 It is thought that the Wagyl/Waugal created the Swan River.The estuary is subject to a microtidal regime, with a maximum tidal amplitude of about , although water levels are also subject to barometric pressure fluctuations.Before the Tertiary, when the sea level was much lower than at present, the Swan River curved around to the north of Rottnest Island, and disgorged itself into the Indian Ocean slightly to the north and west of Rottnest.