Flash Flood Alley

The low-water crossing signs along this road are not decoration. Central Texas, in a crescent following the Balcones Escarpment from west of San Antonio through Austin and up toward Waco, produces more flash floods per square mile than anywhere else in North America. Three things combine to do it. Warm, wet air off the Gulf is forced upward as it meets the rising edge of the Edwards Plateau, which wrings out extraordinary rainfall rates. The ground it lands on is limestone under a few inches of soil, so almost none of it soaks in. And the valleys are narrow and steep, so what runs off arrives all at once. None of it is unusual weather. It is the normal behaviour of this landscape, arriving a few times a decade in any given valley. Some of the heaviest short-duration rainfalls ever measured in the United States have fallen inside this crescent. The practical consequence for a driver is that a creek crossing can go from dry gravel to moving water in well under an hour, often under a clear sky, because the rain doing it fell miles upstream and out of sight. About three quarters of flood deaths in Texas are people in vehicles, and most of those happen at crossings like this one. The instruction repeated on every sign exists for two reasons that are easy to underestimate: water a foot deep will float most cars, and the roadbed underneath it may already have been washed out, so the depth you can see is not the hazard you are judging.