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> What do you think happens when hundreds of millions of gallons of water is separated from the salt?

Where do you think the water that forms the runoff comes from? Rain. Rain from where? Clouds. Clouds from where? The ocean is a significant source of rainwater.

Where do you think the water goes when the river takes it to the ocean? Well, goes back into the ocean.

How many gallons of water does the ocean have? Trillions? A few million will not measurably increase the salt content of the ocean. And, because it's a cycle, it already happens naturally.

So keep the cycle going. Guide the cycle to be more useful to us.

I just looked it up, according to usgc.gov, Californians use on average about 181 gallons of water per day. There are about 22 million Californians. That's several billion gallons per day. Just pointing that out.

>And what happens to the sea life when you suddenly increase the salinity of the water by a lot?

Nothing if you appropriately dilute it or a some very localized damage

Even if all the water california used came from the ocean, that is 0.00000004 of the total ocean volume per year. Will this really increase salinity?

I suppose if you were able to dissolve these tons of excess salt equally across the total ocean volume this wouldn't be an issue. But it has to be dumped back in somewhere, and the local salinity increase in that area even if only temporary should still be considered.

Put the salt in tanker ships and dump it out at sea across a wide area. Cargo ships are massive, and desal plants would likely be in bays (i.e., not on "wild coasts") where there are ample loading docks in nearby harbors.

Again, seems like a solvable problem with minimal incremental infrastructure (particularly given the overall cost of desalination plants with adequate urban capacity).

Please look up how desalination works, you’re not dumping salt in the ocean. It is millions of gallons of brine. For every gallon of water produce, the waste is more than a gallon of brine. It would not be commercially viable as compared to say building a reservoir or not growing almonds and alpha alpha in the middle of the desert.

And they are considered. These are actual considerations that any industrial plant has.

The local increase of salinity can made arbitrarily close to the local salinity by pre mixing it with an appropriate amount of water

That’s not how it works. Think of this as what happens when you drop a couple of cubes of ice in a tub? The temperature of the tub doesn’t go down for sure like you said, but the water around the ice cubes will be very cold. That’s what happens when you release highly concentrated water in the sea, the marine life is decimated but overall the salinity of oceans doesn’t change.

Now you could argue you could dilute the release water, but then why desalinate?

"Now you could argue you could dilute the release water, but then why desalinate?"

....because you dilute brine with seawater so its a simple and (overall) energetically cheap solution.

Why would the salt be dumped back in the ocean? Wouldn’t it be sold to consumers?

The "salt" is in the form of brine, it's a liquid and you'd need huge evaporation ponds to keep up with the outflow if you wanted to dry it all out to sell. If you put your desalination plant near urban areas, where the demand for fresh water is, then the land you'd need for evaporation ponds would be a lot more expensive, and I suspect the profit margins for salt are fairly tight (sea salt competes with salt mines where they just scoop the stuff right out of the ground.)

> dilute the release water

You can mix the brine into treated sewage.