Around 8:15 p.m. on July 31, 1976, Colorado State Trooper Bob Miller was dispatched from Estes Park to check out reports that boulders and rockslides were blocking U.S. 34 in the Big Thompson Canyon. It had been raining for at least an hour, so the rockslides weren’t exactly a surprise and, at that point, not necessarily a cause for concern.

Miller soon found out otherwise. About 20 minutes later, he radioed that his car was trapped by rising water near Grandpa’s Retreat and he was headed to higher ground.

“We’ve got to start taking people out,” he said, according to contemporaneous news accounts. “I’ve got a real emergency down here.”

Miller’s transmission was the first indication to local authorities that something far more serious than a rockslide was underway, but for many of the people in the canyon that night, it was already too late.

By the time the worst of the storm eased in the early hours of Aug. 1, it had dropped up to 12 inches of rain over the canyon’s steep slopes. With nowhere else to go, the water poured into the Big Thompson River, sweeping away homes, vehicles, bridges and everything else in its path.

Ultimately, the flood damaged or destroyed hundreds of structures and more than 400 vehicles. It also claimed 144 lives, making it one of the deadliest natural disasters in Colorado history.

Fifty years later, the Big Thompson Flood remains a defining event in local history. But behind the tragedy lies a compelling scientific story that explains how climate and geography came together and unleashed a flood powerful enough to reshape an entire canyon.

“These kinds of storms are not super uncommon in Colorado,” said Allie Mazurek, a researcher at the Colorado State University’s Colorado Climate Center. “This particular one had very heavy rain rates, which is a bit unusual, but where it fell was also really important. If it had been just a few miles away or the winds were a bit stronger, it might have been a very different story.”

The gathering storm

Residents along Colorado’s northern Front Range woke to sunny skies that Saturday morning, even if it was a little more humid than usual. And while it was an ordinary day from a weather standpoint, it was an otherwise festive day for residents in the Centennial state. On Sunday, Aug. 1, Colorado would mark the 100th anniversary of its admission to the Union in 1876, and the weekend was full of events, activities and celebrations in cities large and small.

Whether it was the holiday atmosphere or just the usual desire to escape the heat of the city, the Big Thompson Canyon was unusually busy that weekend. Larimer County officials later estimated that there were anywhere from 2,500 to 3,500 people there, four to six times its year-round population of about 600.

While visitors camped, fished and explored the canyon, a rare combination of atmospheric conditions was slowly coming together over the Front Range.

Road damage is seen near the mouth of the Big Thompson Canyon following the July 31, 1976, flood. (Joel Radtke for the Loveland Reporter-Herald)

The most unusual ingredient was the amount of moisture in the air. Around sunrise on July 31, a weather-balloon sounding over Denver showed precipitable water values — a measure of the total moisture in a column of air — about 50% above the July average, according to a joint U.S. Geological Survey and National Oceanic and Atmospheric Administration report on the flood.

At the same time, a weak disturbance high in the atmosphere moved north into the region. A shallow front had stalled along the eastern foothills, while a second, deeper front pushed southwest from the plains.

Near the surface, strengthening easterly winds carried humid air toward the mountains. Higher above Colorado, however, the winds remained light, providing little steering to carry developing thunderstorms away.

None of those ingredients was unprecedented on its own. But the exceptional moisture, growing atmospheric instability, weak steering winds and lift supplied by the frontal boundaries and the mountains were converging in the same place at the same time.

The deluge

The place was a narrow band just east of Estes Park, extending roughly 10 miles northward. The time was between about 6 and 6:30 p.m., when the warm, moisture-laden air met the Front Range foothills and was forced sharply upward by the mountains, building towering thunderheads up to 50,000 feet above the Big Thompson watershed.

In news accounts, several witnesses later recalled that rain began falling in earnest — accompanied by frequent lightning — between 6:30 and 7 p.m., as many canyon residents and visitors were eating or had just finished supper.

It was especially bad in Glen Comfort, a small community of homes and campsites about four miles east of Estes Park and halfway to Drake. From about 7:30 to 8:40 p.m., an estimated 7.5 inches of rain fell in the vicinity, as light winds aloft moved the storm cells only slowly and new ones repeatedly formed in roughly the same places.

As the larger storm complex edged slowly northwest, the heaviest rain shifted toward Glen Haven and the North Fork of the Big Thompson. Rainfall rates there were slightly lower, but the downpour lasted longer, continuing until about 10 p.m., though isolated bursts continued through Sunday morning.

A cabin is scattered on top of a bridge which spans the rain-swollen Big Thompson River, which left is banks July 31, 1976. (File photo by Bill Johnson/The Denver Post)

“It was a stationary thunderstorm that had a lot of moisture in it, and essentially it was parked there,” Mazurek said. “So it rained really hard for several hours.”

There were no reliable rain gauges in the heart of the storm, so U.S. Geological Survey scientists later reconstructed the rainfall using radar data, eyewitness accounts and measurements from surrounding stations.

