At some point in the past, something, some type of earth movement or massive earthquake caused a large section, or even multiple sections of solid rock, apparently with a relatively thin covering of dirt and trees, to break loose from the mountains on what is now the Washington side of the Columbia River in the area where Bonneville Dam is now located. The debris from this massive slide was sent crashing over and across the Columbia River, damming it for a period of time and creating that natural geologic feature spanning the river which the Native American stories call "The Bridge of the Gods".
In a picture linked here is seen the Columbia River, which is passing from the right (upstream) to the left (downstream). The Bonneville Dam complex is seen at the left side of the picture. Now, that large area of land protruding into the Columbia River from the upper side of the picture, which bends the river, is the massive Bonneville Landslide, with the other associated landslides. All of that material protruding into the river came crashing down from the mountains seen in the upper-left of the picture. Note that the Bonneville Dam complex actually joins into the landslide material.
Linked here is another picture of the Columbia River, with Bonneville Dam, plus the massive landslide complex. The mountains from which came all of that debris which created the massive landslide complex are more clearly seen in the upper-left of this particular linked picture. Again, note that Bonneville Dam, on the upper, Washington side, is joined into landslide material, rather than being joined into much more stable, solid rock. Linked here is another view of the Columbia River and the massive landslide complex, looking from a different angle.
For those desiring a bit more detail about the overlapping landslide complex in the Bonneville Dam area, just check out the page linked here. The page provides a map of virtually the whole geologic formation. Once again, note that Bonneville Dam connects into landslide material on the Washington side of the Columbia River. And then, linked here is a satellite photo showing the Columbia River, the landslide area, plus the location of Table Mountain and Greenleaf Peak, where the immense amount of landslide material originally came from.
Pictures linked here, here, here and here show the cliff faces, or the shear lines, at the point where the mountains split apart, with the loosened material then crashing in a southerly direction toward and across the Columbia River. Linked here is another picture of the mountains on the Washington side of the river, which mountains were split apart in earlier times as the original, natural Bridge of the Gods was created. An incredible amount of material was ripped from these mountains and thrown across the river by a natural force. Yes, modern insurance agents would likely call it an act of God.
Linked here and here are pictures from the Oregon side of the Columbia River, looking across at the landslide, mountains and cliff faces in the background, from which the landslide debris was ripped during a mighty, geologic event or events in earlier times. Linked here is an aerial view from above the mountains which were ripped apart, showing the cliff faces at the line of parting. Linked here is a photo which shows the generally north-south trending cliff faces where the mountains split apart. It appears that the landslide material started to go in a southeast direction and then turned more toward the south.
With this basic introduction to the massive landslide in the area of Bonneville Dam on the Columbia River, let us now turn to look at some additional details. Then, let us consider some important lessons which can be learned from that which happened in earlier times in the Bonneville area. Let us also consider what these lessons could potentially mean to cities like Portland, Oregon and Vancouver, Washington, plus other cities along the north-south Interstate-5 corridor, when the next Cascadia Megaquake occurs in the days ahead.
The page linked below provides information about the large "Bonneville Slide", plus that which is geologically called the "Cascade Landslide Complex". It is called a Landslide Complex because there are actually four massive, overlapping slides within this large geologic feature. The page states: "The Bonneville Landslide is the youngest of the four landslide lobes within the "Cascade Landslide Complex"." The other three slides are called the "Red Bluff Landslide", the "Mosely Lakes Landslide", and the "Carpenters Landslide". The page also notes the location of these other three slides.
Within the linked page, it is noted that the Cascade Landslide Complex covers about 14 square miles. Regarding the Bonneville landslide, the page states: "The landslide unleashed blocks of rock as large as 800 feet long and 200 feet thick down the mountain, creating a temporary earthen dam more than 200 feet high -- three times the height of Bonneville Dam." And, the lake behind the dam --- or behind the original "Bridge of the Gods", as it was known in Native American stories --- "stretched almost 70 miles (up to the present-day John Day Dam)."
The page linked below notes that there is a controversy about the age of the Cascade Landslide Complex. It states: "The Bonneville landslide has been dated by a combination of radiocarbon dating, dendrochronology, and lichenometry. Initial dating efforts placed its year of occurrence originally at about A.D. 1250 (Lawrence and Lawrence, 1958) and then between A.D. 1500 and 1760 (Reynolds, 2001; Schuster and Pringle, 2002). However, more recent tree-ring dating of living trees on the landslide demonstrated that emplacement had to have occurred prior to about A.D. 1550 (Weaver and Pringle, 2003)."
The page linked above goes on to state: "Subsequently, the technique of wiggle-match radiocarbon dating of tree-ring sequences (Galimberti and others, 2004) was used to acquire a more precise age range (O'Connor, 2004; Reynolds and others, 2015) from wood obtained from a log entrained in the landslide deposit and from two standing dead trees that had been growing along the banks of the Columbia River but had been drowned in the lake formed by blockage of the river by the landslide." So, what was the conclusion that these guys came to?
The linked page states: "Ring patterns show that all three trees were killed in the same year, very likely between A.D. 1421-47 (95 percent confidence interval) and almost certainly between 1416-52 (99.7 percent confidence)." Well, based on the things stated above, it is clear that those in the scientific community seem to each have their own opinion as to when various geologic things happened in the past. Some might jokingly say that there could be almost as many views about exactly when things happened in the past, as there are scientists in the scientific community.
