On October 10, a tropical storm brought unprecedented rainfall to the mountains in Southwest Colorado. Saturated soils meant excess rainwater gushed into the San Juan River, flooding the small community of Pagosa Springs. The flood made history as the third-largest on record for the town. 

Researchers from CIRES’ Western Water Assessment (WWA) responded quickly to create a rapid assessment brief—a scientific resource for town planners and emergency response teams to understand why the flood happened and how they can plan for the future.

“Our goal is to work with the people who can use this information,” said Nels Bjarke, WWA research scientist who co-wrote the report with fellow WWA Research Scientist Seth Arens. “This report provides an assessment of what happened during this flood event and the flood’s historical and local context."

WWA’s mission is to help communities across Colorado, Wyoming, and Utah make the best use of science to manage for natural hazard impacts. Responding to the Pagosa Springs floods exemplifies the team’s work, and the new report provides insight into rainfall data, historic flood context, and the probability of future floods. 

Working with rural communities like Pagosa Springs gives WWA insight into the unique challenges they face: they are exposed to a range of extreme weather events, typically they experience issues with aging infrastructure, and often have finite resources to respond or prepare. WWA researchers use scientific expertise to help communities prepare for their unique local hazard risks ahead of time, through assessments, synthesis reports, and meetings. Weeks earlier, Bjarke and his team presented at the Mountain Towns 2030 conference about wildfire risk, where they connected with local officials from Pagosa Springs to discuss how WWA can support the community in building resilience in the face of natural hazards.

The rapid assessment highlighted precipitation measurements from NOAA’s National Weather Service (NWS) and U.S. Geological Survey (USGS) watershed monitoring stations that measured rainfall leading up to the floods. Bjarke noted that these systems support NWS’s ability to send flood watches and warnings across the Western U.S., and likely saved lives and possibly property in Pagosa Springs. The team also combed through historic data to compare past flooding events, and found the October floods ranked third largest after the first in 1911 and the second in 1927. 

Southwest Colorado has been in a drought for the better part of two decades. This summer, the San Juan River flowed at 42 percent of its average. Beginning in August, the region experienced unusual and increased rainfall, which raised river levels to almost normal levels through October. These uncommon yet frequent rain events left soils saturated as rain began to fall on October 9. 

The Pagosa Springs floods broke two records. On October 10, the San Juan River swelled to over 12 feet in just under two and a half hours, leading to the river overbanking and damaging roadways and water infrastructure. Precipitation paused for a short period, but continued on October 11, causing the river to rise even higher and break the record from the day before. 

With saturated soils, the rain had nowhere to go but directly into the San Juan River, an example of previous research led by Bjarke and CIRES Fellow and WWA Director Ben Livneh. The work found “precipitation intermittency”—how many days have passed between storms—can help predict flood risk. 

This rapid assessment provides context for the likelihood of future floods in the region. Researchers estimate that this event is a 25 to 40-year flood, meaning in any given year, there is approximately a 2.5 to 4 percent chance that a flood of this magnitude might happen. WWA researchers can work with communities like Pagosa Springs to determine how to integrate this type of information into their plans for the future. 

“Our goal for this and other reports is to provide scientific expertise in Colorado, Wyoming, and Utah to help communities like Pagosa Springs understand their local hazard risk and prepare,” Bjarke said. “We hope that this report brings visibility to the potential future risks that communities in the region face.”