Double trouble? Colorado primed for flooding between torrential rains, spring snowmelt

Colorado’s mountain snowpack is starting to melt faster, potentially bringing more high water after recent heavy rain turned some of the state’s typically feeble creeks into torrents.

Big water rose to levels up to 80 times higher than the norm during rain bursts in Colorado Front Range cities this week, forcing police in Denver to warn creekside campers who lack housing to clear out, and scrambling 30 firefighters in Colorado Springs who recovered the body of a person swept away.

More rain was falling Friday — and National Weather Service meteorologists forecast thunderstorms nearly every day next week — saturating soils to the point that water more easily gains momentum.

“The risk of flooding is out there,” said U.S. Geological Survey hydrologist Brandon Forbes, who runs the federal government’s network of 360 gauges on rivers and creeks around the state, which provide cubic-feet-per-second (cfs) water flow measurements every 15 minutes.

“Certainly on the Western Slope, all of our gauge readings will increase as the snow melts over the next few weeks,” Forbes said. “We are preparing for high flows on Colorado’s Western Slope over the next two weeks to a month. For flooding risk, the slower it melts the better. That all depends on the weather.”

The overall rising flows from a combination of melting snow and heavy rain hit Colorado this week amid growing interest in how climate warming is changing water dynamics — driven by intensifying human competition for water across the generally arid Southwest.

Atmospheric scientists long have anticipated a shift away from snow as the primary form of precipitation in Colorado and the high Rocky Mountains toward more moisture falling as rain. Warmer air — temperatures in parts of western Colorado have increased faster than the global average — can hold more water and also release more water as rain, sometimes in bursts during extreme storms. One federal government-backed research project in the mountains above Crested Butte involves measuring how much moisture from mountain snowpack is lost into warming air — the “sublimation” of snowpack converted directly into water vapor, which reduces the water from melting snow that reaches streams.

“The expectation is that a warming climate will result in reduced mountain snowpack runoff owing in part to increased evaporative demand — the atmosphere’s ‘thirst’ for water — which goes up as the temperature rises,” state climatologist Russ Schumacher said, referring to a University of Colorado study in 2021. “‘Compared with a decade ago, there is now substantial evidence from both hydrologic model experiments and analyses of the observed record that recent warming temperatures have already had a role in reducing Colorado River flows.’”

The mountain snowpack in watersheds feeding the Dolores, Animas, Gunnison, Yampa, Colorado and other rivers in the farthest western parts of Colorado this year measured exceptionally high and promise the biggest runoff. Mountains east of the Continental Divide received relatively less snow. The South Platte watershed had snowpack near average, and snowpack in the Arkansas River Basin lagged, peaking at around 74% of the 1999-2020 norm.

Meanwhile, heavy rain — falling in scattered bursts around Colorado since May 9 — has led to unusually high flows in creeks and rivers. On Thursday, the Arkansas River overflowed its banks in southeastern Colorado near La Junta, inundating  U.S. 50. Coal Creek west of metro Denver last week overflowed banks, forcing closures along Colorado 52.

In Denver, intense rain resulted in muddy brown currents coursing through the city in Cherry Creek at 1,300 cfs on May 11, the highest since 1980, federal data show. (U.S. Army Corps of Engineers operators of the dam on Cherry Creek reservoir last week began regularly scheduled season releases of reservoir water through the dam into the creek, adding to infusions from rain. “Having that release occurring means future rain events will cause any possible flooding to be that much higher,” Forbes noted.)

Along Monument Creek, as it flows through the center of Colorado Springs, a USGS team led by Forbes last week measured the flow at 4,000 cfs, 80 times higher than the norm of 50 cfs for this time of year. South of Colorado Springs where Monument Creek joins Fountain Creek, data show a flow of 5,000 cfs, above the norm at that location of 80 cfs.

Water levels in the South Platte River northeast of Denver at Fort Morgan, averaging over 36 years around 300 cfs, hit a high flow on May 14 of 5,930 cfs, data show.

And on the Arkansas River a mile east of Pueblo, flows exceeded the norm of 900 cfs fivefold at 4,780 cfs on May 12.

On Friday morning as rain fell faintly amid fog and smoke spreading from Canadian forest fires, USGS measuring station data showed the following relatively high flows around Colorado from both rain and melting mountain snow runoff:

  • The Cache La Poudre River at Fort Collins: 1,330 cfs, above the norm of 575 cfs,
  • South Platte River in Commerce City: 1,160 cfs, above the norm of 649 cfs,
  • Sand Creek where it meets the South Platte: 178 cfs, above the norm of 121 cfs,
  • Bear Creek southwest of Denver near Morrison: 289 cfs, above the norm of 121 cfs,
  • Big Thompson River below Moraine Park near Estes Park: 288 cfs, above the norm of 153 cfs,
  • The Colorado River at the Utah borders: 35,400 cfs, above the norm of 14,499 cfs,
  • Colorado River at Windy Gap (near Granby): 1,800 cfs, above the norm of 632 cfs,
  • Colorado River at Kremmling: 2,600 cfs, above the norm of 1,930 cfs,
  • Gunnison River (near Gunnison): 4,060 cfs, above the norm of 1,690 cfs, and
  • Dolores River at Bedrock: 4,120 cfs, above the norm of 820 cfs.

Around Colorado, rainfall has saturated soil widely since May 9. That means, the more rain that falls, the greater likelihood of flooding.

“The watersheds are now primed,” Forbes said. “Because the soils are so wet, they don’t have much capacity to hold onto rainfall. Water will run off quicker – so the risk of flooding is higher.”

Mountain snowpack, when it melts, gradually trickles into streams and rivers in western Colorado, often described by city and agricultural water supply managers as a natural, time-release reservoir. In contrast, when precipitation takes the form of rain, creeks can overflow banks in less than an hour as water becomes suddenly abundant.

But high water will not last, Forbes said. “It goes down very quickly.”

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