The region where saltwater and freshwater meet is called an estuary. From your comment, you seem to know that often these types of waters don't mix very well. This is due to the fact that their density is different(saltwater is heavier than freshwater because it has heavy salt molecules in it).

However, these water masses do mix eventually:

• If the estuary has a lot of stirring energy due to tides or wind, then the estuary may be well mixed and the salinity (saltiness) just gets gradually less upstream.
• If this estuary is not well mixed, saltwater may enter underneath the freshwater in what is called a "salt-wedge" (because it is a wedge shape). If you took salinity measurements, the surface would be quite fresh, but the bottom would be salty.

  Dan Berger adds:
  The Amazon river has just such a "wedge" effect; the flow from the river is so large that fresh water can be dipped from the ocean as much as a mile out to sea.

• The estuary can be "partially" mixed so there is again a saltwater gradient upstream, but the saltwater intrudes upstream near the bottom more than if it is does at the top.

More details about salinity.

• Salinity is measured by salt content in parts per thousand by mass denoted by the symbol ⁰/₀₀.
• The salinity of ocean water is approximately 35 ⁰/₀₀.
• This means that in 1 litre of saltwater (10cm cube which has a mass of 1kg or 1000g) there are 35 grams of salt (that's a lot!)
• The salinity of freshwater is close to 0.
• When a water's salinity is reduced by mixing with freshwater it is called "brackish" (usually between 5-20 ⁰/₀₀).
• In hot areas of the world, it is possible for some bodies of water to be extremely salty (for instance greater than 200 ⁰/₀₀) when water evaporates faster than it is replaced by mixing.

Estuary links:

• Karen Grove's Estuary course site
• Estuary Live
• Chesapeake Bay
• EPA estuary program
• San Francisco Estuary Institute
• my project: The CLUE website for some info about how nutrients are affecting estuaries

I hope you find this helpful.