But when the Quran speaks about the divider between fresh and salt water, it mentions the existence of a forbidding partition with the barrier. God has said in the Quran:
( He is the one who has set free the two kinds of water, one sweet and palatable, and the other salty and bitter. And He has made between them a barrier and a forbidding partition. ) (Quran, 25:53)
One may ask, why did the Quran mention the partition when speaking about the divider between fresh and salt water, but did not mention it when speaking about the divider between the two seas?
Modern science has discovered that in estuaries, where fresh (sweet) and salt water meet, the situation is somewhat different from what is found in places where two seas meet. It has been discovered that what distinguishes fresh water from salt water in estuaries is a pycnocline zone with a marked density discontinuity separating the two layers.
3 This partition (zone of separation) has a different salinity from the fresh water and from the salt water4 (see figure 14).
Figure 14: Longitudinal section showing salinity (parts per thousand ) in an estuary. We can see here the partition (zone of separation) between the fresh and the salt water. (Introductory Oceanography, Thurman, p. 301, with a slight enhancement.) (Click on the image to enlarge it.)
This information has been discovered only recently, using advanced equipment to measure temperature, salinity, density, oxygen dissolubility, etc. The human eye cannot see the difference between the two seas that meet, rather the two seas appear to us as one homogeneous sea. Likewise, the human eye cannot see the division of water in estuaries into the three kinds: fresh water, salt water, and the partition (zone of separation).