October 1, 2016 at 7:39 am

The Great Bonneville Flood – Part 1


Image: Lynn Topinka 2005 http://columbiariverimages.com/Regions/Places/wallula_gap.html
Image: Lynn Topinka 2005
In a recent article I told the story of the Teton Dam disaster that occurred on June 5 of the year 1976 in the headwaters of the Snake River, Idaho. In conclusion to that installment I wrote that this modern catastrophic flood “overlapped the region of a gigantic prehistoric flood in the vicinity of American Falls as another juxtaposition of catastrophes.” That ancient flood has left an unmistakable imprint conspicuously etched into the landscape of the Snake River region of Southern Idaho. At its peak the Great Bonneville Flood, as it has come to be called, was about 40 times greater than the Teton Dam Flood at its greatest discharge in the immediate vicinity of the rupture. But there were other factors that put the Bonneville Flood in the category of exceptional floods in the history of the Earth.

The Teton Dam Flood had spent itself within 8 or 10 hours, the length of time it took for the reservoir to drain. Additionally, the peak discharge of one million cubic feet per second was only sustained for the first few hours after the dam collapsed and only in the immediate vicinity of the rupture. The distance encompassing serious damage downstream from the dam was perhaps 40 miles, for the flood wave had mostly spent itself by the time it reached Idaho Falls. By contrast, the Great Bonneville flood sustained peak discharges of about 40 million cubic feet per second for as long as a year, and the distance over which this peak discharge was maintained was more than 1000 miles.

So, what could be the cause of a hydraulic event on this scale?

I am sure that most everyone reading this article has heard of the Great Salt Lake, Utah. Great Salt Lake is a hypersaline lake, meaning it has a high saturation of salt and minerals. The reason for the high concentration of salts is that the modern lake is but a remnant of a much larger lake that once existed at the end of the most recent Ice Age. That much grander lake evaporated away in the warmth of the post-glacial climate and as it did so the mineral content of the water stayed behind and became more concentrated as the volume of water diminished. The modern Great Salt Lake covers about 1700 square miles of the Great Basin region of northern Utah, and only averages about 16 feet in depth.

Great Salt Lake, Utah,
Figure 1 Great Salt Lake, Utah and the Basin and Range Province. To the west and southwest of Great Salt Lake is the Basin and Range geological province. Many giant lakes occupied the basin during the end of the most recent ice age. Most of the flat floors in the intermontane basins were formed by large lakes. See figure 2 for a map of Lake Bonneville.

Utah and the Basin and Range Province. To the west and southwest of Great Salt Lake is the Basin and Range geological province. Many giant lakes occupied the basin during the end of the most recent ice age. Most of the flat floors in the intermontane basins were formed by large lakes. See figure 2 for a map of Lake Bonneville.
The ancient lake was named Lake Bonneville by the American geologist Grove Karl Gilbert after a French born American Army officer, trapper and explorer Benjamin Louis Eulalie de Bonneville, who was the first to explore large regions of the Great Basin and Snake River Plain. Gilbert wrote the first ever monograph for the U.S. Geological Survey in 1890 on the subject of Lake Bonneville. For those readers that have an interest in geology, geography, the outdoors, history, nature or who simply wish to learn more of this fascinating story, Gilbert’s classic monograph “Lake Bonneville” is available as a free download from Google Books.

Now consider this: Ancient Lake Bonneville covered an area of some 20,000 square miles, nearly twelve times greater than modern Great Salt Lake, and in places was up to one thousand feet deep! The volume of this lake at its greatest was more than 2500 cubic miles, slightly less than that of Lake Superior at 2870 cubic miles and more than double that of Lake Michigan. By comparison modern Great Salt Lake has a volume of only about 4.5 cubic miles. In other words, ancient Lake Bonneville was about 550 times more massive than its modern remnant. This massive lake was really more of an inland sea than a lake, one could have sailed over it for days without seeing land.

Lake Bonneville, Ancient

Figure 2. Ancient Lake Bonneville with modern Great Salt Lake shown with dashed lines. Compare with map in Figure 1. Arrow at the top points to the outburst spillway area through which 340 feet of the entire lake drained over a period of several months.

How does one explain the existence of such a vast body of water in a region that is now semi-desert with an annual rainfall of only about 17 inches per year? Quite obviously conditions must have been drastically different at the time Lake Bonneville existed. But, actually, the problem is much greater, for, at the same time there were other vast lakes that came into existence in what are now arid basins of Utah and Nevada. Lake Lahontan, the second largest late Ice Age lake, covered some 8,500 square miles of northwestern Nevada and parts of northeastern California and southern Oregon in areas that are now basically desert. The deepest part of the lake was about 900 feet and this basin today holds modern day Pyramid Lake, the last remnant of Lake Lahontan. In addition there were numerous other sizeable lakes occupying immense areas of the Basin and Range province, or the Great Basin, as it is called by geologists.

