Hello Lunumbra! I am not sure where it originated, but I heard it as a teenager, from a preacher. I took it to be one of those "folksy" sayings that has been around forever, but specifically used by people who live in areas where "low water crossings" are the norm. Those are low bridges, often made of stone, which are submerged with any significant amount of rainfall. They are still found in the Texas Hill Country, and other places, too, I am sure. The same expression would apply to "fords," which are crossings where the water is fairly shallow and the bottom firm.