In a 3-day period starting September 9, 2013, 18 inches (46cm) of rain drenched the Rocky Mountains Front Range, setting off flash floods that roared through Boulder, Lyons, Estes Park, and other Colorado foothill communities. A dozen dams overflowed and six blew out. Walls of water 20 ft. (500cm) high raced down canyons, sweeping away houses and stranding thousands of area residents. As of this writing, the flooding had taken 8 lives and destroyed 1,500 homes.
Average rainfall for the month of September in Boulder is 1.63 in. (3.45cm). So what were the conditions that caused 11 times that amount to fall in 3 days? Many scientists believe that climate change, forest fires, and the severe drought that has gripped the U.S. Southwest for 14 years all played a part.
To begin, a low-pressure center settled over the Great Basin and was held in place by a high-pressure ridge over the Pacific Northwest. The low pressure system tapped into a plume of monsoonal moisture coming up from the Pacific Ocean off Mexico. Since the low was stationary, it kept sucking in the monsoon moisture in a loop, like it was coming in on a conveyor belt. The storm dumped its deluge on the drought-dried Front Range with steep canyons running downhill from peaks exceeding 14,000 ft (4,300m).
Professor Brad Udall, director of University of Colorado’s Wilkinson Center for Natural Resources, said that while current science can’t pin any particular extreme weather event to climate change, this flooding is likely a reflection of global warming. Scientists have warned that as the planet warms, drought and flash flooding will become more prevalent.
According to Sandra Postel, National Geographic’s Freshwater Fellow and noted authority on water use, the drought that has parched the area and gripped the Colorado River Basin for the past 14 years may be partly to blame for the severity of the floods. She said that drought hardens the soil, and when rains do come, the ground absorbs less water and quickly runs off the land.
Postel added that fires lead to worse flooding because they remove vegetation that can slow and trap rainfall. Hundreds of acres of Front Range forest were scorched by the Fourmile Canyon fire in 2010 and the Flagstaff fire in 2012. The burn area from those fires lies directly above the communities hit by the flash floods in September, 2013.
As our climate continues to warm, this same scenario will most likely be repeated in coming years in areas all over the world. Lengthy droughts, severe wildfires, record flooding, and more intense tropical storms are all expected to be part of our future climate menu.