Yellowstone Books

“I like rainbows.

We came back down to the meadow near the steaming terrace and sat in the river, just where one of the bigger hot streams poured into the cold water of the Ferris Fork. It is illegal – not to say suicidal – to bathe in any of the thermal features of the park. But when those features empty into the river, at what is called a hot pot, swimming and soaking are perfectly acceptable. So we were soaking off our long walk, talking about our favorite waterfalls, and discussing rainbows when it occurred to us that the moon was full. There wasn’t a hint of foul weather. And if you had a clear sky and a waterfall facing in just the right direction…
Over the course of a couple of days we hked back down the canyon to the Boundary Creek Trail and followed it to Dunanda Falls, which is only about eight miles from the ranger station at the entrance to the park. Dunanda is a 150-foot-high plunge facing generally south, so that in the afternoons reliable rainbows dance over the rocks at its base. It is the archetype of all western waterfalls. Dunenda is an Indian name; in Shoshone it means “straight down,” which is a pretty good description of the plunge.
...
…We had to walk three miles back toward the ranger station and our assigned campsite. We planned to set up our tents, eat, hang our food, and walk back to Dunanda Falls in the dark, using headlamps. We could be there by ten or eleven. At that time the full moon would clear the east ridge of the downriver canyon and would be shining directly on the fall.

Walking at night is never a happy proposition, and this particular evening stroll involved five stream crossings, mostly on old logs, and took a lot longer than we’d anticipated. Still, we beat the moon to the fall.

Most of us took up residence in one or another of the hot pots. Presently the moon, like a floodlight, rose over the canyon rim. The falling water took on a silver tinge, and the rock wall, which had looked gold under the sun, was now a slick black so the contrast of water and rock was incomparably stark. The pools below the lip of the fall were glowing, as from within, with a pale blue light. And then it started at the base of the fall: just a diagonal line in the spray that ran from the lower east to the upper west side of the wall.
“It’s going to happen,” I told Kara, who was sitting beside me in one of the hot pots.
Where falling water hit the rock at the base of the fall and exploded upward in vapor, the light was very bright. It concentrated itself in a shining ball. The diagonal line was above and slowly began to bend until, in the fullness of time (ten minutes, maybe), it formed a perfectly symmetrical bow, shining silver blue under the moon. The color was vaguely electrical.
Kara said she could see colors in the moonbow, and when I looked very hard, I thought I could make out a faint line of reddish orange above, and some deep violet at the bottom. Both colors were very pale, flickering, like bad florescent light.
In any case, it was exhilarating, the experience of a lifetime: an entirely perfect moonbow, silver and iridescent, all shining and spectral there at the base of Dunanda Falls. The hot pot itself was a luxury, and I considered myself a pretty swell fellow, doing all this for the sanity of city dwellers, who need such things more than anyone else. I even thought of naming the moonbow: Cahill’s Luminescence. Something like that. Otherwise, someone else might take credit for it.”
― Tim Cahill, Lost in My Own Backyard: A Walk in Yellowstone National Park