Depends. You only need a difference in salinity, not non-salty water. If the brine is much more saline than the ocean, there's your energy potential. On the other hand, this kind of power plant also goes well at the mouth of a river. You don't need clean water at all.

I don't know about California, is all the fresh water fully recycled until the rivers are dry? That would be hard to imagine.

I don't think the rivers would run dry if we recycled waste water, as a majority of it is just dumped into the ocean.