GEOMORPHOLOGY REPORT - CRC LEME

EXECUTIVE SUMMARY
In early 2007, an airborne electromagnetic (AEM) survey was acquired along a 450 km reach
of the River Murray Corridor (RMC) in SE Australia. This aim of this survey, carried out
under the auspices of the Australian Government’s Community Stream Sampling and Salinity
Mapping Project (CSSSMP), and managed by the Bureau of Rural Sciences (BRS), is to
provide information vital for addressing salinity, land management and groundwater resource
issues. The study area stretches from where the Murray River crosses the South Australian
border eastwards and south along the Murray River to Torrumbarry Weir. A total of 24,000
line km of AEM data were acquired. The survey area encompasses iconic wetland areas,
national and state forest parks, and areas of irrigation and dryland farming.
Within the Lindsay-Wallpolla and Lake Victoria-Darling Anabranch project area key land
management questions include: 1) What is the potential for salt mobilisation during Living
Murray inundation actions? 2) How is salt delivered to the river? 3) How are the drivers of
floodplain health with respect to groundwater processes to be understood? 4) How are the
high recharge areas in the floodplain to be indentified? 5) What is the extent and thickness of
the Blanchetown Clay and the Coonambidgal Formation? 6) Where is salt stored in the
unsaturated zone?
To address many of these questions, maps of the distribution of salt stores, saline and fresh
groundwater and the hydraulic properties of soil and regolith materials in the shallow subsurface
are required. These are required to provide a 3-D understanding of how salt stores and
saline groundwaters connect to the surface waterways and land surface. Sub-surface
interpretations in the survey areas are hampered by a low density of useful borehole data in
the floodplain in particular, and by a paucity of soil and landscape surface mapping at
appropriate scales throughout the project area. New maps of surface materials, and a new
geomorphic understanding of the RMC project area is required to constrain the interpretation
of the AEM surveys, and address the land management questions.
This report documents the results of new surface materials mapping in the Lindsay-Wallpolla
and Lake Victoria-Darling Anabranch survey area, the methodology used to produce the
maps, and supporting analyses. These data inform interpretation of the surface layer of the
AEM products and provide an understanding of the geomorphic evolution of the landscape.
The Murray River in the Lindsay-Wallpolla Islands reach runs though a valley incised
through a Late Cainozoic succession consisting of the Blanchetown and Loxton-Parilla
Formations. These are mantled by Pleistocene aeolian sands of the Woorinen Formation.
The modern floodplain consists of three distinct generations of meander belt sediments with
scroll bars and oxbow billabongs. A conventional fine-grained floodplain is absent because
of the meander belt sediments extend across the full width of the floodplain within the
confines of the incised valley. However, older meander deposits are draped by floodplain silty
clays with the thicknesses increasing to more than a metre on the oldest deposits. The oldest
floodplain deposits also show distinctively longer meander wavelengths and wide channels
than the modern channel, indicating a diminished flow over time. The soils vary markedly in
salinity and pH, with a general trend of increasing pH and salinity with increasing age.
An extensive terrace occurs along both sides of the incised valley of the Murray River. The
terrace is mantled by aeolian silts and sands and have local outlying dunes of the Woorinen
Formation. Ghosts of channels and billabongs are visible in satellite imagery suggesting an
environment similar to from that which formed the modern floodplain. The soils are potassic,
alkaline, and moderately saline.
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