In 1987, sudden and intense rainfall caused the river to surge at an unprecedented rate in mid-July, leading to a tragedy that killed 10 campers at Pot O' Gold Christian Camp, a summer camp near Comfort, Texas. The campers drowned when their bus attempted to evacuate them and was overtaken by floodwater.

Search efforts continue for the dozens of people still missing after Friday's floods, as questions swirl over what went wrong. Here's what we know so far.

Flash floods happen when heavy rains unleash more water than the ground can absorb, causing that water to pile up and flow to low-lying areas