The inlet channel is long (5 km) and narrow (200 m), and although open all year, sand has to be dredged from it to allow the passage of small boats during late summer. Harvey Estuary, and Peel Inlet communicate across a shallow sill through which a small channel has been dredged to allow the passage of boats.
The basins have approximately the same area (Peel Inlet, 72 km2; Harvey Estuary, 56 km2) and water depth (mean in Peel Inlet, 0.8 m; in Harvey Estuary, 1.0m); maximum depths reach 2.5 m in each basin. Peel Inlet in particular is characterised by large, shallow marginal platforms which are less than 0.5 m deep and occupy more than half the total area. The mean volumes of the two basins are 61 x 106 m3 for Peel and 56 x 106 m3 for Harvey.
The tides of south-western Australia have very small amplitudes.4 The maximum daily range due to astronomic effects is about 1.0 m and is mainly diurnal, with a small, semi-diurnal component. Superimposed on this pattern are water level changes of a similar magnitude but longer time scale (circa 7-10 days), attributed in the main to atmospheric effects and referred to for convenience as a `barometric' tidal component. The total oceanic water level change in the ocean near Mandurah is therefore some 1.5 m. The effect of the long inlet channel is to filter out the astronomic signal but to leave the barometric component essentially unchanged (Figure 3). There is, therefore, little diurnal change in water level in the system, and even less as one moves far from the inlet channel into Harvey Estuary or along the tidal reaches of the Murray River. Such estuaries have been referred to as "microtidal" .