For the period 1920-1997 there were 312 flooding fatalities in Colorado.
Several factors contribute to flash flooding.
The two key elements are rainfall intensity and duration. Intensity is the rate of rainfall, and the duration is how long the rain lasts. Topography, soil conditions, and ground cover also play an important role. Flash floods occur within a few minutes or hours of excessive rainfall, a dam failure, or a sudden release of water held by an ice jam. Flash floods can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels.