When we were at Yosemite National Park a few weeks ago, we happened to be there during peak Horsetail Fall season. We didn’t plan this. It was just a coincidence. Nevertheless, there we were, and so we heard the story of Horsetail Fall two or three times. It goes something like this:

Horsetail Fall was discovered by a hiker/rock climber named Galen Rowell in 1973. He noticed that at just the right time, in just the right month, it would light up so brilliantly that it looked like a firefall. He and his buddies were on the Manure Trail or somesuch when they discovered it, so they jokingly named it Horsetail Fall. They didn’t tell anyone, though, and it remained unknown. Eventually Rowell shared a photo, and it made the rounds of Yosemite insiders. Then social media exploded, and now everyone knows about Horsetail Fall.

This story seems pretty unlikely. Nobody ever noticed it before? You can see Horsetail Fall from the main road exiting the park. Thousands of people have seen it every year for the past century, including a famous photographer or two:

The Ansel Adams picture is from 1959, taken from the same spot as the Kevin Drum picture from 2018. The Adams print is called “El Capitan Fall.”

Obviously this waterfall was no secret. So how is it that nobody before 1973 noticed the brilliant firefall effect during late February sunsets? This is something that everyone agrees about: there’s no record from either native American tribes or the white settlers who came later that they noticed the firefall effect. This is truly strange. Did something happen in the 1970s that changed the color of the sunset? Did the path of the waterfall shift slightly?

Or is climate change to blame? Horsetail is an ephemeral fall, and it appears only in winter and spring, beginning when the snowpack starts to melts. Maybe in the past that didn’t happen until March 1—too late for the firefall effect—but now it typically happens in mid-February thanks to rising temperatures?

This weather station series is for the whole park so it doesn’t tell us specifically about temperatures in February or temperatures in the vicinity of the melting icepack. And there are certainly individual warm years before 1973. Still, the evidence is consistent with the idea that the firefall effect might not have become a regular occurrence until the 1990s or so, and was missed before then because it was so sporadic.

It’s also the case that although Horsetail Fall is easily visible, it’s a fairly minor waterfall in a park full of iconic ones. Most ephemeral falls aren’t named, and they don’t show up on maps either. I can’t find a record of the fall—or even the creek that feeds it—on any map prior to about 2008:

Despite a 1,600-foot drop, Horsetail is apparently too insignificant to show up even on a detailed modern topographic map:

It’s a mystery. Climate change is my best guess for the weird Horsetail Fall tale, but feel free to chime in with your own theories.