What causes the phenomenon where different waters meet but do not mix and thus create a straight line which doesn’t break, even when ships cross over it?

Nothing magical about it, just some cool science.

Firstly, the waters absolutely are mixing! It doesn’t look like it, does it? There seems to be some sort of a clear boundary between the two bodies. On one side, you have murky fresh river water and on the other, you have clear saline ocean water. However, mixing must be occurring… if it was not, then the river water would push the boundary of the ocean outward indefinitely. That is not the case, however- the boundary is relatively stable.

Since the boundary between the clear saline and murky fresh water is stable, we know the rate at which it mixes, the rate at which water crosses over that boundary. It is equal to the discharge of the river. The Fraser River, as shown in the image, has an average discharge of 3,475 cubic meters of water each second. That is the amount of water crossing that fresh/saline boundary every second as well.

When the water crosses that boundary to become saline, it suddenly becomes very clear, which is why the boundary seems to exist between the dirty and clear waters. But now we come to the real crux of the question: why, when the murky fresh water “crosses the boundary”, mixing to become saline, does it suddenly cease to be murky?

Well, that murkiness is suspended sediments: silts and clays, carried by the water. Salt ions, however, bind to suspended particles and cause them to aggregate- they stick together, causing them to become heavier and sink to the bottom of the ocean… that is, to sediment out of the system. So, when the water picks up salt, sediments begin to stick together (a process called flocculation) and drop out, making the water become clear.

To summarize, what you expect to be an impenetrable boundary between two distinct bodies of water is actually a very sharply defined mixing zone where the fresh sediment-rich water picks up salt ions, causing the sediment to drop out, and where (for a stable boundary) the rate of mixing is equal to the discharge of the river.

Nothing magical about it, just some cool science. :)