Thanks for the link! Didn't know it wasn't operating. My understanding is that the treaty says that the salt content should be less than X parts per million and

Thanks for the link! Didn't know it wasn't operating. My understanding is that the treaty says that the salt content should be less than X parts per million and when the Colorado River gets saltier than X, they can turn the plant on. This diverts some of the water out and returns it in with significantly less salt, thus reducing the total salt content of the river as a whole to a level less than X. Basically, imagine you have a glass of chocolate milk. You take a teaspoon of chocolate milk out and put a teaspoon of regular milk back in, you will reduce the chocolate content of the glass. The glass is the river as a whole, the plant only treats the teaspoon.