Given time, most rivers slice ineluctably deeper into the land. But in rivers birthed from moving ice, riverbeds aggrade, or rise, with all the debris filling them. Occasionally, aggradation occurs suddenly in the form of a lahar, also known as a debris flow, a deadly tsunami of mud, boulders, and uprooted trees that is as much solid as liquid and can charge down valleys at up to sixty miles per hour.
In recent years, scientists have discovered that Rainier’s riverbeds are aggrading six to twelve inches per decade. On The Mountain’s south side, the Nisqually River’s bed has risen thirty-eight feet since 1910. Longmire, home to park offices and visitor services, now sits twenty-nine feet lower than the Nisqually River bottom, protected from obliteration by a manmade levee. There’s evidence that the lower Nisqually Glacier is stagnating, moving only a few inches a year rather than a normal several inches per day. Park officials fear a lake may be growing inside the ice, setting the stage for a catastrophic glacial-outburst flood, or jökulhlaup—the only type of flood that has ever overtopped the Longmire levee, in the 1950s.
There has been a growing number of debris flows off Rainier. One tributary of the Nisqually, Van Trump Creek, didn’t see a debris flow in the park’s first 101 years; from 2001 through 2006, four careened down it. In the park’s northeast corner, the White River’s bed now sits sixteen feet higher than White River Road (WA 410).
The era of climate change has made the term “100-year flood” a dark joke among managers at Mount Rainier National Park and on other Pacific Northwest public lands. Many experts, including the Union of Concerned Scientists, expect that higher average temperatures will crank up the hydrological cycle, because air holds more moisture as it warms. Park managers fear the unknown: how severe the biggest storms will get in a warmer climate.
The tempest that began on Nov. 5, 2006, turned into the most destructive storm in Mount Rainier National Park’s 109-year history. Over 36 hours, nearly 18 inches of rain fell—the equivalent of fifteen feet of snow—causing flood devastation that stunned even people who had worked in the park for decades. Major sections of road were gone. Six debris flows scored Rainier’s slopes. The Nisqually River almost topped Longmire’s levee and wiped five acres of Sunshine Point Campground off the map.