Are there saltwater rivers?
yes, a river in Newbrunswick Canada will back up due to tides and there will actually be a reverse saltwater waterfall.
or any river that's polluted enough.
What is area at the rivers mouth where saltwater from the ocean and freshwater from the river meets?
As the question is not precise -  There is Saltwater River on the Tasman Peninsula in Tasmania.  All rivers contain salts of some kind.  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…
There are both freshwater Pike and saltwater Pike. They are different species but are both called pike. I live in Maine where Pike were illegally introduced in the last decade or so. Where the rivers flow into the ocean pike can also be caught. In the ocean near the end of that rivers i have caught pike in full saltwater. It may not be scientifically proven but they can clearly adapt to saltwater