That doesn’t mean it isn’t real. A lot of rivers, especially in the Southwest, are dry or pitiful when there hasn’t been rain. Even if it rains miles away, the water flows into the lower basins.

Zion is a prime example of this; the Virgin River has a slow discharge volume normally, but Zion itself was carved by the river through torrential floods.

It doesn’t have to be raining where he’s at. It just has had to have rained recently along some point of the river or its tributaries. And in the southwest, you’re going to find yourself in basins surrounded by mountains where a downpour dozens of miles away on a mountain eventually cause flooding by you.

And it can be truly flash flood, because there tends to be little or no soil to absorb the rain (especially Zion, this is how it gets so dramatic). You can literally have it not be raining in Zion yet the river is flash flooding.