Downstream Changes in Floodplain Character on the Northern Plains of Arid Central Australia

Along the Sandover, Sandover-Bundey and Woodforde Rivers on the Northern Plains in arid central Australia, floodplain landforms, processes and sediments differ between confined upper and middle reaches (where channels and Holocene floodplains are flanked by indurated, Pleistocene alluvial terraces) and unconfined lower reaches (where channels are flanked by extensive Holocene floodplains). In confined reaches, terraces up to 5 m high restrict lateral channel migration and floodplains are typically less than 50 m wide. Channel avulsions, splays and distributary channels are rare and overbank vertical accretion is the main process of floodplain formation. Downvalley, terrace heights decline and they are eventually buried by younger, relatively erodible, floodplain silt and sand. In these unconfined reaches, termed floodout zones, channels are more laterally active, floodplains are up to 6 km wide and numerous splays, distributary channels and palaeochannels are present. Channels decrease in size downstream and end in flood-outs, a term used to describe sites where channelized flows terminate and floodwaters spill across adjacent alluvial surfaces. In flood-out zones, floodplains form mainly by vertical accretion, lateral point-bar accretion and abandoned-channel infilling. Stratigraphical and sedimentological features characteristic of flood-out zones include: 1 thin veneers of Holocene sediments over Pleistocene alluvium or bedrock; 2 a paucity of sedimentary structures preserved in channel or floodplain deposits; 3 the surficial nature of many floodplain features; 4 incorporation of aeolian sediments in the predominantly fluvial deposits; 5 channel sand and gravel bodies encased in fine-grained overbank deposits; 6 a general down valley decrease in the ratio of channel sands and gravels to overbank fines; Although an absence of channels makes it difficult to reconcile floodouts with conventional definitions of 'floodplain' or with existing floodplain classifications, fluvial landforms and deposits in floodout zones are best regarded as part of a continuum of floodplain types.