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Mega-Floods of the Recent Past

So much of geology is detective work. In the 1920's a young geologist, J Harlen Bretz, put forth an outrageous theory. He wrote that a monster flood of biblical proportions had torn across northern Idaho and into eastern Washington, dramatically altering the landscape.

Geologist J Harlen Bretz [Credit: Geological Society of America]

He received a lot of ridicule for his views, but eventually, most geologists came to accept the evidence. Later, they accepted the notion that, along the Clark Fork River, an ice dam several thousand feet high had forced water to back up into present-day Montana.

"It would have measured thousands of feet thick and several miles long," says geologist Bill Bonnichsen. "And this caused the development of a lake in the Missoula valley of Montana. And this lake just filled up and filled up and filled up. And then for some reason — whether it caused the dam to float up, or whether it burrowed underneath the ice dam, it eventually let go with a huge deluge of water."

That deluge tore apart everything in its path. Even mountains were not spared. The top speed of the water is estimated at 70 to 80 miles an hour. For two days, it flowed at a rate of 500 million cubic feet per second, or ten times the flow of all the rivers in the world combined.

And this Missoula Flood did not happen just once. "This process happened many, many times," said Bonnichsen. "Some estimates are as high as 80 times; others are like 30 times. We don't know exactly how many, but there were many successive Missoula floods, and in the process of the ice coursing through the valley where lake Pend Oreille is, it dug deeper and deeper, leading to a lake that is on the order of 1,200 to 1,500 feet deep.

Map showing geographic reach of Lake Missoula flood(s) [Credit: Cassandra Groll]

"That's the cause of Lake Pend Oreille. When that process was all done, here was this deep basin that the glaciers had dug out. And where the glacier terminus had been is where the Rathdrum Prairie is. That is then filled in with material, so the lake has been impounded ever since."

Today there are no glaciers left in Idaho, so it's hard to comprehend just how heavy an ice field can be. We have to travel to Alaska for any comparisons. But the weight of a great thickness of ice can push the earth's surface down hundreds of feet.

"And you can see some really interesting effects of that in the drainages of the rivers in northern Idaho and on up into Canada. At one time all that stuff had to flow south and now all of those rivers flow north as the southern part came up. And as near as I can figure out, a lot of that has to do with the isostatic rebound of the land as the continental glaciers were receding with time."

The last Missoula flood swept through northern Idaho a mere 15,000 years ago, burying parts of Oregon and Washington in hundreds of feet of water. So it is conceivable that someone actually witnessed the greatest flood ever documented by scientists.

Map of Lake Bonneville [Credit: Cassandra Groll]

About the same time as floods were tearing through northern Idaho, a great cataclysm was brewing in southern Idaho. Water was backing up in Utah, nearly a thousand feet above the current level of the Great Salt Lake. At Red Rock Pass, southeast of Pocatello, the rupture occurred. Four hundred foot flood waters tore through the Portneuf Narrows. In other places the deluge broke free across the plain.

"When the Bonneville flood entered the area where American falls reservoir is, it went out into a fairly flat area," explains Bonnichsen, "so there's a large delta of material deposited there — the American Falls delta — which is a relic from the time of the Bonneville flood."

Further west the flood followed the path already established by the Snake River, spilling out above the canyon rim. "And it's almost like a mud flow down at the bottom of a channel like that. It has great erosive capability," said Bonnichsen.

It must have been an amazing sound. Boulders the size of cabins, being ripped from the canyon walls, bumping and grinding along the river bottom, carried along by the incredible force of the flood.

Melon Rocks [Credit: Scott DeHart]

"One of the interesting things you find are great big sandbars or gravel bars," says Bonnichsen. "They're really boulder bars, just like you would see in a river, but they're much larger in size, made out of boulders, boulders as much as ten feet across."

Huge blocks of basalt traveled three to six miles before eventually coming to rest high above the river. Now considerably more rounded, their resemblance to petrified watermelons caused one geologist to refer to them as melon gravel. Most of us consider them just one more example of the intensity of the Bonneville Flood.

Shoshone Falls, one of the great spectacles of North America, is another example of the power of that flood. It carved the canyon much deeper as the waterfall migrated upstream to its present location, where it is 212 feet high, forty feet higher than Niagara Falls.

Eventually, the flood waters receded, and life returned to normal for the Snake River canyon and southern Idaho. But what is normal, when you're talking eons of time? The ice age that had stretched over two million years was coming to an end. Another climate change was in the works, as ice melted and lakes and swamps and gullies dried up, leaving beds of silt and sand at the mercy of the wind.

Ages End [Credit: Painting by Stev H. Ominski]

For those who wonder which came first, the Bonneville flood or the Missoula floods, Bonnichsen has this to say. "Lewiston is the one place in the state where you really see the overlapping deposits from both the Bonneville flood, which came down the Snake River, and the results of the Missoula flood, which came across northern Idaho and then across central Washington. Evidence has been found in this area that the Bonneville flood came through early on, and the successive Missoula floods affected this area after that."

For modern-day humans, it's interesting to know that these floods were all fairly recent — almost yesterday, in geologic terms — and that we can still clearly see their effects on the landscape, with just a bit of effort on our part.

Text by Bruce Reichert