During the peak of the Millennium Drought (1997-2011) inflows for the Murray-Darling Basin were the lowest on record and the lower River Murray received only 19% of its long-term average inflow during 2008-2009. As a consequence the pool-level in the lower Murray fell to 1 meter below sea level. This event triggered an episode of widespread mass failure in the alluvial river banks and ground subsidence in river-adjacent floodplain deposits between Blanchetown and Lake Alexandrina.

Multi-beam surveying of the channel and submerged river banks has been undertaken in sections of the river where large bank failures threatened private housing or public infrastructure. This data demonstrates that slumps and slab slides from the Murray’s submerged banks are widespread and common, and three different types of bank-failure morphology have been recognised. These are: Type 1, ‘recent’ (2009-2011) deep-seated rotational slumps cresting 5 to 10 metres inland from the waterline, characterised by distinct, sharply-defined failure scars and associated debris fields of angular blocks shed from the failure site; Type 2, ‘mature’ shallow planar-failures, with less well-defined smoother failure scars and associated debris fields of smoothed or rounded blocks; and Type 3, ‘relict’ shallow planar-failures characterised by subdued-relief scars that do not present an associated debris field. It is suspected that successive high-flow events progressively erode and redistribute material, smoothing the landslide scars and redistributing the slide-debris deposits.

Bank-failure and the delivery of slide material into the channel is interpreted to be an ongoing and long-term geomorphic characteristic of the lower River Murray. It is strongly suspected that the recent Type 1 failures are amplified equivalents of the older Type 2 and 3 failures, with the amplification being a consequence of anthropogenic modifications of the river channel and banks, e.g. levee and embankment construction on the channel margins, as well as the construction of barrages at the river mouth, which enabled the recession of the pool-level to reach 1 meter below sea level during the Millennium Drought. The deep cresting geometries of the Type 1 failures pose an increased risk to river users and a challenge for the management of this hazard.