I've learned that hiking with kids is a bit like trying to predict the wind; things can seem to be in your favor, or not, and then abruptly reverse direction. I watch them for subtle hints of fatigue or hunger, such as a lagging pace. They lack an adult's self-awareness. They drink when I urge them, but not always enough. They'll complain one moment that hiking embodies the most inhumane form of "torture," and "this is the worst day of my life!" The next, they'll dash ahead, suddenly excited. They are imprecise barometers of their own physical condition.

So I read them like an ancient mariner searching the night sky, trying to anticipate their needs before trouble arises. Parenthood is complicated like that.

Also with us is my 12-year-old nephew, Marco, who shows no signs of tiring but is visibly nervous about the exposure and keeps his distance from the railing; and my mom, Joanne, an active seventy-three-year-old who has hiked in many places with me, among them the Tetons, the Grand Canyon, and the Presidential Range. She has returned to Yosemite Valley to repeat hikes she and I shared fifteen summers ago this month. But even my unflappable mother has already told me, "This feels a lot tougher than it did fifteen years ago." Now she rises slowly to her feet and starts upward.

The trail turns a corner, where a breeze stirs the air. Then Upper Yosemite Falls comes into view, a column of water falling through more than a quarter mile of air.

I ask Marco, Nate, and Alex, "What do you think of that?" But they only stare, saying nothing.

It's easy to be hypnotized by the complex liquid topography of Upper Yosemite Falls. At its apex, a thick, brilliantly white pillar drops perhaps a hundred feet and crashes onto a big ledge, sending a horizontal geyser ricocheting straight outward. This eruption breaks up into a broad curtain raining watery tracer bullets that arc downward. Strong gusts occasionally blow the curtain to the left, a pendulum swing in defiance of gravity. The waterfall blossoms in its earthward plunge, spreading ever wider, the curtain shredding like tissue paper.

Below the falls, the creek drops another 675 feet through the middle cascades, mostly hidden in a gorge. It then pours over 320-foot-tall Lower Yosemite Falls, which is visited by exponentially more people than the upper falls because it's just a short, flat walk from the road. Yosemite Falls drops a total of 2,425 feet, making it the sixth-highest in the world. The upper falls alone ranks among the world's twenty tallest.

While Nate and Marco scurry ahead, Alex continues gazing silently at the waterfall, her brow knitted in thought. Finally, she asks me, "How does the water go up the mountain?"

What a great question.

To the eyes of a seven-year-old, Upper Yosemite Falls appears to materialize inexplicably from the top of this 1,400-foot cliff. So I explain to Alex that up there, beyond sight, a lot of melting snow fills that creek with water. But I don't try to explain, yet, that Yosemite Creek, like many High Sierra streams, is ephemeral--this creek and waterfall dry up by July or August every summer. A rare heavy rain in autumn may temporarily resuscitate the creek. It alternately trickles and freezes through winter. But as spring liquefies the prodigious high-country snowpack, a rejuvenated Yosemite Creek bulks up again, building to a crescendo in May and June.