They estimated that about 10 inches fell near Glen Comfort between roughly 6:30 p.m. and midnight, while in Glen Haven, about 9.5 inches fell. Additional showers continued overnight and into the next two days, bringing the three-day total at one gauge west of Glen Haven to 12 inches.

The river rises

All of that rain had to go somewhere, and the canyon’s steep, rocky terrain gave it only one direction to travel — toward the Big Thompson River and its North Fork.

Fed by runoff from dozens of gulches, tributaries and draws around Glen Comfort and Glen Haven, the normally placid river rose with astonishing speed, spilling over its banks. Rather than arriving as a single wall of water, it just continued to rise and spread as runoff from one drainage after another poured into the river.

There was no single point where the destruction began.  As the rain-swollen river raced downstream from Glen Comfort, it tore rocks and trees from the canyon walls before sweeping up bridges, vehicles and entire buildings.

Among the most haunting sights survivors later recalled were propane tanks, ripped from their foundations and carried downstream with an eerie whistle.

The destruction intensified downstream. Situated just below the confluence of the Big Thompson and its North Fork, Drake was especially vulnerable, even though it received only about 4.5 to 5 inches of rain itself.

But Drake’s own rainfall was only part of the threat, as runoff arrived from both flooded watersheds. Roughly 30 minutes after the surge from the main channel swept through the community, a second crest arrived from the North Fork, compounding the destruction.

From there, the torrent barreled east through Cedar Cove, The Narrows and the lower canyon, destroying more homes, businesses and bridges, and washing away long stretches of U.S. 34.

In the span of a few hours, the Big Thompson had been transformed into a river unlike anything previously measured. By late July, it was typically 15 to 20 feet wide and 1 to 3 feet deep. Near Drake, it was flowing at just 137 cubic feet per second before the rain began.

During the flood, its flow swelled to an estimated peak discharge of more than 31,000 cubic feet per second just below Drake — nearly four times greater than the highest flow ever recorded there.

Legacy of the flood

It might have been the deadliest in recorded history, but the 1976 flood was neither the first major flood on the Big Thompson nor the last.

An archived pages of the Reporter-Herald newspaper following the Big Thompson Flood of 1976. (Jenny Sparks/Loveland Reporter-Herald)

In September 2013, nearly a week of persistent rainfall once again swelled the river, destroying homes, washing out U.S. 34 through much of the canyon and cutting off communities for days. Two canyon residents, were killed during the 2013 disaster.

Despite the similarities, the two floods were driven by very different weather patterns — a short-lived, but intense burst in 1976, while the 2013 flood built over a period of days. Together, the two disasters have helped scientists better understand the different ways Colorado’s mountains and atmosphere can combine to produce catastrophic flooding.

Today, hydrologists generally classify the July 31, 1976, Big Thompson Flood as a 1,000-year flood event, which, contrary to popular conception, does not mean another flood of similar magnitude can’t happen for another millennium.

Rather, it means a flood of that size has about a 0.1% chance of occurring in any given year based on historical records and statistical analysis, said Mazurek.

But that estimate is not necessarily static, she continued. As more flood data become available, geologists studying prehistoric flood deposits are extending the canyon’s flood record back thousands of years.

“It’s definitely an active area of research,” Mazurek said. “We’re developing what we call exceedance probabilities with better measurements, and, as we have data that spans a longer period of time, we can make updates to probabilities.”

The advances haven’t been limited to flood-frequency research.

Today, meteorologists can identify dangerous conditions earlier and issue warnings sooner through the use of Doppler radar and satellites, computer models and networks of real-time stream gauges that weren’t present in the canyon in 1976.

Even so, Mazurek said technology alone cannot eliminate the danger from a flash flood.

“Being in a canyon in the summer in Colorado, especially in July, that’s just something that you need to have awareness of,” she said. “If you know that there’s precipitation forecast that day, have a way to get warnings.”

A sign warning people to climb to higher ground during a flash flood is pictured on U.S. 34 at the mouth of the Big Thompson Canyon Juily 17. (Jenny Sparks/Loveland Reporter-Herald)

But even absent an official warning, mountain visitors need to understand the hazards of canyons, recognize threatening weather and know to seek higher ground when flooding threatens.

“Flash floods tend to be less flashy — pardon the pun — than something like a tornado,” she said. “A flash flood is just water rising, so people don’t realize the danger. I think they underestimate the power of water, too.”

Many of those lessons will be explored Friday during the Colorado Climate Center’s Big Thompson Flood 50th Anniversary Symposium at the Estes Park Events Complex.

Researchers from the National Weather Service, NOAA, Colorado State University and the U.S. Geological Survey will discuss the meteorology behind the storm, advances in forecasting and flood science, paleoflood research and how the disaster continues to shape flood preparedness a half-century later.

But the central lesson may be the simplest, Mazurek said. Colorado’s climate and topography provide the right ingredients for extraordinary weather when the conditions align.

“It all goes back to Colorado being a state of extremes,” she said.