Sand slugs formed by large-scale channel erosion during extreme floods on the East Alligator River, Northern Australia

In February/March 2007 extreme rainfall occurred over a four-day period in the 7000 km2 East Alligator River catchment in Arnhem Land, northern Australia. The resultant large flood caused extensive bank erosion, channel widening, stripping of point bars and floodplain, resulting in large amounts of sand transport. This sand was largely deposited in the downstream river channel as a sand slug, and as deep overbank sand splays where the valley abruptly widened immediately downstream of an island anabranching, bedrock-confined reach. Interpretation of a time series of aerial photographs and satellite images from 1950 to 2012 showed that there have been considerable channel changes along the study reach. The aerial photographs show that extensive sedimentation in the same reach as in 2007 also occurred in 1975, 1981 and 1984. Each time, the sand slug was reworked rapidly over succeeding years by subsequent smaller floods, and the channel deepened naturally as sand supply declined due to revegetation of the upstream riparian zone and the reformation and stabilisation of point and lateral bars. Sand slug formation at an intermediate floodout is an episodic process dependent on the supply of large volumes of sand by extensive channel erosion during extreme floods. A conceptual geomorphic model was developed to highlight the differential effectiveness of extreme versus moderate floods.