Mad Kaw, Having never seen it I can only imagine.
The Park Service does have a spot next to a huge arroyo with a sign that discusses that very thing, and I did find myself looking around for thunder clouds. Some of the arroyos we went through on day two were exceptionally large.
All the campsites in Big Bend are designated by the Park Service, but to be honest, I never considered whether they might be subject to flooding. Camp de Leon is on higher ground, tho.