 square kilometres and it includes a number of major rivers and tributaries including the Hawkesbury-Nepean, Colo, Coxs, Grose, McDonald, Wollondilly and Warragamba rivers. When weather events such as East Coast Lows generate heavy, widespread rainfall across the catchment, a significant amount of water is drained into the Hawkesbury River in a short period of time.
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. Longneck Lagoon is located in a low point of the flood plain and is significantly impacted by these flooding events.
Watch the following video to gain a better understanding of the 'bathtub effect' and its impact on residents in the Hawkesbury and Penrith areas.
Source 7:Photograph of sandstone cliffs at Sackville Reach. These large cliffs are the cause of the 'bathtub effect' and the extensive flooding that occurs further upstream. https://www.sl.nsw.gov.au/stories/dyarubbin/chapter-5-gurangatty-story/5
Suggested lesson activity
Suggested lesson activity
Complete the topographic map questions in the embedded worksheet below.
Longneck Lagoon topographic map.pdfSources:
Geology and Water Quality of the Longneck Catchment (Date unknown), Cattai Management Committee and the Hawkesbury Nepean Catchment Management Trust.
https://www.environment.nsw.gov.au/resources/salinity/allaboutsalinity.pdf
https://wetlandinfo.des.qld.gov.au/wetlands/ecology/processes-systems/sediment/
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