Mirror Lake

Every spring, the tenaya creek washes tons of silt and mud down the canyon and into Mirror Lake, and in summer the lake dries up. Until the early 1970s, the National Park Service dredged the lake periodically to preserve it as a popular tourist attraction, but since then the lake has been rapidly filling up with silt; it's destined to become what it once was—a small forest with a creek running through it.

Every American should visit Yosemite National Park at least once. But some summer weekends, it seems they've all decided to come at the same time. The park is only 31/2 hours from San Francisco and 6 from Los Angeles, and that accessibility, coupled with the park's justly famous natural beauty, spells tourist crowds. There's no two ways about it.

Air pollution is the gravest environmental threat Yosemite faces (auto emissions on the crowded park roads are only one cause). Ozone damages the foliage of certain sensitive pine trees, and acid deposits in the snow or rain periodically overwhelm the lakes and rivers. Still, you can't blame pollution for what's happening to the postcard view at Mirror Lake.

The Yosemite Valley loop road visits several iconic vistas—Bridalveil Falls, the great Glacier Point overlook, the rounded summit of Half Dome, Yosemite Falls, and the awesome 7,549-foot-high (2,300m) sheer rock face called El Capitan, the world's tallest granite monolith. As snows melt from the surrounding peaks, the crystal-clear waters spill over the canyon walls in some of the most spectacular waterfalls on earth. (Upper Yosemite Falls is the tallest waterfall in North America; Ribbon Fall and Sentinel Falls join it on the list of the world's 10 tallest falls.) Among these, Mirror Lake is a particularly beloved sight. Like the name says, it offers up a near-perfect reflection of the surrounding mountain scenery, your classic Yosemite photo op. The trouble is, these days it's rapidly filling up with silt, and shrinking every year. Soon it may be more appropriate to call it Mirror Meadow.

To understand what's happening, remember that glaciers carved this valley out of the Sierra Nevada range, shearing vertical faces in the granite and scooping out lake beds. Formations like El Capitan and Half Dome are made of granite, a particular durable rock—that's why they thrust out so dramatically from the landscape. But as glacial ice moved through this valley eons ago, it also carried along rubble and debris, some of which still lies loosely scattered around the park. Carried off the peaks in snowmelt, this rubble and debris eventually ends up in the lakes. Ages ago, this already happened in many spots; Yosemite Valley itself used to be a lake. And in our lifetimes, Mirror Lake is doomed to follow this same fate.

You can't park a car at the trail head to Mirror Lake, so take the shuttle bus (Valley Stables stop) and follow the trail for 1 mile (1.6km) to the lake. From the shore you can still get a breathtaking view of Half Dome when spring snowmelt raises the water high enough. The sooner you get here, the better.