The Lake Missoula megafloods

[a post by Anne Jefferson]
If I had a time machine and could go back to any point in geologic history, as supposed in this month’s Accretionary Wedge call, the event that I’d most like to see is the repeated flooding of the Pacific Northwest at the end of the last Ice Age. These “Missoula floods” are among the largest floods in Earth history and they irrevocably changed the topography of Washington and Oregon. My time machine would be an aircraft capable of flying with the floodwaters as they raced from Montana to the Pacific Ocean, and my time machine would also have LIDAR capabilities for collecting pre-, syn-, and post-flood digital measurements of topography and water surfaces and allowing an unparalleled determination of flood magnitudes and erosive volumes.
Our story begins 19,000 years ago, shortly after the Last Glacial Maximum, deep in the heart of the Bitterroot Mountains in western Montana (Figure 1), as melt water from the waning glaciers began to pour into Clark Fork River valley and its tributaries. Water in the Clark Fork ponded up behind an enormous ice dam from a lobe of the Cordilleran Ice Sheet, and reached a maximum depth of 600 meters as Glacial Lake Missoula. The lake contained more than 2000 cubic kilometers of water, more than the modern volume of Lake Erie and Lake Ontario combined. Pedestrians in the modern day town of Missoula might notice a strange horizontal striping to the hillsides surrounding town (Figure 2)…these are the traces of the shorelines of the ancient lake.

Figure 1. USGS map of the Pacific Northwest between ~12 and 19 thousand years ago.

Figure 2. Traces of Glacial Lake Missoula in Missoula, Montana, February 2009. Photo by the author.

A little after 19,000 years ago, water in Glacial Lake Missoula ruptured the ice dam, and the collected water went rushing downstream at speeds reaching 100 km/hr. Peak discharge in the Spokane Valley has been estimated at 17 +/- 3 million cubic meters per second, and drainage of the lake took several days.
Today, the Clark Fork River drains into the upper Columbia River and then into the Pacific Ocean. At the time of the Missoula Floods, the upper Columbia was buried under glaciers and that route was blocked to the floodwaters from Glacial Lake Missoula, so instead they were forced to take an overland route, carving new channels into the fertile Palouse loess deposits and the underlying 17 million year-old Columbia River basalts of eastern Washington. These channels are spectacular (Figures 3 and 4): up to 182 m deep and 32 km wide, with dry waterfalls, scoured potholes, and streamlined islands. Granite boulders the size of small cars were carried by the flood from the Idaho batholith and deposited in central Washington. Even for geologists, it can be hard to appreciate the full extent of these floods from the ground, but satellite photos (Figure 5) show the huge aerial extent of the erosion, covering a region we now call the Channeled Scablands.

Figure 3. Dry Falls and Grand Coulee near Coulee City, Washington, June 2004. The falls are 122 m high and 5.6 km wide. Photo by the author.

Figure 4. Grand Coulee downstream of Dry Falls as viewed from Lenore Caves, June 2004. Photo by the author.

Figure 5. Landsat view of the Channeled Scablands region. This image was taken on August 31, 1972 and shows about 34,250 square kilometers of eastern Washington. The dark green features are the channels, the light green is the wheat-farmed Palouse region and the modern Columbia River is at the top right. Image retrieved from a National Park Service website.

Several times along their way across Washington, the floodwaters found basins in which to spread out and slow down. The fertile, wine-growing soils of the Yakima and Walla Walla valleys are slackwater flood deposits of loess from the Palouse valley. In a few special places, we can see the thick layers of silt, separated by thin horizons of soil (Figure 6). The number of layers preserved in these places help constrain the number of floods, and ash layers in the soils help constrain their timing. The best estimate is that there were more than 40 late Pleistocene megafloods that crossed eastern Washington, though not all of them may have come from from Glacial Lake Missoula itself. The floods began after 19 thousand years ago, many occur after 15 thousand years ago, and some post-date a Mount Saint Helens eruption 13 thousand years ago. More than 25 floods had peak discharges exceeding 1 million cubic meters per second.

Figure 6. Missoula flood deposits in the Walla Walley valley, Washington, June 2004. Photo by the author.

Where the Columbia River turns east and runs along the modern-day border of Oregon and Washington, the Missoula Floods were once again confined to a single channel, at a place called Wallula Gap. As immense as the Columbia River Gorge is (Figure 7), this channel was a major constriction on the flood, only able to transmit 20% of the peak discharge. The waters therefore backed up behind the gap, flooding the Pasco Basin and creating a temporary lake – Lake Lewis – around the tri-cities region of Washington. Floodwaters in the gorge were more than 215 m deep, as evidenced by the flood deposits found on surrounding ridgelines. The town of Lyle, Washington, in the Columbia River is built on a giant eddy bar left by the floods (Figure 7).

Figure 6. Columbia River Gorge, looking upstream from Rowena Crest, Oregon, June 2004. Photo by the author.

Figure 7. Lyle, Washington, as viewed from Rowena Crest, Orego. Photo by the author in June 2004.

When the floods reached Portland, Oregon, the waters filled the Willamette Valley, producing a lake 100 m deep, approximately 50 km wide, and 175 km long. Today the lake bottom is seen in the flat topography and fertile soils of the valley. Icebergs, probably remnants of the glacier that dammed Lake Missoula, were carried this far, because the valley is pocked by glacial erratics (Figure 8), rocks that had fallen onto or into the ice, were rafted downstream, and left behind when the icebergs melted. These rocks are not rounded, so it is clear that are not material moved by the flood itself.

Figure 8. Glacial erratic on the edge of the Willamette Valley in Erratic Rock State Park, Oregon. Photo taken May 2008 by aboutmovies and used under Creative Commons license.

