That could trigger more stringent construction regulations and require more homeowners to get flood insurance.
The flood caused $3 billion in damage and killed 10 people. Early estimates called it a 1-in-500 event, meaning the chances of such a deluge in any one year are 1 in 500.
The Colorado Water Conservation Board says new studies show it was mostly a 1-in-100 event. Officials say that means homes, roads and other infrastructure could be more vulnerable than previously thought.
The new data will likely mean changes in officially designated flood plains. If flood plains expand, more people would have to get flood insurance and stricter construction codes would kick in.