The amount of housing and development next to waterways over the past several decades is the factor and building in remaining open spaces elsewhere overall. While an increased amount of precip may very well have happened, if we hadn't built 10 fold, I'm sure the billions number would have been drastically reduced. Take NJ for example. The coastline is loaded with homes 10 feet from an Ocean. Then, the mandate for low income housing is causing communities to chop down their remaining land and put up more housing to reach an arbitrary % agreed to by the state gov. My old town alone, if you look at the open space from 1990 vs now, it is almost non-existent. It is all becoming concrete and the water has no place to go.