The water carries minerals from the soil back into the river, which get more concentrated each time. Salt is a major concern (this is the source of ocean salt),

The water carries minerals from the soil back into the river, which get more concentrated each time. Salt is a major concern (this is the source of ocean salt), and in major irrigation rivers like the colorado is a major issue that is controlled carefully for downstream users, for whom it would otherwise be too salty to use for irrigation. The US treaty with mexico dictates the salt content of the river at the border for this reason. The southwest US used to be home to a widespread desert irrigation agricultural people, the Hohokum, and failure to manage soil salt content is afaik still the leading hypothesis for their disappearance. So no unfortunately water that flows back into the river isn't neutral.