What Are the Odds of Being a Rockfall Casualty in Colorado?

Monday's tragic rockslide in Chalk Creek Canyon highlights the random nature and low probability of rockfall causing casualties. What if the hikers had started 10 minutes earlier or 10 minutes later than they did? What if the rockfall happened later in the evening?  There are no answers for why the rocks let loose exactly when the hikers were directly below at that same moment in time. My book, Colorado Natural Hazard Events has a chapter on rockfall that covers where rockfall has caused casualties in the past in Colorado. Steep rock slopes of more than 40 degrees are found throughout the mountains of Colorado and virtually every canyon where creeks drain from the mountains in Colorado has slopes that steep. Rockfall is more likely in the non-summer months when rain or snowmelt penetrates cracks and fissures in the rock and freezes overnight. The ice expands and causes "ice-jacking", which can loosen a large block of rock off the side of a cliff face and get a rockslide started. At least 500,000 people climb or hike in rockfall-prone areas in Colorado each year. Between 1998 and 2011 there were 25 hikers or climbers killed and 18 injured, which works out to an average of 1 casualty for every 190,000 hikers and climbers in rockfall-prone areas. So, in the 2 years since 2011, 5 or 6 casualties would be the number expected based on the historical rate. Two hikers were injured by a rockslide in Deer Creek Canyon west of Littleton in January, making 8 casualties total after Monday's tragedy and 2013 isn't over yet, but the historic rate continues on at more-or-less the same pace. Should  you avoid hiking in rockfall-prone areas? Life is full of risks that aren't in your control. Visiting beautiful places that give people a great deal of pleasure involves a low level of risk, like driving an automobile to get there. You are about 1,000 times more likely to be a casualty in an automobile accident than being hit by rockfall in Colorado.