Splay formation along the lower reaches of ephemeral rivers on the northern plains of arid central Australia

Splays are common features of many rivers and deltas, forming as a result of overbank flooding and sediment deposition, but they have rarely been described from modern dryland settings. This paper investigates splay formation along the lower reaches of ephemeral rivers on the Northern Plains of arid central Australia. Here, splays vary from small, lobate or tongue-shaped features < 1 km long, to larger, elongate features up to ∼ 4 km long, and are supplied with rare floodwater and sediment through well-defined breaches in the upper parts of the main channel banks. In proximal and medial parts, splay channels are incised up to ∼ 2 m into pre-splay, fine-grained floodplain sediments but remain elevated between ∼ 0.5 m and 2 m above the main channels. Only in the very distal parts are splay channels less deeply incised, either grading into the floodplain surface or terminating as sediment lobes that are deposited upon the floodplain. Aerial photographs and field observations indicate that many splays develop rapidly during large floods and are modified during infrequent, subsequent floods. Four main stages of splay development are proposed: (1) aggressive overbank flow results in bank-line breaching and channel scour on the floodplain, but little sediment deposition; (2) coarse bed sediment is diverted from the main channel, preventing further deepening at the bank-line breach and slowing channel scour; (3) increasing coarse sediment transport through the bank-line breach and scoured channels leads to the formation of distal sediment lobes; (4) subsequent floods divert around these distal lobes, sometimes leading to further channel scour and lobe formation. On the Northern Plains, splays contribute to downstream decreases in the discharge and size of the main channels, and splay deposits form local accumulations of gravelly sand in the predominantly fine-grained floodplain successions. Some splays ultimately may evolve into distributaries or ana-branches. The dominantly erosional bases of these splay channels contrast with typical descriptions of many humid-region splays, but they do not display any geomorphological or sedimentological features that are necessarily diagnostic of formation in a dryland setting.