Maybe I'm not seeing it but wouldn't it be just easier to pump the river water back up instead of letting it flow on the sea?
Except that the amount of water needs to be augmented as it disappears through usage and evaporation. So you’d run out pretty quickly.
Large scale desalination might be necessary but I think the article is downplaying the brine issue. I don’t think we’re running out of table salt so trying to use it for harvesting metals will be an environmental challenge at the scale being proposed here.