Barrier between Fresh and Salt Water

The Quran describes the existence of a barrier between bodies of salt and fresh water, which do not readily mix. This could be interpreted as describing the phenomenon of estuaries where fresh and saltwater meet but do not immediately mix due to differences in temperature, salinity, and density. These Quranic references highlight the complex hydrological processes that govern the Earth’s waters.

۞ وَهُوَ ٱلَّذِى مَرَجَ ٱلْبَحْرَيْنِ هَٰذَا عَذْبٌۭ فُرَاتٌۭ وَهَٰذَا مِلْحٌ أُجَاجٌۭ وَجَعَلَ بَيْنَهُمَا بَرْزَخًۭا وَحِجْرًۭا مَّحْجُورًۭا

And it is He who merged the two seas; this one fresh and sweet, and that one salty and bitter; and He placed between them a barrier, and an impassable boundary.

The Holy Quran · Al-Furqaan (The Criterion) · 25:53

مَرَجَ ٱلْبَحْرَيْنِ يَلْتَقِيَانِ

He merged the two seas, converging together.

The Holy Quran · Ar-Rahmaan (The Beneficent) · 55:19

بَيْنَهُمَا بَرْزَخٌۭ لَّا يَبْغِيَانِ

Between them is a barrier, which they do not overrun.

The Holy Quran · Ar-Rahmaan (The Beneficent) · 55:20