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.
Nor can I figure out, at the moment, how to describe for her one certain outcome of a warming climate: less snow in Yosemite’s future. Snowfall has declined measurably for decades in virtually all parts of the world that receive it. In the Sierra, as across the Mountain West, snow melts out and streams reach peak runoff two to four weeks earlier than a half century ago. The upshot is that this waterfall and every other one in Yosemite will reach peak runoff weeks or months earlier in the year by the time Alex is grown up. The profound effects of this seasonal shift in water flows will reverberate throughout ecosystems across the western United States, the region that’s home to many of our big wilderness parks.
Seeing and hearing the upper falls, it’s hard to believe it dries up every summer. I remember a late-summer day years ago, when I looked toward Yosemite Falls and it wasn’t there. For an instant, I assumed I must have been looking in the wrong spot, but I wasn’t.
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