Plans to develop a flood monitoring system in the Texas county hit hardest by deadly floods were scheduled to begin only a few weeks later.

Also: San Antonio mourned the victims in a Travis Park vigil; UTSA said one of its teachers died in the Guadalupe River flood; Kerrville officials said a privately owned drone collided with a helicopter conducting search and rescue operations.

Over just two hours, the Guadalupe River at Comfort, Texas, rose from hip-height to three stories tall, sending water weighing as much as the Empire State building downstream roughly every minute it remained at its crest. The force of floodwater is often more powerful and surprising than people imagine.

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.

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.

Texas flooding map, update: Thousands face new flash flood risk Ann-Margret, 84, Makes Rare TV Appearance and Shares 2 Secrets for Her Dance Moves

Follow for live updates in the Texas flooding as the death toll rises to 120, as rescue operations start to shift to recovery phase

Here's what to know about the deadly flooding, the colossal weather system that drove it and ongoing efforts to identify victims.