At least 81 people are dead and at least 41 more are missing after devastating flash floods slammed Texas Hill Country, with water rescues taking place along the Guadalupe River, which rose rapidly early Friday morning to the height of a two-story building.

Here’s what to know about the deadly flooding, the colossal weather system that drove it in, and around Kerr County, Texas, and ongoing efforts to identify victims.

When too much rain falls for the ground to absorb, it runs downhill, pulled by gravity into streams, creeks and rivers.