Chasing the flood: sediment architecture of the dryland De Grey River after a tropical cyclone in the Pilbara Region, western Australia

The dryland, ephemeral, and tidally influenced De Grey River in the Pilbara region of Western Australia is notable for its exceptional preservation of individual flood deposits. In February 2025, Cyclone Zelia produced a historically large flood that substantially altered the fluvial environment across proximal, medial, and distal reaches of the system. This event provides a rare opportunity to document the sedimentologic and geomorphic impacts of an extreme discharge event within a dryland river.

Pre- and post-flood satellite imagery, including July 2025 datasets, was used to identify cyclone-specific deposits and guide the selection of trench sites. At each location, 1-m-deep trenches were excavated to analyze post-flood stratigraphy, grain-size distributions, sedimentary structures, sorting, and textures. Imagery also revealed the formation of a new gooseneck cutoff as one of the major geomorphic changes associated with the flood, though environmental constraints prevented detailed mapping.

Sedimentological observations show that cyclone deposits are dominated by medium well-sorted sands and macroscale gravel bar forms produced during high-magnitude bedload transport. Most preserved deposits reflect lower-flow regime conditions, including planar and trough cross-beds and dune-scale accretion surfaces. Thin gravel laminae interbedded within these sandy units record brief high-energy pulses capable of transporting coarser material before flow rapidly returned to lower-energy bedload conditions. Additionally, a newly developed tidal bar on the downstream end of the inside chute cutoff channel contains sigmoidal cross-sets that record tidally modulated flow reversals enhanced by a localized backwater zone created by the new gooseneck cutoff. Upper-flow regime structures such as isolated concavo-convex antidune sets, chute-and-pool features, and a basal gravel anti-dune train are present but atypical, indicating formation only within narrow high-velocity flow corridors during peak discharge. Fine sediment was largely flushed to the delta rather than retained within the channel belt.

These findings show that extreme floods in dryland rivers may preferentially preserve lower-flow regime deposits and that extreme floods drive geomorphic reorganization that governs sediment transport pathways.