Well, I introduced them to reading, so they can write it down for later.
They might decide later they have a compelling reason to divert a river. Rivers eventually change course anyway; perhaps they could just ask you to put it where they want it, and then they wouldn't be tempted to try damming or anything like that?
Maybe they could put a message in a bottle? Would you notice that; do you read? Men are in a hurry all the time; they have to be, they don't live very long.
Okay, so up to once annually they can send you river-related requests, which you will consider?
Thank you. Would you like to make up a name so they have something to call you other than 'river Maia'?
But perhaps you could tell them what the problem is, and then they'd be able to think of other ways to get what they need and propose a revision the following year.
...Will you in fact be able to read the Men's language when they send messages?
And when did Thuringwethil pick it up. And what kind of favor. I could try teaching you now, if you like. She was around when the written language was invented and can probably get through basic vocabulary with only intermittent Allspeak use.