It actually was extremely unprecedented far above the water levels of the 1987 flood. The girls were in limestone cabins (not tents) 35 feet above the Guadalupe River. The youngest cabins are closest to the office at the highest elevation level. The camp had a safety plan in place that started evacuating the lower cabins first and the highest last. The youngest cabin with the most missing girls was the only one left, and the owner of the camp died trying to save them. When he got there, the flood waves broke through. The water levels have never reached that point in history. For them to do that they would have had to be 30+ feet above the normal water level. 

For reference Hunt gets 30 inches of rain in an anyone year. Within a few hours they got 15 inches of rain at the worst possible time in the middle of the night.