According to NOAA, floods kill an average of 90 people each year in the US., the highest average of any type of natural disaster. Most flood deaths occur as people are swept away in cars and other vehicles. The reasons vary with climate and topography.

In the middle of the country, tributaries of the Missouri and Mississippi rivers routinely overspill their banks, sometimes causing catastrophic flooding such as this spring’s deluge in Nebraska, Iowa and adjacent states. As seasonal temperatures rise, snow melts early and inundates the river system at the same time spring rains hit. The water has nowhere else to go.

In other places, intense rains can trigger flash floods in areas where the terrain funnels water into a narrow space. Ellicott City, Md., has suffered two 1,000-year floods in the past three years because it sits at the bottom of a hill where several streams converge.

Areas within a wide band of Texas, from north of Dallas to south of San Antonio, are so prone to flooding that the entire zone is referred to as “Flash Flood Alley.” Steep canyons and valleys of the desert southwest routinely channel torrents of water from storms at higher elevations.