If we were taking salinity readings from an area in an estuary where a river is flowing into the ocean water, we would find the freshest water at the surface. this is true because freshwater is

more dense than saltwater

less dense than saltwater

colder than the saltwater

a better solvent than saltwater

When fresh water and salt water face in a Tide water, they do not regularly mix very quickly. The freshwater usually floats on top of the denser seawater because fresh water flowing into the water is less salty and have less density than water from the ocean. When salt is dissolved in water, that dissolved salt combines to the volume of the water and makes the water denser than it would be without salt.