In tide-dominated sedimentary systems, close relationships exist between tidal hydrodynamics, sediment transport and geomorphology. Tropical coastlines contain many tide-dominated mangrove creeks, yet few studies to date have examined the detail of such relationships for these environments. Time-series observations of tidal height, currents and suspended sediment concentrations were taken between 1992 and 1996 in Cocoa Creek, a mangrove creek system near Townsville, NE Australia. The creek and surrounding mangrove swamps and salt flats were surveyed with an echo-sounder and total survey station, respectively. For ‘within-channel’ tides, the flood tide is always the fastest, at up to 0.5ms−1. In contrast, for overbank tides (i.e. tidal height >+1.5m Australian Height Datum, AHD) ebb currents are fastest in July, December and January, but flood currents are fastest in August and September, at up to 1ms−1 in both cases. The tidal asymmetry of overbank tides in Cocoa Creek is controlled by the interaction between offshore tidal forcing and the intertidal storage effect of the mangrove swamps and salt flats, with the result being that during certain periods of the year there tends to be a predominance of either faster flood or ebb velocities on overbank tides. Significant tidal suspended sediment transport in the channel is only initiated at overbank height. On overbank tides, measured net suspended sediment fluxes in the channel are mostly seaward-directed (up to 180t per tidal cycle). However, the net flux measured over a neap–spring period may be either landwards or seawards (up to 465 and 60t, respectively). Furthermore, on the larger overbank tides (where the maximum tidal height >+1.85m AHD) net sediment fluxes may be reduced because of a limited supply of available material. Thus hydrodynamic and sediment sampling durations of up to a month may not be representative of long-term trends. Given that our large dataset has not identified a clear long-term net transport direction within the creek system, we conclude tentatively that the geomorphology of Cocoa Creek may be near a long-term equilibrium.

Variation in freshwater discharge into the tropical estuaries of northern Australia is extreme between wet and dry seasons, and the resultant effect upon sedimentary processes in relation to the geomorphological evolution of the coastal plain is largely undocumented. Observations of tidal height, currents and suspended sediment concentrations were taken over a 3-week period in July 1994 (dry season) in the mesotidal Normanby River estuary, northeastern Australia. The estuary and adjacent inner shelf were surveyed using a 3.5 kHz seismic system and an echo-sounder, and sampled with a total of 175 grab samples and 17 vibrocores. Sediment sampling occurred in July 1994, December 1994 and March 1996, the latter following the 1995/96 wet season. Flood tidal currents dominate the estuary at spring tides with peak velocities of 0.85 m s−1 recorded at the mouth. Measured tidally driven sediment transport is landward at the mid-estuary site, and extrapolation gives estimates of 15,000–30,000 tonnes of bedload and ca. 50,000 tonnes of suspended load moved landward per year. The surficial sediments in the main estuarine channel are inferred to be Holocene, and are: (1) delta-front sediments (which extend to at least 6 km seawards of the mouth); (2) lower estuarine silts (located at 2–11 km landwards of the mouth); and (3) channel dune sands (13 to >50 km, i.e. nearly to the tidal limit). All dunes surveyed on the river bed during the dry season showed a strong asymmetry, with lee faces consistently facing landward, whether surveyed at high or low water, indicating landward bedload sediment transport in the dry season. An underlying hard regional surface is exposed at 11–13 km landward of the estuary mouth. In the absence of direct measurements we have estimated sediment transport during wet season freshwater flood events. A major freshwater flood event may transport only 6000–32,000 tonnes of bedload seawards, and modern wet season events appear incapable of supplying sand in significant quantities to the inner shelf. We infer that for the late Holocene, the frequency, magnitude and duration of wet season events has been insufficient to reverse the landward bedload transport driven by tidal currents in the dry season.