On the tidal stretches of the Medway, the salt water being denser than freshwater flows close to the bottom, whereas the freshwater is higher near the surface.
Of course mixing occurs as the current hits obstacles etc., but it's sometimes possible to catch saltwater species fishing near the bottom and coarse fish by fishing shallow in the same swim.
During spring tides, when the saltwater retreats out quite a way, and when there is a lot of freshwater coming down the river, roach have been caught down as far as Rochester feeding amongst the crabs and bladderwrack.
As the tide runs twice a day, I guess that the resident fish have learned the ropes and know how to stay in the strata that best suits them. This tendency of salt and freshwater to stratify without too much mixing might mean that the fish of the broads are not so vulnerable to saltwater incursion as might be expected, perhaps?