GEOMORPHOLOGY REPORT - CRC LEME

metre of the ground surface (Chen 1995). The Darling River and its Anabranch also enter the
Murray River in this reach at Wentworth, and approximately 15 km west of Wentworth,
respectively.
3 KEY LAND MANAGEMENT QUESTIONS
The main identified issue in this iconic reach of the Murray River is the downstream impact
of salt mobilisation during flood recessions. Research to date indicates that the Lindsay and
Wallpolla Islands accumulate large amounts of salt during low flow periods and that
significant floods could act to liberate much of this salt, moving it downriver where it could
severely affect agricultural areas in South Australia. Accordingly, the key identified
requirements are the design of an environmental watering regime and planning of
revegetation strategies to optimise floodplain health and limit the salinity impact on the
Murray River. This will entail gaining a more detailed understanding of the floodplain
characteristics, including flush zones and salt stores. Identification of major salt influxes to
the river and possible interception zones is also desired. These concerns can be expressed in
the following questions (from Lawrie 2006), with conceptual geophysical targets shown in
Figure 3. Figure 4 shows a cross-section of the Murray River floodplain and Wallpolla Island.
1. What is the potential for salt mobilisation during Living Murray inundation actions?
This requires identifying holes in the Coonambidgal Formation – target 1. Salt stored
in the sub-surface (targets 2 and 5 in the model below) may be mobilised through
connected pathways to the river and surface (through targets 1 and 3). This question
addresses the act of mobilisation and is therefore concerned with the source or initial
position of the salt and what is causing it to move. This requires identifying high salt
stores that are at risk of being mobilised. Therefore, we need to identify high
conductivity salt stores, particularly in the Coonambidgal Formation and upper
Monoman Formation (target 5), and we need to identify low conductivity zones
where water preferentially feeds into the floodplain sediments to potentially mobilise
these salt stores, namely flush zones (target 3) and floodplain recharge areas (target
1). In the model below. Questions 2 and 3, below, deal with the destination of the
mobilised salt. See Figure 3.
2. Delivery of salt to the river. How salt is being delivered to the river requires
identification of high salinity zones (similar to previous question) and relatively
permeable zones or pathways for groundwater movement back to the river or
anabranches once the flood recedes – targets 5, 4 and 2 in Figure 3. Salt being
delivered to the river now will be interpreted from mapping salt stores (eg targets 2
and 5) and their connection to the river either directly or indirectly through
preferential flow paths. Additionally, salt being delivered to the river will also be
mapped by identifying areas where Blanchetown Clay is thin or absent, giving rise to
potential higher saline influxes to the river from saline groundwaters in the Loxton-
Parilla Sands (target 4 in Figure 3).
3. Understanding of the drivers of floodplain health with respect to groundwater
processes. This is matter of identifying elements of floodplain and groundwater
processes – targets 1, 2, 3 and 4 in the model below. This is a matter of identifying
elements of floodplain composition (including salt), groundwater levels and
groundwater processes. AEM can provide important baseline data in resolving this
question by defining the distribution of fine and coarse grain floodplain lithologies
(target: whole of floodplain, particularly top 5 metres), high salinity zones (target 5),
high recharge zones (target 1), flush zones (target 3), permeable pathways delivering
salt to prone areas such as anabranches and other depressions (target 2) and exposure
to saline fluxes from the Parilla Sand (target 4). Recharge zones (target 1), salt stores
(2 and 5) and flush zones (target 3) are conceptualised in Figure 3.
3