Slowly the water would drain from the Willamette Valley and upstream lakes and empty completely into the Pacific Ocean through the Columbia River. Vegetation would encroach on the new sediments, and soil would begin to develop. In the meantime, another Glacial Lake Missoula would be forming behind a new ice dam on the Clark Fork River, getting ready to repeat the process again. Presumably floods of varying magnitudes continued until the glaciers had retreated north of the river. The last floods probably occurred less than 13 thousand years ago.
Floods over, measurements completed, my time machine and I would return to the present, ready to fill in the details of the floods that shaped the topography, soils, and agriculture of the Pacific Northwest. Even without a time machine, there’s so much more I could tell you about the Missoula Floods, but I’ve already gone on long enough. Instead I’ll refer you to the numerous scientific papers on the subject beginning with those of J. Harlan Bretz in the 1920s. When Bretz proposed a megaflood as the source of the Channeled Scablands, it flew in the face of the ruling uniformitarian paradigm and his work was not accepted for several decades. The casual audience may want to check out the Ice Age Floods Institute website, an on-line USGS publication on the The Channeled Scablands of Eastern Washington, the book Cataclysms on the Columbia, and, if you can find it airing sometime, the excellent Oregon Public Broadcasting documentary Ice Age Floods. If you live in or are traveling to the Pacific Northwest, also keep an eye out for sites along the newly-approved Ice Age Floods National Geologic Trail.
Categories: by Anne, geology, outcrops, past worlds, Pleistocene

Comments (14)

  1. BrianR says:

    Very nice post … I still need to go visit these landforms for myself someday. I gotta make that happen sooner rather than later.
    One aspect I’d like to add here is that the power of the floodwaters didn’t simply stop when they reached the ocean (at least not always). While this part of the story is more difficult to investigate than the onshore part (the evidence is on the deep sea floor) it is significant. A paper by Normark & Reid (2003) show late Pleistocene flood-derived turbidity current deposits 1000 km (!!) from the mouth of the Columbia way out on the Pacific plate (see their Fig. 21).
    For anyone interested, here’s the full citation:
    Normark and Reid, 2003, Extensive Deposits on the Pacific Plate from Late Pleistocene North American Glacial Lake Outbursts: The Journal of Geology, v. 111, p. 617-637

  2. I visited the “Scab Lands” on a GEO field trip in 2006, and they were fascinating. I have some nice shots of the dry falls myself. (You are welcome to them if you ever need them) Great post.
    I myself would want to back to the point where the Atlantic was just opening up. The weather patterns would be fascinating. It would be neat to look across the Atlantic, and see Europe on the horizon!

  3. Diggitt says:

    I am not a geologist, but I blogged about the Spokane Floods last month The Accretionary Wedge prompted a different reflection though but it was a tough choice.

  4. doug l says:

    Fascinating story. I never tire of hearing it well told, as was this post. Great pics.
    These kinds of geologic events, particularly when the are contrary to the conventionally held ideas are the best.
    I’m currently fascinated by the 12.9KYA impact event as I’m living in the midwest. Talk about taking wishing for a time machine.

  5. Lockwood says:

    Oops… missed this. I’ll slip it in real quick, and no one will notice. The wedge is up, and I hope I can accrete this before you get there.
    As noted above, great description and pictures. My favorite is taking people up to Crown Point, letting them take in the view, then explaining that the flood topped that pinnacle, 800 feet high.

  6. coconino says:

    Great minds think alike! And, good pictures and graphics!
    I spent a good portion of my childhood seeing all the evidence of the Missoula (Spokane, Bretz) floods, from the Touchet beds, to the scablands, to the shorelines near Missoula. I always thought it would be pretty phenomenal to have actually seen them. I still have a hard time imagining all that water backing up against Wallula Gap (which helped the deposition of the Touchet beds) and rushing through, down the Columbia River valley, etching their way into the landscape.

  7. llewelly says:

    Thank you (Anne Jefferson) for writing this article – Lake Missoula is one of my favorite lakes of all time.

  8. Gary Kleinknecht says:

    Excellent post! To gaze upon the Channeled Scablands, dry cataracts, giant current ripples and other features left by these catastrophic events is an awesome experience. I encourage all to visit the floods region.
    If you ever get that time machine working, I want to reserve a seat.
    Gary K

  9. CMB says:

    Glacial Lake Missoula was my favorite topic in college, especially since I got my geology degree from the University of Montana in Missoula. For more information, check out David Alt’s book “Glacial Lake Missoula and Its Humongous Floods”. Alt is great teacher and one of his favorite subjects is Glacial Lake Missoula.

  10. Excellent post. I have been to some wildland fires in the area and had wondered about the geologic history. Thanks.
    On a related topic, had the Grand Canyon in Arizona been formed by a massive flood (Noatic, of course), I guess it would show the same erosion: lots of channels accross the Kaibab and Coconino Plateau rather than one really deep canyon. I’m bookmarking this one for the next time someone claims the Grand Canyon was formed by Noah’s Flood.

  11. zayıflama says:

    I always thought it would be pretty phenomenal to have actually seen them.

  12. aldo says:

    It’s stunning…I have written just today a post about the end of Agassiz Lake.
    I ask you if these floods could have rearranged the net of oceanic currents, as the Agassiz flood did

  13. Amy Smith says:

    I really enjoyed your article on the Lake Missoula megafloods. The Nova special on the subject was great also. I too have run into “the ruling uniformitarian paradigm” in my attempt to describe a Caribbean Megaflood.
    Still, it was a great article and a nice style of telling it.

  14. warren schmidt says:

    My wife and I are scheduled to visit June 2010 – Any suggestions for reading resources – background from little geology to major in college