The Lower Hawkesbury River Estuary is a drowned river valley, which was cut into Hawkesbury sandstone bedrock during the ice ages when ocean levels were much lower than they are now.
There are several major tributaries to the Lower Hawkesbury Estuary, including Berowra, Mangrove, Mooney Mooney, Mullet, and Cowan Creeks. With its tall, highly weathered vertical sandstone cliffs and gorges, open waterways, secluded bays and expanse of natural vegetation, this area is one of the most spectacular river valleys in New South Wales.
The drowned river valley, with its steep sandstone slopes and chiselled gorges, has meant foreshore areas are largely only accessible by boat. Those foreshores that are relatively flat are mostly occupied by private development or natural mangrove barriers.
The Lower Hawkesbury River is part of the Sydney Basin, created in the Permian and Triassic ages (200-250 million years ago), and consists almost entirely of horizontally bedded sedimentary rocks. At this time, the basin was a large lake filling with sediment, before being uplifted at the end of the Triassic age. Following this, the uplifted sandstone bedrock was eroded over millions of years to form deep V-shaped series of valleys bordered by steep cliffs. These valleys were then drowned during the last sea level rise (about 20,000 to 7,000 years ago) to form the changed valley shape that we see today.
Channel widths of the Hawkesbury River valley vary from just over 100 metres in the upper reaches to nearly 1km at Brooklyn. Downstream from the Brooklyn Bridge the water depth is 8 to 9 metres.
Many islands were created by the drowning of old ridge lines, such as Bar, Milson, Triangle, Peat, Long and Spectacle islands. You can see Triangle Island to the left of GZ across from and in the channel.
The tributaries of the Lower Hawkesbury estuary are also deeply incised gorges characterised by riverine and marine sediment bars at or near the confluence with the main channel. The shallow nature of the upper reaches of tributary creeks has led to the formation of extensive shoals with deep scour holes around the outside of bends or against extruding rock edges. The drowned tributary valleys tend to form backwater areas to the main channel, for example, near the township at GZ.
Mangrove species in the region are dominantly grey mangroves and river mangroves. The muddy sands rich in organics have tended to accumulate in little bays on the river edges forming muddy shores and shallow muddy bays. They are a productive habitat for mangrove growth.
EarthCache - Logging Requirements:
Please email the CO with your answers to the following:
1. From the coordinates walk towards the end of the jetty. Describe what you see on the other side of the river.
2: Turn 180 degrees. The little township of Spencer is built on what geographical formation?
3. Compare the level of the river to what you have observed when answering questions 1 and 2. How do you think this discrepancy occurred?
4. (Optional) Based on the information gained on the sign at GZ, where in the universe is Spencer?
*Please note, you do not have to wait for approval from the CO to log your find, and you may do so as soon as you have sent your email, however logs may be deleted if no verification email is received within a week.
Congratulations to Mr&MrsFish for their First to Find.
References:
Hawkesbury Estuary Program: Lower Hawkesbury Estuary Management Plan, 2008
http://www.hornsby.nsw.gov.au/media/documents/environment-and-waste/water-catchments/estuary management/Lower_Hawkesbury_Estuary_Management_Plan.pdf
Wikipedia: Hawkesbury River