The heavy rain that turned a river in Texas into a raging wall of water was fueled by unique atmospheric conditions, according to meteorologists and climate scientists.

More than 850 people have been rescued by emergency responders. Many of the stories have been astonishing: A Coast Guard swimmer saved 165 people at the summer camp; a 22-year-old woman was rescued after being swept 20 miles downriver and clinging to a tree overnight; and a young girl was found after floating on a mattress for hours.

Texas Hill Country, where much of the flooding took place this time, is often referred to as “flash flood alley.” It's home to steep terrain and a lack of vegetation that creates a perfect storm for waters to rise quickly.