Channel function shift around a recently-colonised estuarine mangrove shoal

Large sediment loading to the coast can accelerate estuarine infilling, which promotes the establishment of salt marsh or mangroves. Bio-geomorphic feedback ensues where channels and shoals gradually evolve in tandem as the vegetation gains a foothold. The aim of this work is to explore the governing dynamics of a recently evolving estuarine, mangrove-covered channel-shoal system in Whitianga Estuary, Aotearoa New Zealand. Observations of water level, flow velocity, suspended sediment concentration (SSC) and bed sediment characteristics were used to infer flow asymmetry and sediment transport pathways around the shoal. Historical images were combined with the measurements to qualitatively link the observed, short-term processes to the influence of expanding vegetation on multi-decadal evolution. Comparisons of flow asymmetry in two contrasting drainage channels (in close proximity) on the shoal show that rapid colonisation by mangroves can quickly change the flow asymmetry from flood to ebb dominant. Mangroves opportunistically colonise the higher ground at the centre of the shoal. Meanwhile they become eco-engineers creating their own environment as evidenced by the abrupt change in the sediment textural environment, even outside the immediate area of new vegetation. Although many studies have focused on spatial flow patterns associated with mangrove vegetation, our observations show evidence of the speed at which colonisation fuels geomorphologic change. • In complex channelised estuarine environments, hydrodynamics are highly spatially-variable making representative in situ sampling challenging. • Evidence of a regime transition in channel properties is presented and associated with forest expansion. • Vegetation causes strong gradients in flow asymmetry between channel and shoals.