Brisbane River
Brisbane River's source is located in the Great Dividing Range, east of Kingaroy. It then makes its way south, past Mount Stanley, before being joined by the Stanley River, just south of Somerset Dam.
The river runs from there into Lake Wivenhoe, created by the Wivenhoe Dam. Beyond the dam, the river meanders eastward, meeting the Bremer River near Ipswich, then making its way through Brisbane's western suburbs, including Jindalee, Indooroopilly and Toowong.
Water from the highest point in the catchment has fallen from Mount Langley in the Conondale Range, 868m above sea level. The Brisbane River then flows Bulwer Island and Luggage Point through the Port of Brisbane and into southern Bramble Bay an embayment of Moreton Bay.
The river travels 344 km (214 mi) from Mount Stanley to the mouth. The river is dammed by the Wivenhoe Dam, forming Lake Wivenhoe, the main water supply for Brisbane.
Before European settlement, the Brisbane River was spiritually important and a vital food source for the Aboriginal people of the Turrbal nation with fishing and firestick farming in the upper reaches where there was freshwater, depending on the season.
Alluvial Plains
An alluvial plain is a largely flat landform created by the deposition of sediment over a long period of time by one or more rivers coming from highland regions, from which alluvial soil forms. A floodplain is part of the process, being the smaller area over which the rivers flood at a particular period of time, whereas the alluvial plain is the larger area representing the region over which the floodplains have shifted over geological time.
As the highlands erode due to weathering and water flow, the sediment from the hills is transported to the lower plain. Various creeks will carry the water further to a river, lake, bay, or ocean. As the sediments are deposited during flood conditions in the floodplain of a creek, the elevation of the floodplain will be raised. As this reduces the channel floodwater capacity, the creek will, over time, seek new, lower paths, forming a meander (a curving sinuous path). The leftover higher locations, typically natural levees at the margins of the flood channel, will themselves be eroded by lateral stream erosion and from local rainfall and possibly wind transport if the climate is arid and does not support soil-holding grasses. These processes, over geologic time, will form the plain, a region with little relief (local changes in elevation), yet with a constant but small slope.
At the posted coordinates, you can see the remnants of alluvial plains of the Brisbane River in the high bank. Large stones rounded and shaped by the force of the water have been deposited on the valley floor and over time, silt and sediment has built up around these stones forming what is now this high bank on the shore of Lake Wivenhoe.
Logging the Earthcache
To log this earthcache, you must answer the following questions. Please send your answers to the CO. Do not include answers in your log.
- Please describe what evidence you see to indicate that this was once an alluvial plain?
- What height do you estimate the embankment to be?
- Why do you think this high bank is now above the water level of the lake?
- Optional: please include a photo of your group in your watercraft.
You do not need to wait for a response before logging your find. I will contact you if there are any problems with your answers.
