James, K.F.; Bourman, R.P., and Harvey, N., 2015. Rapid evolution of a flood tidal deltaic island in the River Murray estuary, South Australia: A canary in the cage of river management.
Bird Island, at the oceanic outlet of Australia's largest exoreic river system, the Murray-Darling, did not exist before 1940. Originally, flood tidal sediments were moulded by tides, waves, wind, and river flows in the back-barrier lagoon, landward of the migrating River Murray Mouth. The estuary was fluvially dominated, and the terminal lakes were mainly fresh. Following more than a century of European settlement and freshwater abstraction, salinity levels increased, prompting the construction of barrages (completed in 1940) near the Murray Mouth to restore the former predominantly freshwater character of the estuary. Further reduced river flows, a restricted tidal prism, and management strategies produced irreversible changes in the dynamics of the River Murray Mouth. These are archived in the landforms and vegetation of Bird Island. By the mid-1950s, sand shoals were enlarging and a small patch of vegetation was established, forming the core of an incipient island. Progressively, dunes and salt marshes were established, reflecting intimate associations between landform evolution, vegetation colonisation, and island stability. As the mouth migrated to the NW, so did the flood tidal shoals, the sources for newer generations of dunes, thereby developing a clockwise establishment of dunes on the expanding island. Approximately 1 km in diameter and carrying more than 80 plant species, Bird Island assisted mouth closure in 1981. During the 2002–10 drought, dredging maintained the mouth and improved lagoonal water quality. Subsequent healthy river flows cleared the mouth, highlighting the important role of river flows in maintaining oceanic access, and sediment removal, while flushing the river system. The evolution of Bird Island as documented in this study demonstrates how anthropogenic activities can cause rapid and irrevocable changes to coastal–estuarine environments.
