The ferocious flooding which devastated Colorado this week has happened before, and it will happen again. Although there is no simple explanation for the massive, record-breaking deluge of the past few days, experts are able to pin much of the blame on Colorado’s natural assets. Colorado’s geography and weather are gifts which make the “Centennial State” one of the world’s most spectacular tourist destinations, but when stormy conditions prevail – especially after a dry spell – they can form a potentially lethal combination.
Narrow canyons form natural basins for heavy rains which inundate rivers and pour down steep-sided mountains, and towns nestled in the foothills – such as Boulder – are particularly vulnerable. On most occasions, this is not enough to cause a disaster, but meteorologists have pinpointed two extreme factors which have turned the September downpour into one of the state’s worst ever floods. The rains have not just been unusually heavy; they have come at the worst possible time.
It is no surprise that many locals have described the recent rains as “biblical” as more than 17 inches hit the northern part of Colorado in the week beginning Sept. 10. On Sept. 12 alone, Boulder received over nine inches of rain, which almost doubles its previous one-day record. So far this year, Colorado has been hit by more than 30 inches – another unwelcome record broken.
Weather experts say that a strong, slow-moving storm ran headlong into a front of extremely humid monsoonal air from Mexico which got trapped over the Rockies. Light winds and cool air then teamed up to empty the water from the sky directly over the foothills. It poured down the slopes into the major tributaries of the South Platte River – including Boulder Creek – and couldn’t find any escape on ground already soaked by earlier rains, causing flash floods.
A combination of long-term drought and 2012’s Flagstaff Fire are also partly to blame, according to National Geographic’s Sandra Postel. The Colorado River basin has been unusually dry for about 14 years, she says, and when the soil hardens it can’t absorb as much runoff water. Additionally, large blazes remove vegetation which is critical in slowing down and trapping rainfall.
This has not been a typical storm. September is traditionally one of Colorado’s driest months, and major floods in the past – such as the Big Thompson Flood of 1976 which killed 143 people – tend to happen in late spring or summer. But scientists are warning that this latest disaster is an anomaly which is likely to be repeated in years to come.
According to Brad Udall, director of the University of Colorado’s Getches-Wilkinson Center for Natural Resources, Energy and the Environment, global warming is partly to blame. As a result of climate change, wildfires and floods are likely to become more common, and moisture being held in clouds by warmer air will also mean heavier rains. Udall says the connection between climate change and the latest floods “might be 10 percent or it might be 90 percent, but it isn’t zero percent.”