The Great Missoula Floods
Exhibit by: Jessica Sterling
Ice Dammed Lake Missoula
The Portland region owes its rich agriculture and beautiful geography to a series of massive ice-age floods that burst from an ice dam about 15,000 years ago, stripped soil from eastern Oregon and Washington, ripped cliff walls off the Columbia gorge, and floated house-sized boulders downstream.
During the last great stage of the Ice Ages, the Cordilleran ice sheet extended south along the Canadian Rockies and the Coast Ranges, and just reached into the United States. Ice reached thicknesses of over 3,000m (over 9,842 ft, or 5 ½ World Trade Center towers). One of the greater ice streams, the Purcell ice stream developed in the fault-bound trench (of the same name), and gathered ice and snowfall. The lower reaching lobe completely blocked the Clark Fork, a major tributary of the Columbia River, in what is now northern Idaho.
Fed by snowfall and glacier meltwaters from the Rockies in Montana, the Clark Fork became ponded behind the
ice barrier, and formed Glacial Lake Missoula. The huge lake reached over 250km (155mi) to the southeast, and backed up along many tributaries. The volume in Lake Missoula reached about 2,170km3 more than Lakes Erie and Ontario of today.
Ice dams are not permanent, so the Purcell ice dam failed, probably due to rising pressure in Lake Missoula and thinning of the ice as the climate changed. Another possible explanation to the dam failure is the jökulhlaup floods draining down from sub-glacial basins further north beneath the ice sheet.
jökulhlaup: (“yer-kul-hloyp” an Icelandic term that has been adopted by the English language) Is a glacial outburst flood.Triggered by geothermal heating and occasionally by a volcanic subglacial eruption, it is now used to describe any large and abrupt release of water from a subglacial or proglacial lake/reservoir
The Great Flood
When the ice dam failed, it created an enormous flood pulse as Lake Missoula completely drained out. The volume and velocity is estimated by the size of the transported boulders, and the amount of soil stripped from the valley sides. It is estimated to be 38 cubic kilometers (over 9 cubic miles) of water per hour, which is about 600 times the mean flow of the Amazon River today. It reached speeds of about 80km/h (almost 50m/h). This catastrophic outpouring had some spectacular effects, which carved the landscape and deposited debris far downstream. Giant waves were created downstream within the massive flow. They shaped huge ripples in the bed of this giant flood, 30m high and 100m apart (98ft high and over 300ft apart), similar to the way the beach sand forms waves from the moving tide. They can still be seen today in Camas Prairie.
The floodwaters poured away to the south, dumping gravel and debris in a basin before overtopping a col and cascading into Glacial Lake Columbia, another very large ice-dammed lake.
col: A pass between two mountain peaks or a gap in a ridge.
A massive plateau of Columbia River Flood Basalts covered by Palouse loess, created in the Tertiary times, stands south of Glacial Lake Columbia. During the great Missoula flood the water flows instantly stripped the channels down to the basalt rock, but loess remained in higher ridges. Farmers settled in the 1800s took advantage of the remaining loess in the hills, recognizing the strips of scoured basalt as almost useless, thus calling them the scablands.
The Scablands and Coulees
The Cheney-Palouse scablands (south of I-90) appear to have been formed in the first great flood. The northwestern scablands were not formed in this event, probably because the Columbia ice lobe extended further south, almost cutting Lake Columbia in two, reducing the impact of the flood. Some channels were cut deeper into the basalt forming coulees.
coulee: a dry stream valley, especially a long steep-sided gorge or ravine that once carried melt water from a glacier.
The Floods Repeated
With Lake Missoula gone, the Clark Fork reduced to a modest river, the Purcell ice lobe advanced again, recreating the ice dam, and a new Lake Missoula started to fill up, until the dam failed again. This whole cycle of filling and emptying was repeated over and over. Evidence of this comes from sediments of Lake Columbia exposed along Latah Creek, south of Spokane.
Perhaps due to the slow thinning of the Cordilleran ice sheet throughout the years, each filling of Lake Missoula only reached a level a little below that of its predecessor. Counting the annual varves across disjointed exposures, there are more than 2000 recognized in the sediments left in the one northern arm of Lake Columbia. Within this there are more than 80 gravels, each of which may represent a flood pulse from each failed Lake Missoula.
varve: a typically thin band of sediment deposited annually in glacial lakes, consisting of a light layer and a dark layer deposited at different seasons
Some volcanic ash and shell sand exists within a few dated layers. This gives an indication of how old the floods were. Dating the floods, the first was about 15,300 years before present, while the last one was about 12,700 years before present. Sediments indicate that Lake Missoula existed for a total of about half of this period, at about 50 year intervals in the earlier events, and only about 10 years in the later events when the ice was not so large.
Formation of Portland and Willamette Valley
Downstream of the gorge, the main flood pulses poured through the Portland Basin with depths of over 100m (over 300 feet), and water flowed into the Willamette Valley, depositing sediment which now makes up the nutrient rich agricultural valley floor. During the rushing floods, Portland would have been under about 400 ft of water. These phenomenal flows of water also led to the deposit of large boulders being swept downstream of the flood, as well as gravel deposits, which formed the landscape of the Portland region today. Alameda Ridge is one example of sedimentary land formation in the city.
Allen, John Elliot, Burns, Scott, Burns, Marjorie. Cataclysms on the Columbia: The Great Missoula Floods. Ooligan Press. 2009.
Cataclysm on the Columbia – Marjorie and Scott Burns Lecture. Portland State University Library. Youtube.com. 2011
Channeled Scabland Eastern Washington Ice Age Floods Lake Missoula. http://hugefloods.com/Scablands.html
J. Harlan Bretz. The Evidence for a Catastrophic Flood http://www.detectingdesign.org/?page_id=703#The-Evidence-for-a-Catastrophic-Flood
Oregon Field Guide – Missoula Floods. Oregon Public Broadcasting. 1999. http://www.opb.org/programs/ofg/segments/view/1660
Waltham, Tony. Classic localities explained 5: Lake Missoula and the Scablands, Washington, USA.Blackwell Publishing Ltd, The Geologists’ Association & The Geological Society of London, Geology Today, Vol. 26, No. 4, July–August 2010. http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2451.2010.00763.x/abstract