The morphodynamics and recent geological evolution of a small barrier
estuary were investigated. The estuary is located on the macrotidal ce
ntral Queensland coastline and is characterised by an almost infilled
back-barrier basin colonised by mangroves. The tidal channels inside t
he estuary account for less than 20% of the total estuarine area. The
vertical tide in the estuary is characterised by a positive asymmetry,
where a rapid rise in water level during the flooding tide is followe
d by a slow fall in water level during the ebbing tide. The positive t
idal asymmetry is predominantly ascribed to the truncation of the low
tide level within the estuary due to the presence of a natural sill at
the mouth of the estuary. The tidal asymmetry increases with increasi
ng tide range and would suggest that the morphodynamics of the estuary
are governed by flood tidal currents and flood-dominant sediment tran
sport. However, flood-dominance is only observed during neap and mean
tides. During spring tides, a large volume of water inundates the mang
rove area at high tide and the consequent drainage of the mangrove are
a during the falling tide induces significantly stronger ebb flows and
ebb-dominant sediment transport. Although the proportion of tidal cyc
les during which net ebb-directed sediment transport prevails is limit
ed, on an annual basis the estuary can be considered ebb-dominant. The
stratigraphy of the estuary is characterised by a seaward-thickening
wedge of open-marine sand up to 3 m thick, embedded between two layers
of muddy sediments. The open-marine sand accounts for a significant p
art of the infilled volume and suggests that hood-dominant sediment tr
ansport has occurred during the infilling process. Hydrodynamic measur
ements indicate ebb-dominance for much of the estuary, suggesting that
a hydrodynamical change from flood- to ebb-dominance has occurred dur
ing estuarine evolution. This hydrodynamical change may have resulted
from an increase in intertidal water-storage capacity, predominantly t
hrough lateral mangrove-expansion, causing faster flow velocities duri
ng the ebbing tide. Ebb-dominance was probably first established at th
e head of the estuary and migrated downstream as the estuary became in
filled.