On Wednesday, hundreds prayed, wept, and held one another at a prayer service, among the first of many somber gatherings to come in the weeks ahead.

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.