Washington Scablands

This image is about 200 miles from east to west. Lake Pend Oreille is at the upper right, with lake Coeur d’Alene below it. Lake Chelan, about 55 miles long, snakes out of the Cascade Mountains at the upper right. The ice age flood waters gushed out of Montana through Lake Pend Oreille and passed over Spokane. What overflowed the Spokane River valley on its way to the Columbia River (the county boundaries nicely follow the drainage’s path) spilled southwest across the Columbia Basin, scouring away the topsoil and leaving scars clearly visible from space. Before the floods, the area was covered in the same soil that still covers the area – rich Palouse Loess. Beneath the loess is a bedrock of dark basalt rock.  The table mountains of dark basalt are hallmarks of the Scablands – flat because they originated in vast lava flows. The basalt from those flows underlies most of the soil in this image.
Rock Creek Recreation Area is shown by the small blue square southwest of Spokane.

Scablands Satelite Image

Palouse – looking north from Kamiak Butte

Steptoe Butte from Kamiak Butte

In the afternoon light, you can see  the hills’ original dune shapes. In a much drier age, dust blew off of the Columbia Plateau and formed these hills. Ash from the Cascades’ frequent eruptions is responsible for the Palouse’s super-rich soils. As the climate became wetter and less windy (partly due to those rising mountains), the dunes stopped their eastward march, like ocean waves frozen in time.

The hill in the distance is Steptoe Butte.

Lake Bonneville and an Earlier Flood

Related to the floods from glacial Lake Missoula is a single flood event that took place around 14,500 years ago. Lake Bonneville covered much of the Great Basin area in present-day Utah, along with parts of Idaho and Nevada. At it’s greatest extent the lake was about the size of lake Michigan, but much deeper (1,000 feet). Although Lake Bonneville was created by the wetter climate of the ice age, it wasn’t glacier-caused like Lake Missoula. (It was what is called a pluvial lake.) There was no ice sheet in northern Utah holding the lake back, but instead a mountain pass. When the lake finally spilled over Red Rock Pass, in northern Utah, it started a sudden erosion event that rapidly cut away the soil and rock at the top of the pass. Within a few weeks, about 1,000 cubic miles of water spilled out through the Snake and Columbia River drainages. This was twice the amount of the largest floods from Lake Missoula, but the Missoula floods are still considered the larger event because of their repeated occurences.

Most of the scars from the Lake Bonneville floods in Washington were erased by the later Missoula floods, but its results can still be seen along the Snake river south of the Lewiston/Clarkston area. Like in eastern Washington, exposed basalt bedrock can be found along the river, and water erosion lines seen high up on valley and canyon walls. And just upstream from Glenns Ferry, Idaho, there is a magnificent bank of flood-rounded boulders, some a yard or more thick, deposited in a bend of the Snake river more than a hundred feet above the current water level. That’s a powerful flood.



It’s been a while since I’ve posted, but since I’ve been walking a lot lately, and writing a little, and because the following has some overlap with the Oregon Trail trip of 2009 (wow that’s a long time ago!) I thought I’d post it here. For the last few years, I’ve been in love with the scablands of eastern Washington, especially after having walked through so much similar country in Oregon and Idaho in 2009. But the scablands of eastern Washington have a unique history, and I’ve found that knowing it makes the walking even more fascinating than usual.

Lake Missoula and the Ice Age Floods

The glacial Lake Missoula covered a vast area of western Montana at the southern edge of the continental ice sheet at the end of the last ice age. The lake began to fill 16,000 or so years ago, when an arm of the ice sheet advanced far enough into northern Idaho to dam the Clark River around present-day Lake Pend Oreille. At its deepest, Lake Missoula was about 2000 feet deep, and its waterlines can still be seen on the sides of valleys in western Montana. I say waterlines because the lake flooded and refilled dozens of times.

The lake broke through the ice dam when the water level rose high enough to lift the ice (even glacial ice floats). Once that happened, the rest when pretty quickly. In a period of about 2 weeks, most of the lake’s 500 cubic miles of water drained across eastern Washington and through the Wallula gap and Columbia Gorge to the Pacific Ocean. It took soil, rocks, trees, animals, and maybe even people along with it.

In the 1920’s, a geologist named J. Harlan Bretz thought there was something very unusual about the scablands of eastern Washington. For one thing, they seemed way too fresh to have been done by any process that he knew of; 10-15,000 years of weathering is nothing compared to the millions of years geologists often deal in when talking about erosion. For another, the channels that make up the scablands were obviously water-ways of some type, but not only were they totally out-of-proportion with the rivers and streams that flow through them today (these streams are called “misfits” by  geologists), but out-of-proportion with any known river systems. He eventually concluded that a catastrophic flood must have created the scablands.

There was a lot of resistance to the idea of a flood of that scale, and it was years before Bretz was vindicated. What he was describing was unlike any recorded event, and remains the largest such flooding ever discovered. Confirmation started with the discovery of the vanished Lake Missoula. Soon it was found that the lake had refilled at least 40 times as the ice sheet advanced (and burst the ice dam for the last time about 10,00 years ago). Surveys of ancient shorelines on mountainsides in Montana, along with soil layers from the floods’ many slackwater areas in Washington, confirm the frequency of flooding.

I imagine it is every geologist’s dream to go back in time to witness events like the ice age floods. Imagine an enormous lake filling the valley’s of western Montana breaking through a tall dam of glacial ice and draining hundreds of miles to the ocean in a matter of 2 weeks!

The water rushed by at 60 miles per hour, carrying soil and ice and trees, rounding off large boulders so that they look like river rocks. In some places, bars of rocks and boulders can be seen going hundreds of feet up valley walls — an entire southern Washington town was built on one such bar. In some places the water was forced through valleys, which it widened, and at other places it spread out, or got backed up into temporary lakes. Rocks originating in the Rocky Mountains can be found as far away as western Oregon, transported there by icebergs caught up in the torrent. Those icebergs jammed up near Portland, Oregon, and flooded the northern half of the Willamette Valley up to 400 feet deep.

The floods also scoured away hundreds of feet of topsoil (mostly Palouse loess). Some of the hills in eastern Washington were tall enough to escape the water, and still stand out like islands among the scablands, their forms lengthened and their flanks steepened by the rushing water. Erosion lines can be spotted on these flanks several hundreds of feet above the rocky channels. A few tall ridges that stood perpendicular to the stream show notches in their otherwise flat profiles where the water broke through and spilled over. The Saddle Mountains north of Yakima were presumably named for the notched in the ridge just east of the Columbia River.

In places, the water left ripples exactly like those seen in sandy stream beds — but these ripples are 30 feet tall, spaced hundreds of feet apart from crest to crest, and can only be fully appreciated from a distance or from the air (or with a lot of walking and a good imagination).

The most famous remnant from those floods is the Columbia Gorge on the Washington/Oregon border between Hood River and Troutdale, Oregon. This was another bottleneck for the flood waters, where water levels rose to about a thousand feet during the flood events (for only a day or so). When Lewis and Clark came through the Gorge just over 200 years ago, they dubbed the mountain range “Cascades” for the many waterfalls that spill into the Gorge. These falls — Multnomah Falls, Bridal Veil, and many others — came about when the flood waters “amputated” the last few hundred feet of these streams’ courses.


Main source of info:

Roadside Geology of Washington, by David D. Alt and Donald W.  Hyndman. Mountain Press, Missoula, MT: 1994.

More good info: