Admittedly, I didn't initially plan on the river outlines appearing in the desert. But the more I thought about it, the more I decided that they would make useful water channels, wadis and I might even convert one of the courses into a canyon of some sort.

Deserts rarely have to consist of miles upon miles of burning sands rolling across a great expanse, but merely a climatic zone where it rains too far too little for there to be the sort of vegetation found in more temperate zones. So I decided to leave them in, because it actually didn't make sense not to have large watercourses (no matter whether how much of the time of the year water would flow through it).