A swift-moving flood that swept through the Hill Country of Texas on Friday, killing at least 79 people and leaving many more missing, was a flash flood.

Hurricane Katrina is the deadliest flood event in the U.S. in the past 25 years. The storm crashed into the Gulf Coast and caused devastating flooding when levees failed in New Orleans, where people had to be rescued by boat and helicopter from rooftops.

One of Texas' most popular rivers flooded over the weekend and contributed to Texas' catastrophic flooding that claimed over 80 lives, including two dozen children at Camp Mystic. Here's what to know about the layout of the river and the role it played in one of the state's deadliest flash floods in modern history.