this cap of the topo is pretty much the entire drainage watershed of the south fork above mystic (marker B)
marker A is 6.5 miles southwest of mystic
it's dry as a bone for years at a time, but it clearly has a geologic history of being subject to the same event that happened thursday night:
the road at the top is 39
the area between the "river bed" and 39 is the floodplain from erosion events like thursday night
here is the streetview of the house on the right: https://maps.app.goo.gl/741GBi5MGCgvn9z18
the dry riverbed, erosion escarpment from water accelerating through the curve in the path (see: the bernoulli equation), and obvious floodplain clearly indicate this same event has transpired for thousands of years and i have no doubt that this exact same flood has happened at least twice since the settlement of kerr county in the 1840s, but the institutional memory expired before all of the camps were built post-war
the drainage watershed of the guadalupe south fork is relatively small but it obviously can become very powerful
i'm going to stop there because this line of inquiry is heading towards an indictment of good people in a time of tragedy