As the question is not precise -
[1] There is Saltwater River on the Tasman Peninsula in Tasmania.
[2] All rivers contain salts of some kind.
[3] All tidal rivers will contain saltwater - When the seas tide comes in and the river 'flows' in the opposite direction to its natural flow until it is stopped by an obstruction [such as a weir] or by the rivers own flow being stronger or the tide going out.