Geomorphic evolution of Coorong, Lower Lakes and Murray Mouth

Last week, Celine Steinfeld from the Wentworth Group and I had the pleasure of a field trip to the Coorong, Lower Lakes and Murray Mouth (CLLMM) with two eminent coastal geomorphologists, Bob Bourman and Nick Harvey. They were able to show us many sites to illustrate the Late Quaternary evolution of the area and the dynamics of the river mouth.

The south east part of South Australia has one of the most complete Cainozoic records of sea level change in Australia if not the world. The record extends into NSW and Victoria. As an undergraduate I was exposed to this incredible story through reading the work of Reg Sprigg, one the giants of Australian geology and oil and gas industry. His discoveries have been the subject of many subsequent studies including that of Doug Schwebel, Peter Cook and especially Colin Murray- Wallace. Even Peter Roy has had an interest with his work on heavy mineral deposits contained in older parts of the sequence in southwest NSW. That the sequence of marine and coastal sediments are so well preserved in this region is largely due to its tectonic history. Quaternary strandlines are well-preserved because of the presence of carbonate sands that become calcified.

The uplifted strand plain is more restricted near the mouth of the Murray River. Here one can observe late Pleistocene beach and dune deposits representing the Last Interglacial (Stage 5e) and the Penultimate Interglacial (Stage 7). We stood in awe of a dramatic Stage 7 outcrop at Clayton on the banks of the Murray River. Bob showed us with great enthusiasm several sites in and around Hindmarsh Island that were formed during Stage 5e. One can see how the deposition of this large dune-beach system “pushed” the channel of the Murray to the west to exit near Goolwa during Last Glacial times (Murray-Wallace et al., Quaternary Geochronology, 2010, vol. 5, 28-49).

Bob and Nick have both been involved in publishing papers related to the formation and depositional features of the Holocene barrier that blocks off the Coorong and Lower Lakes. This barrier is divided into two parts, to the east of the Murray Mouth it is named the Younghusband Peninsula, and to the west the Sir Richard Peninsula. It is the site of many archaeological discoveries. What is intriguing is how this dune-capped sand barrier grew during the last 7000 or so years with the termination of the Holocene transgression into a more stable phase of sea level and regressive mode of sedimentation. Within this barrier the Murray Mouth has migrated.

Much of the work of Bob, Nick and their students has been on rapid geomorphic changes that have taken place since the construction of the barrages. These barrages were built to restrict the entry of saline waters into Lake Alexandrina in the late 1930s. The aim was to allow river waters to sustain a freshwater system for use for irrigation and human consumption. Their work showed how an estuary was transformed by this massive human intervention (Bourman et al., Marine Geology, 2000, vol. 170, 141-168; and James et al., Journal of Coastal Research, 2015, vol. 31, 1103-1119). In particular, the growth of flood-tidal delta deposits at the mouth is an amazing transformative sedimentary structure associated with the decline of the tidal prism for around 100 to 10 square km. This has allowed onshore sand transport to accumulate sand landward of the Murray Mouth. In recent years the sea has moved sufficient volumes of sand to require dredging to keep the mouth open to river flow.

These authors have commented on how the rapid evolution of the flood-tidal delta at the Murray Mouth should be seen as “a canary in the cage of river management” (James et al., 2015). They note that Bird Island at the outlet of this great river did not exist before 1940. Changes have been well-documented by air photos and field surveys. River water extraction has been a factor along with the restricted tidal prism. They note that by the mid-1950s sand shoals were enlarging and vegetation was becoming established leading to salt marsh and low dunes being formed. Bird Island now carries more the 50 plant species. Given the continued presence of the barrages and restrictions on river flows, this island is symbolic of how human activities can cause rapid and irrevocable changes to coastal-estuarine environments.

Travelling with Nick and Bob makes me more aware of how important it is to continue to work on an area over a long period of time. They have devoted decades to learning more and more about the dynamics of CLLMM and the value of this work cannot be underestimated. River managers must learn to appreciate the end of the system; my concern is that in the case of the Murray-Darling Basin they have not.

Words by Prof Bruce Thom. Please respect Bruce Thom’s thoughts and reference where appropriately: (c) ACS, 2017, posted 19 September 2017, for correspondence about this blog post please email admin@australiancoastalsociety.org

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