Remembering the One Colorado Flood To Rule Them All—Still

Growing up in the Southwest U.S., you’re taught to fear flash floods like few other things. If a storm is moving in, don’t go near the creek. Other threats like black widows, lightning, and rattlesnakes are all more self-evident as acute dangers, but a flash flood is born over tens or hundreds of square miles and if some kid is playing battleships in an arroyo or drainage ditch (my neighborhood creek was a combination) when a flash flood arrives, it’s already too late. Any child with access to open-space and reasonably attentive parents knows that the appropriate response muddy running water—one of the only flash flood warning signs is mud in the water—is to back away apologizing.

The water turns muddy because excess water is washing away formerly dry places. A desert waterway has a vast network of tributaries feeding it that are usually just some dirt or sand in a long dry crack until a storm passes through, filling them all for a short time. The sudden rush of water grabs up large swaths of land with it, making the resulting flood an angry liquid rip in the earth filled with red dirt, wood, and some amount of actual water. A flash flood is made of a substance that could have no stagnant life; the torrent needs energy to keep all of these liberated solids in suspension and, once stalled, the dirt and other assorted detritus falls out, leaving, once again, just water.

I was pretty little when I heard about the Colorado flash flood, the one that remains to this day the worst in the state’s history by many metrics (including the most important one: death toll) and one of the most destructive anywhere ever. I must have been on a weekend or day trip with my dad to the town of Estes Park or somewhere else around Rocky Mountain National Park, and I don’t think the story was meant as a cautionary tale. We were just driving through Big Thompson Canyon and the disaster was still pretty near timewise. It looked then like a lot of arid, rock-walled Front Range canyons, but nonetheless, there it was: everyone here died.

By last Monday, Boulder, the news epicenter of this month’s Colorado flooding, received just over 17 inches of rain since the first of the month, or about 10 times the monthly average for September. On the 16th itself, the day when that 30 foot wall of water charged down Boulder Canyon in the middle of the night, the city got upwards of 6 inches. This busted the city’s previous daily rainfall record by two inches, an event with only a .1 percent chance of occurring, leading to the somewhat misleading designation as a “thousand year flood.”

In 1976, an area of Big Thompson Canyon, lying between the northern Denver exurb of Loveland and the mountain town of Estes Park, received 10 to 14 inches of rain in a couple of hours. Hours. Because inches of rain are somewhat hard to visualize, let’s convert that to snow. It’d be about 10 feet. So, figure a new foot of snow every 10 or 15 minutes. Unlike the recent Colorado rainfall, these 10 to 14 inches fell in a very small area in a very small amount of time. This is actually the opposite of what just happened and why this week’s floods will likely go down as the most costly in state history: coverage area (from the southern-middle of the state to the Wyoming border) and duration (12 days so far, with some brief interruptions).

A precipitation map of the area around Big Thompson Canyon for the night of July 31, 1976 between about 9 and 11 pm looks like a dartboard with the bullseye directly over the upper canyon. In fact, in parts of the canyon it was even dry that night, at least until the floodwaters began rising. That happened quickly; many parts of the canyon are no more than a giant stone pipe or aqueduct, a deep gash in the bedrock. With little soil or vegetation, water has no place to go but down its steep course. And it is steep: from top to bottom, the creek loses nearly a half mile of elevation.

The Big Thompson River is really just a creek most of the year, subject to a maze of diversions and a generally arid climate. Before this month’s floods, the Big Thompson was running at about 75 cubic feet per second (cfs), which is a relative trickle. During the recent floods, it peaked at over 10,000 cfs, enough to cause “monumental” damage. In 1976, the water level rose to 32,000 cfs at a rate fast enough to give nearly zero warning to tourists and residents in the canyon. The crux of that 32,000 cfs arrived in a 20-plus foot wall of water moving at 14 miles per-hour.

With warning and an escape route, that’s escapable. But there was little that night, and the floodwaters killed 144 people in a very brief period. In addition, it destroyed 400 cars, 418 houses, and 52 businesses, along with most of U.S. Highway 34. The loss of the highway cut off the only means of escaping the flood as the sheer rock walls prevent any sort of route upward in many places. People died in tents, campers, homes, and cars.

In a piece last week in The Denver Post, Allen West, a member of the 1976 Big Thompson rescue cohort, recalled, “At the old hydroelectric plant where my family had once enjoyed Sunday picnics, the brick building had vanished. Only the turbines and concrete foundation remained. In a nearby tree, amid the branches maybe 10 or 15 feet off the ground, hung a lifeless body.”

Last week, even I had an hour or so of warning that a flash flood was charging down Boulder Canyon, and I’m a thousand miles away, via Twitter. Many areas of Boulder and other vulnerable communities had already been evacuated by the time serious danger hit. The death toll now stands at 10 people, though total costs from this month’s flooding will dwarf the 1976 Big Thompson flood. Much of the difference in death tolls (this month and otherwise) likely has to do with a network of rainfall sensors installed in the foothills above Front Range communities, such that if 10 to 14 inches of rain were to fall in a remote area above hundreds of thousands of unsuspecting people, forecasters and local authorities would be given advance notice.

The Front Range has been booming for decades now and its communities make up some of the fastest-growing in the nation. What were once rural towns surrounded by pasture are now full-on suburbs and exurbs choked with subdivisions, malls, and vulnerable fracking facilities. The truth is that the southwest all over remains a land-use nightmare, sensors or not. Fires and floods: it will never happen here. Rainfall sensors exist to keep people from dying, not to keep them from losing all of their worldly possessions or to keep governments at every level from hemorrhaging massive amounts of cash on rebuilding communities at the outlets of runoff pipelines.

That last one is a rather unempathetic statement, I think. After all, my greataunt and uncle are still stranded on the side of a mountain (in their half-flooded house) above Boulder Canyon. The town where I went to high school, Manitou Springs, Colo., has been hit by devastating flash floods twice this summer, both of them magnified exponentially by a massive burn scar looming over the hamlet. (The effect of wildfire burn scars is in essence to make formerly tree-lined canyons into runoff pipelines like Big Thompson Canyon.)

The ugly truth is that there’s no other preventative strategy on par with good land-use planning. You can’t reengineer an entire mountain range of rivers and streams, though Colorado has certainly tried. Places like Boulder Canyon, Big Thompson Canyon, and Fountain Creek Canyon above Manitou Springs only exist because they channel rainwater and snow runoff, yet they all feature massive developments at their bases. Gilbert White, a geographer considered by many as the "father of floodplain management," noted in 1945, “Floods are 'acts of God,' but flood losses are largely acts of man."

@everydayelk