Abstract : Field studies were carried out on the water and sediment dynamics in the tropical, macro-tidal, Daly Estuary. The estuary
is shallow, very-turbid, about 100 km long, and the entrance is funnel-shape. In the wet, high flow season, normal tidal ranges
can be suppressed in the estuary, depending on inflow rates, and freshwater becomes dominant up to the mouth. At that time
a fraction of the fine sediment load is exported off- shore as a bottom-tagging nepheloid layer after the sediment falls out
of suspension of the thin, near-surface, river plume. The remaining fraction and the riverine coarse sediment form a large
sediment bar 10 km long, up to 6 m in height and extending across the whole width of the channel near the mouth. This bar,
as well as shoals in the estuary, partially pond the mid- to upper-estuary. This bar builds up from the deposition of riverine
sediment during a wet season with high runoff and can raise mean water level by up to 2 m in the upper estuary in the low
flow season. This ponding effect takes about three successive dry years to disappear by the sediment forming the bar being
redistributed all over the estuary by tidal pumping of fine and coarse sediment in the dry season, which is the low flow season.
The swift reversal of the tidal currents from ebb to flood results in macro-turbulence that lasts about 20 min. Bed load transport
is preferentially landward and occurs only for water currents greater than 0.6 m/s. This high value of the threshold velocity
suggests that the sand may be cemented by the mud. The Daly Estuary thus is a leaky sediment trap with an efficiency varying
both seasonally and inter-annually.