When the Mississippi River water flows into the Gulf of Mexico the fresh water does not mix immediately with the salt water. Instead, it takes a while and the fresh water forms a plume and does not mix for a while.
As in the case of all rivers which flow into seas or oceans, the Nile River mixes its fresh water with the salt water of the Mediterranean at its delta. There is no exact point at which a river ceases to be fresh water and become salt water.
Yes. Because even though rivers run to merge with salt water rivers flow down so there is no salt water in a river making it 100% fresh water. Yes. Because even though a river runs to merge with salt water, rivers flow down. Therefore, making a river 100% fresh water.
The Charles River is Fresh water but has traces of salt from the locks the separate the river from Boston Harbor. When the locks open and close very little salt water gets into the river, but all in all, it is fresh water.