[mapt]

Why not?

Existing ocean currents in the Gulf of California driven by wind, tides, and thermohaline circulation are on the order of 10cm/s, tens of kilometers wide, hundreds of meters deep. Very little of that is associated with local rivers, which are relatively tiny because the whole region is extremely arid. Are you going to be desalinating 0.1 km^2 of water per second?

The Sun pumps a thousand watts a square meter onto the surface of the Earth on a 24-hour cycle and this tends to shake things up.

[viburnum]

The brine doesn’t spread out automatically, it sinks, and it’s deadly to aquatic life.

[mapt]

It does spread out automatically. We call it diffusion.

Diffusion can also be assisted with very modest power requirements.