STRONGLY SEASONAL RIVERS IN THE SUBHUMID TROPICS OF NORTHEAST AUSTRALIA – DISCHARGE VARIABILITY AS A CONTROL ON DEPOSITIONAL PROCESS AND PRODUCT

Rivers such as the Burdekin River of northeast Australia experience strongly seasonal discharge with high inter-annual variability in response to spatially heterogeneous, often intense summer precipitation. The combination of the firm yet erodible, clay-rich character of the floodplain and intense precipitation renders the response time between onset of precipitation and surface runoff a matter of minutes. The Burdekin River experiences rises in discharge over minutes to hours from a base flow of a few m/s to over 30,000 m/s. The rising limb of the hydrograph is precipitous, with the flood wave often travelling at 2-5 m/s. The peak discharge is typically maintained for several hours, after which it declines rapidly. Because of the short duration of individual flow events, the river-floodplain system is unable to attain equilibrium conditions with the result that the river bed typically reflects the most recent flow event or events. The river bed, exposed for much of any year, is composed of texturally mature but mineralogically immature sand with subordinate gravel. Mud (the dominant constituent of the solids discharge) is carried as washload and flushed out to sea. Rapid changes in discharge during the falling limb facilitate the preservation of Froude-supercritical bedforms on the surface and transcritical flow structures in the shallow subsurface. The bed of the river is vegetated by trees that are able to survive repeated inundation by fast-flowing, sediment-laden waters during flow events. These trees form flow-parallel groves that nucleate linear bars. Repeat ground-penetrating radar surveys over the same transect suggest that the river may mobilize several m of alluvium during major flow events, redepositing it during the falling stage of those events. The subsurface architecture of the Burdekin River alluvium does not resemble that of perennially-flowing rivers in that it typically lacks macroform structure, shows complex lateral facies variability and preserves an abundance of flow-transcritical sedimentary structures. It also preserves in situ, downstream-inclined tree stumps, pedogenically modified mud layers, and numerous internal erosion surfaces.