Alhamdullilah for the fresh water.
The verses at the heart of the controversy are: Qur'an 25:53 and 55:19-20.
Qur'an 25:53 reads: "It is He Who has let free the two bodies of flowing water: One palatable and sweet, and the other salt and bitter; yet has He made a barrier between them, a partition that is forbidden to be passed."
Qur'an 55:19-20 reads: "He has let free the two bodies of flowing water, meeting together: Between them is a Barrier which they do not transgress:"
However,
any reasonable person knows that if you take a glass of salt water and a glass of fresh water, pour them in a single container, they will mix. So where did people get the idea that salt and fresh water will not mix? Actually, they get it from the Qur'an itself, amazingly enough. But, does the Qur'an actually say that salt water and fresh water will not mix?
The word translated barrier in the Qur'an is, Barzakh. According Mostmerciful.com, Dr. Abdullah Nadwi in his 'Vocabulary Of The Holy Qur'an' defines Barzakh as; "Litt : a thing that (n.) intervenes between any two things". Therefore, Barzakh is commonly translated as barrier: Something that prevents one thing from infringing upon another.
Prof. Ibrahim Khalil, states: "Modern Science has discovered that in the places where two different seas meet up, there is a barrier between them. This barrier divides the two seas so that each sea has its own temperature, salinity, and density. For example, Mediterranean Sea water is warm, saline and less dense, compared to Atlantic Ocean water. When Mediterranean Sea water enters the Atlantic, it moves several hundred kilometers into the Atlantic at a depth of about one kilometer with its own warm, saline and less dense characteristics, because of the barrier that distinguishes between them. Although there are large waves, strong currents, and tides in these seas, they do not mix or transgress this barrier."
The author fails to mention, however, that the water does eventually comingle; besides, it is cleadr that the Qur'an is talking about fresh and salt water-not salty and less salty seas.
The barzakh occurs also in the evaporation process. According to the USGS: The rain that falls on the land contains some dissolved carbon dioxide from the surrounding air. This causes the rainwater to be slightly acidic due to carbonic acid. The rain physically erodes the rock and the acids chemically break down the rocks and carries salts and minerals along in a dissolved state as ions. The ions in the runoff are carried to the streams and rivers and then to the ocean. Many of the dissolved ions are used by organisms in the ocean and are removed from the water. Others are not used up and are left for long periods of time where their concentrations increase over time.
So when the sun evaporates sea water, the salt is left behind. Fresh water is transported by clouds back over land and decends as rain-replenishing our fresh water rivers and streams and then the whole process begins again.
Had Allah (swt) not put this Barzakh in place, our rivers and streams would now be just as saline as the oceans.
The Qur'an: Yusuf Ali Translation.
http://ga.water.usgs.gov/edu/whyoceansalty.html