Correction: I could not imagine her perspective-- I sure didn't anticipate that question, anyway. But after she utters it, it strikes me as a perfectly logical inquiry for someone who hasn't conceptualized that uphill from this liquid tower, beyond sight, sprawls a high country of forest and meadows. Up there, an exceptionally deep snowpack from this winter and spring continues melting well into summer, feeding Yosemite Creek and this waterfall. To Alex, the water appears to materialize inexplicably from the top of this cliff.

We visited Yosemite Valley recently to hike to some of the most spectacular waterfalls on the continent in early summer, when mountain snowmelt fattens them up so much that they create something like a very localized rainstorm, even on a sunny day. Besides Alex and me, our three-generation party consisted of my nine-year-old son, Nate, my 12-year-old nephew, Marco, my wife, Penny, and my 73-year-old mom, Joanne, who hiked to these same waterfalls with me 15 years ago.

We scrambled to the banks of the Merced River, a thunderclap of foaming whitewater coursing around boulders the size of SUVs. We walked to Vista Point below Bridalveil Falls, getting showered by its mist. We dayhiked to the brink of Upper Yosemite Falls, 2,700 feet above the valley floor, to peer over its dizzying edge. And on our last day in the park, we hiked what may be the most popular 2.6 miles of footpath in North America, the John Muir Trail and Mist Trail to Vernal Fall and Nevada Fall—both of them exploding with runoff from a Merced River that was under a flood watch shortly before our arrival.

Watch a video of our hikes here.

Seeing Yosemite's famous waterfalls was the second of 11 adventures I'll take with my kids, over the course of 12 months, to national parks that are likely to be very different places due to climate change by the time my kids are my age. Yosemite's world-famous waterfalls naturally swell in spring, as mountain snows melt, and dwindle by autumn, after a dry, hot summer. But they will suffer pronounced shrinkage under even the best-case scenario envisioned by researchers: a 30 percent decline in Sierra Nevada snowpack in this century if we take aggressive measures to reduce our carbon emissions. If we don’t, projections call for snowpack to diminish by up to 90 percent, which would really change the view from Yosemite Valley.

Fed by a seasonal creek, Yosemite Falls often dries up by late summer, a visual absence as conspicuous as seeing the New York City skyline before and after 9/11. The Mist Trail may one day be misty in name only. Alex's question about how the water gets up the mountain might have real-world relevance by the time she has a daughter or son.

I'll chronicle our year of adventures in a book, Before They're Gone, to be published by Beacon Press. Read more about my book project here.

Next up for my family: five days of sea kayaking in Glacier Bay, Alaska.