With more than 170 still missing, communities must reconcile how to pick up the pieces around a waterway that remains both a wellspring and a looming menace.

In what experts call "Flash Flood Alley," the terrain reacts quickly to rainfall steep slopes, rocky ground, and narrow riverbeds leave little time for warning.

The Guadalupe River has a history of deadly flooding. Here is a look over the decades. (AP video Marshall Ritzel/Albee Zhang)

More than 100 people have been confirmed dead since July 4, when the Guadalupe River in central Texas swelled overnight and triggered flash floods that swept through an area known locally as “Flash Flood Alley.

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.