Traveling north on Montana Highway 200 -- 15,000 years ago.
Glacial Lake Missoula was dammed by ice 15,000 years ago (MT Dept. Transportion).
How big was the lake?
JT Pardee of the US Geological Survey was mapping the geology of northwest Montana when he came across old lake shorelines high on valley slopes (Pardee 1910). He concluded there had been an immense lake during the last glacial advance, and called it Glacial Lake Missoula. The highest of the shorelines stands 4250 feet above sea level, 900 feet above Missoula and 2000 feet above Sand Point. Based on the high-water mark, the lake covered some 2900 square miles and contained 530 cubic miles of water.
| Relief map hand-painted along high-water contour line to recreate Glacial Lake Missoula.|
Click on photo to view detail. From Montana Natural History Center in Missoula.
How big was the dam?
The ice that impounded Lake Missoula was a lobe of the Cordilleran ice sheet that extended south via the Purcell Trench and blocked the Clark Fork River near Sand Point. Estimates put the size of the dam at 2000 to 2500 feet in height and more than 30 miles across.
|Mouth of Clark Fork River at Lake Pend Oreille; arrow marks approximate location of ice dam.|
How big were the floods?
It’s not easy to measure floods that took place 15,000 years ago, but we do know a few things. They reached the Pacific Ocean after traveling on the order of 500 miles from Sand Point. They didn’t take a direct route, but rather spread out in huge braided networks. Sometimes the floods encountered tight spots, temporarily impounding large lakes. Allen et al. estimate that more than 16,000 square miles were flooded and scoured out.
Scablands of eastern Washington. Tan wheat fields and grassland; green irrigated fields (lower left); brown areas of basalt bedrock exposed by floods. From Google Earth; click on photo to view.
Flood depth varied with topography, but was on the order of hundreds of feet. For example, it’s thought that 700 feet of water flowed over today’s Dry Falls at peak flood, with the “falls” visible only as a bump in the raging torrents.
Dry Falls back in the day, from the Montana Natural History Center. At peak flow, the surface of the water is thought to have been even higher -- well above the tops of the cliffs.
Dry Falls in the early 21st century. On this hot September day it was very difficult to imagine what it was like during Pleistocene times!
How fast were the floods?
Glacial Lake Missoula probably “emptied” in two or three days, not counting lakes and ponds left behind after the level dropped below drainage divides. It took only a few days for these huge masses of water to reach the Pacific Ocean. Speed estimates range from 30 to 80 miles per hour, depending in part on width of the flood path. Considering the volume of water and debris, this is scary! It’s thought that water, ice, dirt and rock “surged” out of Lake Missoula at around 9.5 cubic miles per hour (Allen et al.).
One way to calculate speed is to look at the size and form of features created by the floodwaters, like the giant current ripples of Camas Prairie. These are up to 45 feet in height and several hundred feet apart, and many are antidunes -- evidence of very rapid flow. The raging waters scraped off 150-200 feet of soil and dirt, and plucked boulders weighing tons out of the underlying bedrock. Allen et al. estimate that almost 50 cubic miles of soil, sediment and rock were removed.
|Giant current ripples form alternating dark-light pattern in mid-ground.|
How many floods were there?
J Harlan Bretz, who argued that the bizarre landscapes of eastern Washington were created by catastrophic flooding long before anyone else accepted the idea, proposed a single mega-flood which he called the Spokane Flood (he didn’t know of Glacial Lake Missoula at the time). Current thinking is that there were multiple floods, perhaps as many as a hundred, over a period of about 3000 years. Not all were mega-floods, and there may have been other sources. But the flood of 15,000 years ago probably was the largest, and the main water source was Glacial Lake Missoula.
What do all these numbers really mean?
While it’s easy to write that 530 cubic miles of water and debris flowed from Glacial Lake Missoula to the Pacific Ocean at speeds sometimes approaching 80 mph, it’s pretty hard to wrap my brain around these dimensions, though they certainly sound catastrophic. Perhaps we can better comprehend the enormity of the mega-floods by comparing them with things we know.
Glacial Lake Missoula often is compared to existing lakes. At its largest, it contained half the volume of Lake Michigan, and as much as Lakes Erie and Ontario combined. The ice dam across the Clark Fork River was so tall that “with room to spare, two Empire State Buildings could have stood one atop the other against Glacial Lake Missoula's ice dam” (PBS NOVA). The biggest of the floods contained ten times the flow of all rivers of the world and 60 times that of the Amazon -- and it reached the coast in just a few days!
Allen et al. (Appendix C) compare Lake Missoula floods with 22 examples of more familiar catastrophes in terms of energy expended. The largest Missoula flood probably was similar in magnitude to the meteorite impact credited with the Great Extinction 66 million years ago. It is estimated to have had almost 400 times the energy of the 1906 San Francisco earthquake, 75 times that of the largest fusion (nuclear) bomb, more than ten times the energy expended during the first eight hours of the 1980 Mt. St. Helens eruption, and 26% of all energy produced in the USA in 1970.
What would it have been like to experience a mega-flood?
By Stev Ominski, from the Ice Age Floods Institute.
There’s another question that comes up repeatedly -- did humans witness the catastrophic floods? Whether people lived in the Pacific Northwest at that time is still debated. If there were witnesses, they left no record. Fortunately, being human, we can imagine what it might have been like to hear a distant escalating roar, and then to see an immense mass of swirling water, ice, rocks and dirt approaching at frightening speeds.
“... depending on how far away you were, there would have been a lot of stress and a lot of noise from boulders banging together. You might also have seen rather bizarre things—huge waves, for instance, perhaps even particles flying out of the flow. That would have been rather disconcerting.” Vic Baker interviewed on PBS NOVA
“Imagine the kind of velocity the water had when the flood came through, the tremendous air blast caused by the wave front. Its roar would have built for a half-hour at least ...” Leonard Palmer, Northwest Magazine, June 26, 1983 (in Allen et al.)
“In a scene belonging more to the realm of science fiction than to reality, this towering mass of water and ice ... literally shook the ground as it thundered toward the ocean” From poster at information center in northwest Montana
“how can we imagine a torrent of air exploding into existence, driven by a wall of water hundreds of feet high and moving at 60 miles per hour ... [and] this wall may well have come at night.” Marjorie Burns (in Allen et al.)
“The lake, he realized, was moving, and currents of muddy water were starting to tear at the banks. He watched, stunned, as the lake level started to drop. From his vantage point on the high ridge, he could see water surging through the gap in the hills, tearing away at the soil and rock. He could hear giant boulders bouncing in the depths.” Geotripper; for more, read In the Land of the Great Draining.
This post is part of a series about the Ice Age Mega-floods of the Pacific Northwest.
Allen, JE, Burns, M, and Burns, S. 2009. Cataclysms on the Columbia. Portland State University, Ooligan Press. NOTE: This is recommended reading: information-packed, interesting, enjoyable.
Pardee, JT. 1910. The Glacial Lake Missoula, Montana. J. Geol. 18:376-386.