Monte Morin, writing for the LA Times:
Though about a decade of drought has diminished vegetation that might otherwise work to slow rain runoff, urban development has also created areas where rainfall moves rapidly across populated areas.
“Of course, over the years, Boulder has grown and grown and like most cities, the more it grows, the worse the runoff gets,” Patzert said. “That’s because there’s more parking lots, more driveways, more roads, etc.”