Cadillac Desert_ The American West and Its Disappearing Water - Marc Reisner [100]
To geologists, the age of the Columbia River Basalts was a particularly exciting time. The vulcanism lasted ten million years or so, and covered a broad area. You can see the evidence in the cindered lava beds of Idaho and western Oregon, in the columnar basalts of Devil’s Postpile in the Sierra Nevada, in the smoking cones of the Cascades. The Cascade volcanoes, which formed recently—Mount St. Helens is probably no more than fifteen thousand years old—are the last embers of a giant bonfire which began to end, according to the available evidence, about seven million years ago. By then, the Pacific and North American plates had begun to equalize, grinding against each other like teeth and causing a chaos of earthquakes and volcanos beyond anything imaginable in our time. The Columbia River flowed during the whole period of eruption; constantly smothered by lava dams, it must have changed course hundreds of times. As the vulcanism subsided, the river began to enjoy the first quietude of its long existence, which lasted several million years—until the ice came.
The continents of snow that slid down from the North Pole during the Ice Ages stopped somewhere along a latitudinal line defined by Seattle, Spokane, and Great Falls, Montana. Where the topographic conditions were right, however, some of them went farther, huge peninsulas of ice that protruded a hundred or two hundred miles south. Near the present location of Lake Coeur d’Alene in western Idaho, an ice lobe laid itself across the path of the voluminous melt pouring from the mile-high glacial walls and blocked it, forming what may well have been history’s most prodigious dam. Confronted by a wall of ice thousands of feet high, the runoff pooled and backed into a reservoir referred to by geologists as Glacial Lake Missoula. Frigid, ephemeral, hundreds of feet deep, the lake covered an area roughly the size of Lake Michigan and contained half as much water. At some point, as the lake deepened behind the ice dam, the dam must have begun to float—ice being lighter than a corresponding volume of water. The flood probably came in a sudden instantaneous release, like the collapse of Teton Dam, and emptied Lake Missoula within a couple of weeks. The volume of the flood is anyone’s guess; Larry Meinert, a geologist at Washington State University at Pullman, says a reasonable estimate is ten times the combined flow of all the rivers in the world. The modern topography of the Northwest was pretty well formed by then; most of Lake Missoula searched out the main stream of the Columbia as its route to the sea. Inundated by a flood surge of 230 million cubic feet per second, the Columbia’s spacious canyon was a thimble holding a dinosaur egg. In the upper stages the flood was probably twenty miles wide, confined by steeper valleys, but as it poured across the old lava plains of central Washington it spread into a flowing tumult as wide as Indiana. In places, the water excavated canyons overnight, extensive channels scoured through bedrock that remain such a dominant feature of the landscape that central Washington is more often referred to by geologists as “the channeled scab-lands.” The big channels are known as coulees—Rocky Coulee, Lind Coulee, Esquazal Coulee. The biggest of all—seven hundred feet deep, five miles across, more than fifty miles long—is called the Grand Coulee.
Lake Missoula—greater and lesser incarnations of it—formed and reformed at least six times. The last time was about seventeen thousand years ago; by then there may have been humans living in the region. All of the land swept by the floods was stripped absolutely to bedrock. The glaciers, however, had left behind mountains of fine silt—the ground-up surface of Canada—and the winds distributed it around the region with a generous universality. The silt, known as loess, makes for extremely good farmland, and in some parts of Washington, such as the Palouse region below the Blue Mountains, it accumulated to