If I asked you to name the deadliest single-event natural disaster to strike the western half of the United States in recorded history, what would you answer?
If I had to hazard a guess, most people are going to suggest the 1906 San Francisco earthquake. This was a bad one, no doubt about it; an estimated three thousand people died, and most of the city was destroyed by the quake and the fires that followed it. Another one that might come to mind is the eruption of Mount Saint Helens in 1980, but that one comes in a distant follower at fifty-seven casualties.
The worst natural disaster in the western United States -- by a significant margin -- is one a lot of people haven't heard of. In the winter of 1861-1862, an atmospheric river event turned the entire Central Valley of California into an enormous lake, submerging once dry land under as much as ten meters of water. Over a period of forty-five days, a hard-even-to-imagine three meters of rain fell in the Sierra Nevada Mountains and the surrounding area, draining down into the lowlands far too fast to run off. Rivers overflowed their banks; some simply vanished under the expanding lake. Although the middle part of the state bore the worst of it, devastating floods were recorded that year from northern Oregon all the way down to Los Angeles.
The exact death toll will probably never be known, but it's well over four thousand. That's about one percent of the entire population of the state at the time.
A man named John Carr, writing in his memoir thirty years later, had this to say:
From November until the latter part of March there was a succession of storms and floods... The ground was covered with snow a foot deep, and on the mountains much deeper... The water in the river ... seemed like some mighty uncontrollable monster of destruction broken away from its bonds, rushing uncontrollably on, and everywhere carrying ruin and destruction in its course. When rising, the river seemed highest in the middle... From the head settlement to the mouth of the Trinity River, for a distance of one hundred and fifty miles, everything was swept to destruction. Not a bridge was left, or a mining-wheel or a sluice-box. Parts of ranches and miners cabins met the same fate. The labor of hundreds of men, and their savings of years, invested in bridges, mines and ranches, were all swept away. In forty-eight hours the valley of the Trinity was left desolate. The county never recovered from that disastrous flood. Many of the mining-wheels and bridges were never rebuilt.
Many of the smaller towns never were, either.
What seems to have happened is that in rapid succession, a series of narrow plumes of moist tropical air were carried in off the Pacific. These "atmospheric rivers" can carry an astonishing amount of water -- some of them have a greater flow rate than the Amazon River. When they cross over land, sometimes they dissipate, raining out over a wide geographical area. But the West Coast's odd geography -- two mountain ranges, the Coast Range/Cascades and the Sierra Nevada Mountains, running parallel to each other with a broad valley in between -- meant that as those plumes of moisture moved inland, they were forced upward in altitude (twice). The drop in pressure and temperature as the air rose caused the water to condense, triggering a month-and-a-half-long rain event that drowned nearly the entire middle of the state.
The reason I bring this up is because the geological record indicates the Great Flood of 1861-62 was not a one-off. These kinds of floods hit the region on the order of once every century or so.
Only now, the Central Valley is home to 6.5 million people. And one of the predictions of our best models of climate change is that the warm-up will make atmospheric river events more common.
When people think of deadly disasters, they usually come up with obvious and violent ones like earthquakes and volcanoes. Certainly, those can be horrific; the 1976 earthquake in Tangshan, China killed an estimated three hundred thousand people. But the two most dangerous kinds of natural disasters, both in terms of human lives lost and property damage, are flooding and droughts -- two opposite sides of the climatic coin, and both of which are predicted to get dramatically worse if we don't somehow get a handle on the scale of fossil fuel burning.
I saw a quip making its way around social media a while back, that every disaster movie and horror flick starts with someone in charge ignoring a scientist. There's some truth to that. Unfortunately, we've not been very good at taking that message to heart. We need to start listening -- and fast -- and learning from the lessons of the past. Disasters like the Great California Flood will happen again, and now that we've stomped on the climatic accelerator, it will likely be sooner rather than later.
Let's hope we don't close our eyes to the potential for a catastrophe that will dwarf the one of 170 years ago by several orders of magnitude.
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