Science in the Glorious Quran: The Salt and Sweet Water Barrier

“وَهُوَ الَّذِي مَرَجَ الْبَحْرَيْنِ هَٰذَا عَذْبٌ فُرَاتٌ وَهَٰذَا مِلْحٌ أُجَاجٌ وَجَعَلَ بَيْنَهُمَا بَرْزَخًا وَحِجْرًا مَّحْجُورًا” (الفرقان:53).

“And it is He who merged the two seas, one palatable and sweet and the other salty and bitter, and He placed between them a barrier, and an impassable boundary” (Qur’an, 25:53).

In estuaries, where fresh (sweet) water and salt water meet, what distinguishes fresh water from salt water is a pycnocline zone with a marked density, separating the two streams. This zone has a different salinity from both fresh and saltwater. The Qur’an mentions this phenomenon in the above verses.

Longitudinal section showing salinity in an estuary

An estuary with a marked density discontinuity between fresh and salt waters