Rapid coastal geomorphic change in the River Murray Estuary of Australia

Abstract

The River Murray Estuary, a Ramsar Wetland Site, has experienced considerable rapid coastal change during the Quaternary. The interplay of aeolian processes, river flows, tidal oscillations, wave action and variations in relative sea-level due to global sea-level changes and land subsidence, provides the energy for the ongoing dynamism, often accelerated by human impacts. The estuary is the terminus of the Murray-Darling catchment, which covers 1.073 million km2 of the Australian continent. Terminal Lakes Alexandrina, Albert and the Coorong Lagoon are Holocene features, occupying tectonically subsiding Quaternary interdune areas. They formed in response to eustatic sea-level rise following the Last Glacial Maximum (ca. 20 ka). The last interglacial shoreline (125,000 yr BP) parallels the modern shoreline several kilometres inland. Dislocation of the last interglacial shoreline demonstrates ongoing tectonic subsidence, as does historical seismic activity. The northern half of Hindmarsh Island formed during last interglacial times when it was the sink for dominant longshore transport from the southeast, which pushed the River Murray westward, partly explaining the large bend in the River Murray at Goolwa. The modern coastal barriers of Sir Richard Peninsula and Younghusband Peninsula formed from 7000 yr ago, following glacier melt and sea-level rise. Subsequently, the barriers have migrated landward, sporadically exposing lagoonal sediments on the ocean beaches. Differential loading of the soft lagoonal sediments by advancing dunes, possibly in conjunction with seismic events, has deformed and elevated them to up to 10 m above present sea-level (APSL). During the mid-Holocene an extensive sand flat, with associated dunes, formed immediately inland of the coastal barrier. At least six generations of Late Pleistocene dune systems occur in the region. For example, during last glacial times the climate was drier, colder and windier than at present and a system of parallel, west-east trending, yellow-red desert dunes developed around the lakes. Aeolian processes remain important with occasionally up to 5000 tonnes of sand being in motion along 10 km of the modern shoreline. During mouth migration, dunes up to 2 m high have been formed and vegetated in 12 months, directly inland from the mouth, and replicating the formation of older dunes on Hindmarsh Island. Elsewhere sand blown directly from the barrier system infills channels. Barrage construction on the beach facies of the last interglacial shoreline transformed the estuary into freshwater lakes with permanently raised water levels and reduced the tidal prism by 90%. Increased deposition, upstream and downstream, accompanied barrage construction, as have accelerated lakeshore erosion and the growth and consolidation of the flood tidal delta (Bird Island). These human accelerated changes provide rapidly formed analogues of older Quaternary features, and aid their interpretation. The shape and location of the Murray Mouth is constantly changing, migrating over 1.6 km since the 1830s. Migrations of up to 6 km over the past 3000 yr have influenced sedimentation on the landward shore of the back-barrier lagoonal system. (C) 2000 Elsevier Science B.V.