From my reading I knew that this was a classic salt wedge estuary. Winter rains typically force a strong freshwater flow down the system. Eventually (in wet years) this fresh almost totally displaces the brackish tidal water .Pushing it back to the shallow, silted mouth. As run off rates fall in spring, each successive incoming tide pushes more salty, oceanic water back into the river and this forms a saline ‘wedge’ underneath the surface freshwater. This wedge of soft water gradually migrates up steam and becomes thicker. In a dry summer and autumn, the salt wedge can travel scores of kilometer up river. Come winter and the first heavy rainfalls, the whole cycle begins again.
A wild card is thrown into the equation by the fact that the deeper, salty water-which would appear to be ideal habitat for many marine species-loses its dissolved oxygen though summer and can become almost anoxic(lacking in oxygen)by autumn, particularly in drought year. This has a significant impact on fish location and can mean that the bottoms of deeper holes are almost lifeless. The dynamic and complex nature of salt wedge hydrology and the sheer scale of the Glenelg mean that its fish stocks can and do move around a great deal, both vertically within the water column and-more significantly-up and down some 40 to60 km of potentially tidal river. Put simply, they aren’t always easy to find.