There’s a rock on a mountainside. Maybe it’s smaller than a golf ball, maybe it weighs 20 tons. To gravity, size doesn’t matter. Whether the rock stays put or tumbles downhill will be determined by an eons-old calculus of shape, location, and slope angle. Soil stability makes a difference, of course, as does the amount of vegetation stitching the earth in place. Rainfall can grease the skids, snowmelt too. And retreating glaciers alter the entire equation, introducing multipliers with enormous erosive force. The rocks on Mt. Rainier have behaved fairly consistently since humans first set foot on its sprawling slopes 7,000 years ago. But something has changed in recent decades, scientists say, something that could profoundly alter the experience–and even the safety–of the millions who live and recreate on the Northwest’s most beloved peak. This is the story of The Mountain, as locals affectionately call 14,410-foot Rainier, and the storms that have begun to tear it down.
All mountains crumble to the sea, but glaciated stratovolcanoes like the dozen or so in the Cascade Range have been in the accelerated program since their giant ice fields started receding 150 years ago. From Washington’s Mt. Baker to California’s Mt. Shasta, the clatter of rockfall loosed by melting snow is an ever-increasing soundtrack.
Rainier is no exception. It wears the most massive cape of ice in the contiguous United States–26 glaciers spanning 35 square miles. However, like most of the planet’s glaciers, virtually all of Rainier’s are shrinking, a direct and escalating consequence of a warming climate. Seen from Seattle, 40 miles away, the fifth-highest summit in the Lower 48 looks immutable and eternal. In reality, it’s as transitory–in epochal terms–as a sand dune.
Across the Cascades, receding glaciers leave behind steep, barren moraines of loose dirt and friable igneous rock that slide at the slightest disturbance. But Rainier’s massive size and height spawn weather that intensifies the erosion. Storms that brew deep in the Pacific pound the mountain with up to 90 inches of rain each year, and the popular trailhead at Paradise, located at 5,400 feet on the south side of the peak, averages 58 feet of snow annually. All that precipitation transforms Rainier into a 100-square-mile fire hydrant come spring. Seven rivers and hundreds of streams burst from the melt–470 miles of waterways within park boundaries alone.
The rivers carve sediment from the earth as they descend, and carry it downhill. But as on other glaciated volcanoes, Rainier’s water transports so much dirt and rock that its riverbeds rise over time, or aggrade, with the tons of debris that gradually get driven downstream.
This process is normal–a geological cause-and-effect that scientists say happens whenever ice melts. But what’s happened on Rainier in recent years isn’t normal. Rapidly warming temperatures have changed the seasons, creating more rain (vs. snow) and melt (vs. freeze) days at both ends of winter. The result is a higher incidence of destructive floods. It goes like this: Big rainstorms (that should have fallen as snow) abruptly liquefy so-called "wasting" glaciers–essentially perennial snowfields–creating steroidal stream levels. When enough water starts moving, it can scour the aggraded riverbeds, sweeping surplus sediment at frightening speeds. In extreme scenarios, a flood becomes a massive debris flow known as a lahar–a deadly tsunami of mud, boulders, and uprooted trees that can gouge 100-foot-deep gullies, flatten forests, and jump longstanding banks.
If that weren’t enough, lahars also raise riverbed levels farther down the mountain. That may seem innocuous, but it’s a concern in places like Rainier, where the human infrastructure–buildings, trails, and campsites–hasn’t kept pace with the changing terrain. The town of Longmire now sits 29 feet lower than the Nisqually riverbed, protected by a man-made levee, like an alpine New Orleans.
Paul Kennard, Rainier’s geomorphologist, or river specialist, speaks with a scientist’s detached calm about the guillotine suspended over Mt. Rainier National Park. He sees debris flows more frequently now–about four a year for the past five years–and says conditions are ripe for a cataclysmic flood. But his biggest worry isn’t a flood so much as what researchers don’t yet understand: namely, how bad the storms may get, and how soon they’ll arrive.
Many, many rocks sit poised to tumble off Rainier. All that’s needed to unleash them is a really big rainstorm.