Nationwide, First Street found more than twice as many Americans live in dangerous flood-prone areas than FEMA's maps suggest, leaving many homeowners and even local officials unaware of the risk.

By the time I watched the video, I’d seen the images of upturned bunk beds, muddied stuffed animals, tiny T-shirts snagged on branches.

Thousands of first responders were still combing through piles of mud-covered debris in Texas Hill Country on Thursday, hoping against long odds to find survivors six days after flash floods swept th...