"Oh, that's convenient," observes the other sorceress, voice echoing up to Kithabel. "Thank you."

She doesn't need to be on the ground anymore, the water is good and cowed, now. She leads it to the river it came from, and then wills the river to keep it. What's that river? Yes, that's right, no complaints, you do what you are told. The sorceress deepens the river, just for good measure. And it will stay that way, now won't it? Yes. Yes it will. Good river.

Once that's taken care of, she flies back and starts telling houses that they should start doing what she says, too. They obey.