Mediated equilibrium: The influence of riparian vegetation and wood on the long-term evolution and behaviour of a near-pristine river

Analysis of the contemporary geomorphology, vegetation and in-channel wood within the relatively pristine Thurra River in southeastern Australia provides insight into river behaviour prior to European disturbance. This sand-bed river has an extremely low channel capacity with a pronounced pool-riffle morphology. Lateral migration rates are low (11-24 mm a^-1), as are floodplain aggradation rates (average = 0.27 mm a^-1). Sedimentological evidence is used to place contemporary channel dynamics within a 16 ka evolutionary framework. The floodplain has continuously aggraded over this interval, despite a number of avulsions and numerous meander cutoffs. Avulsions occur on a timeframe of once in 5 ka or more, while cutoffs occur around once in 1 ka. The morphology and evolution of the Thurra River are appraised in terms of a mediated equilibrium condition, in which channel capacity, hydraulics, bedload transport rates, bank erosion rates and in-channel deposition are substantially influenced by vegetation and wood within the channel and on the floodplain.