Excuse my language, but: Holy. Shit. That's what all of us natural disaster-curious Internet voyeurs were thinking last week, our jaws giving in to gravity as we clicked through images from Colorado's Front Range of people trudging through baseball fields covered hip-high with water, roads sliced apart by whitewater, and cabins transformed into riverine islands. Weather Channel CEOs were, no doubt, rubbing their hands together and cackling, while the usually staid National Weather Service called the rains "biblical." "There’s no scientific definition of 'biblical,'" reported Climate Progress, "but the flooding has been unlike anything local residents have ever seen before."
Well, yes and no. The amount of rain that fell on communities like Boulder and Lyons was indeed historic, smashing those cities' monthly and daily precipitation records, and devastating many residents. In some locales, it's being called a 1,000-year rain event, meaning there's a 1 in 1,000 chance of such heavy rains in any given year. But it should come as no surprise that Boulder flooded. Crack open the history books and you'll find that the Front Range has always been flood prone. According to the Boulder Area Sustainability Information Network (BASIN), when it comes to flooding, "the Boulder Creek drainage is considered among the most hazardous in the entire western United States." The city sits smack at the mouth of Boulder Canyon, and because its floodplain is so developed, the risks to human safety and property are especially high here.
Boulder owes its vulnerability to geography. It's pushed up against the foothills of the Rocky Mountains and steep-sided canyons. When storms get stuck above it, as they did last week, rains can careen off pitched slopes into the canyons, where creeks swell and eventually burst forth onto the city itself. One of the earliest documented instances of this was in 1894, which incidentally, was also Boulder's last 100-year flood event. Homes, bridges and train tracks were swept away, and the girth of Left Hand Creek apparently grew to a half-mile. Significant flood events have occurred more than a dozen times since.