Figure 3. View of the Wasatch Mountains, looking east. The water erosion from Lake Bonneville is clearly visible on the lower section of the mountain flanks. View looking east from roughly 6000 feet altitude. Photo by Randall.

Once again, I recommend using Google Earth. The characteristic landscape of linear mountain ranges separated by lower regions, the basins, in the western half of Utah and most of Nevada is easily seen in Google Earth and is especially apparent when using the terrain maps feature of the program. To the west and southwest of Great Salt Lake lies the Great Salt Desert, which shows up very distinctly in Google Earth. Here is the extremely flat, planar landscape of the famous Bonneville Salt Flats, for nearly a century the site for motorcar racing, high performance vehicle testing and setting new land speed records. The exceptional flatness of the terrain is totally the result of the former presence of the great lake that once existed here, and, which, over some 5 or 6 thousand years slowly evaporated away leaving the immense salt pan as an end product.

It must be understood that the Great Basin is like an enclosed bowl with no outlet to the ocean and no streams or rivers flowing out of it. Such a feature is called a hydrologically closed basin. During the latter part of the Ice Age the Great Basin was more temperate than now with substantial rainfall and numerous lakes that formed when conditions of precipitation and evaporation were suitable. Rivers flowing into these lakes carried a considerable amount of sediment eroded from the mountains and uplands. It is this mineral rich sediment that formed the bottom of Lake Bonneville which is now hard desert floor virtually impervious to high performance vehicles racing over it at 500 to 600 miles per hour.

Lake Bonneville has apparently come and gone a number of times. Whether there is a periodicity to this extreme environmental alteration, remains to be seen. What is known is that the end of the last incarnation of Lake Bonneville, which coincided with the end of the Ice Age, was marked by an event of extreme magnitude. The lake rose suddenly and precipitously until it breached a mountain pass in northern Utah. The water that spilled over and through this low spot in the lake perimeter, now called Red Rock Pass, was a deluge, gushing in massive torrents out onto the Snake River plain where it quickly overwhelmed whatever pre-flood Snake River was flowing in the area now occupied by American Falls Reservoir. The water tore across the immense volcanic plain of southern Idaho, ripping out hundreds of feet of bedrock in the process, until it turned north and slashed its way through Hell’s Canyon, ultimately connecting with the Columbia River in Washington State and beyond that to the Pacific Ocean.

Figure 4. Red Rock Pass – the outlet for the great Bonneville Flood. The town of Zenda sits right in the spillway carved by the draining floodwaters. Flow was towards the north, following the pathway now occupied by Highway 91.

For several months the massive inland sea called Lake Bonneville poured through the breach eating its way down through the bedrock spillway of Red Rock Pass. The lake level dropped 340 feet until it temporarily stabilized when the sill at the outlet reached a layer of harder rock. Two conspicuous shorelines still remain to testify to this catastrophic draining of the lake. It is estimated that over those several weeks to months more than a thousand cubic miles of water poured through the pass. The story of the Great Bonneville Flood is quite remarkable in its own right but even more importantly are the implications and the questions raised regarding the nature of global change and what these events might teach us about this dynamic planet upon which we live. In the second part of this report I will describe the remarkable effects a flood of 40 million cubic feet per second has upon the land over which it passes. I will also show why it is almost certain that this flood was in reality only a regional response to a much larger scale event and was most likely witnessed by early Americans.

Figure 5. Two shorelines on the Wasatch Mountains northeast of Salt Lake City. These shorelines, or strandlines to use the geological term, are evidence of the great body of water that once existed in this basin. They also show the lowering of the water level from the draining of the lake. Photo by Randall.

More soon,

– Randall

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Thank you and Godspeed! – Randal and Camron (Astromonk) ]

Read part 2 here: The Great Bonneville Flood part 2.

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[editors note] Learn a bit more about the region and taking in the incredible scale of the geological features left in the wake of the global superflood of ~13k BP here.

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  1. Wow, super helpful supplementary video. Thanks for embedding that

  2. Thank you Randall. Your articles and studies are always scientific, geologically sound, and massively historic. (Part 2 of the Great Bonneville Flood is also a must read).

    We also appreciate SGI opening the door on the cataclysmic end to our last ice age and the role it played in forming the Wasatch Front and our present day hiking windfall – called the Bonneville Shoreline Trails.