In the early days of July, pieces of weather systems were converging to create a disaster over Texas Hill Country that would transform the Guadalupe River into a monster raging out of its banks in the pre-dawn hours of July 4, claiming the lives of more than 129 people. At least 160 are still missing.

A hydrologist explains why the region is known as Flash Flood Alley and how its geography and geology can lead to heavy downpours and sudden, destructive floods.