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.

State and local officials said they did their best to coordinate evacuations and rescues, but better cellphone service might help in future floods.

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.