This 1972 weather event is the biggest flood to hit the U.S. in the last 100 years. In the eastern Black Hills of South Dakota near Rapid City, a stationary group of thunderstorms began around 5 p.m.

Meteorologist Scott Sistek said some rivers are approaching their worst-ever levels, with 12 waterways reaching “major” flood stage across the region. Western Washington is experiencing one of its ...