A little over half way down in the page linked above, there is a section titled Dating the Bonneville Landslide. According to that section, findings of some scientists tend to indicate that the Bonneville Landslide may have occurred somewhere between 250 to 400 years ago, or between the years 1670 and 1760. Within those time-windows is located the massive and destructive Cascadia earthquake, which occurred in the year 1700. By the way, the page calls that earthquake "the powerful offshore Cascadia Subduction Zone earthquake." And yet, it may have shaken apart mountains in the Cascades.
Now, the page linked below mentions the Bonneville and Red Bluffs landslides, which are both part of the Cascade Landslide Complex. It states: "Both slid into the Columbia River within the last 600 years, and evidence suggests that the Bonneville landslide moved rapidly (tens of feet per second)... The Bonneville slide temporarily dammed the river and formed the 'Bridge of the Gods' that is known from Native American oral histories." And, at the end of the section which follows, let us look at additional things which are spoken about in the linked page.
Within the page linked below is an ABSTRACT which declares: "The 14-km2 Bonneville rock slide-debris avalanche temporarily dammed the Columbia River some 190-450 calendar yrs BP (AD1500-1760), forming a short-lived 'land bridge' that Native Americans called the 'bridge of the gods.' At this narrow site on the river, the U.S. Army Corps of Engineers in 1937 completed the 60-m-high, concrete-gravity, hydroelectric/navigation Bonneville Dam, using the toe of the landslide as the right abutment and for much of the foundation of the dam. During the 1970s, a second powerhouse for Bonneville Dam was constructed on a foundation excavated into the lower landslide."
The ABSTRACT continues with these words: "Construction of the dam, the second powerhouse, and appurtenant structures caused only minor 'construction' slides and no major reactivation of the Bonneville landslide. Conversely, the right end of the dam serves as a 'buttress' that helps to stabilize the toe of the landslide." Well, this may sound acceptable to people at the present time, but what happens when a massive earthquake hits and the landslide and dam become somewhat unstable? If earthquakes caused the massive slides in the Cascade Landslide Complex, truly, what could happen the next time around, once the incredible shaking begins? A failed dam? Portland and other downstream cities partly underwater in the flood?
There is more to consider in this story. About one-third of the way down in the page linked below is a section titled Current Hazards. That section begins with these words: "The Cascade Landslide Complex is perhaps one of the more dangerous landslides in Washington State. The Bonneville Dam sits on the landslide debris of the Bonneville Landslide. Major pipelines, powerlines, and transportation routes cross the landslide. Future movements (potentially during a Cascadia Subduction Earthquake) would result in major disruptions in utilities and the potential for the redamming of the Columbia River."
If a massive earthquake were to cause a large slide which slammed into Bonneville Dam, that could bring about the failure or end of the dam. If a massive earthquake were to ever cause a redamming of the Columbia River at some point, the linked page brings up the possibility for flooding of low-lying communities along the river. And, considering things further, if Bonneville Dam were to fail in a massive earthquake and slide/damming scenario, it could potentially create quite a mess for cities and communities, both up and down the river...especially with a sudden loss of electrical energy from the dam's powerhouses.
The link below accesses a U.S Army Corp of Engineers (COE) photo which is found on the U.S. Geological Society (USGS) website. The photo shows the Bonneville Dam site, with the Table Mountain Landslide (Bonneville Landslide) shown on the Washington side of it. In the photo, things presently look fairly serene, even with the included Table Mountain Landslide. But, in an incredible earthquake, possibly in a Cascadia megaquake, there is the potential that things could change very quickly. Is there another massive landslide waiting to happen in this area?
Looking once again at the page from the end of the preceding section, which is again linked below, it shows what is lurking in the Columbia River Gorge, in the area around the Bonneville Landslide. The new mapping methods show that there are "previously unrecognized landslides beneath dense forest cover" in this area. The page states: "The lidar imagery used to make the map reveals that landslides occupy about two-thirds of the 222-square-kilometer (86-square-mile) map area, expanding the area of previously known unstable terrain by 60 percent."
The USGS page then states: "The majority of these landslides show evidence of multiple movement episodes: a reminder that some old landslides can reactivate and threaten nearby communities. At least one large landslide appears to have crossed the Columbia River at high speed -- a scenario that has serious hazards implications if it were to be replayed." Then, these words: "The western Columbia Gorge has been long recognized as an area susceptible to landslides. Abundant rainfall, steep terrain, geologic structure and erosion by the Columbia River combine to create topography capable of ground movement."
The page linked below notes that "landslide occurrence in the map area [around Bonneville Dam] has spanned thousands of years; about a quarter of the landslides are estimated to have moved within the last 1000 years, and 12 have moved within the last 20 years or are currently moving." The page also states: "Further analysis of potentially unstable terrain in the western Columbia Gorge is warranted, especially in light of the potential for a large Cascadia Subduction Zone earthquake in this region." And, "Ground motion from earthquakes can trigger landslides in terrain that might otherwise remain stable."
Looking further into the official government page linked above, it states: "Depending on the size, speed, and runout distance of any future landslide, vulnerable infrastructure could include a major natural gas pipeline; high-voltage power transmission lines from Bonneville Dam; road, rail, and river barge transportation corridors along the river; and Bonneville Dam itself." These are all things to consider, when it comes to the next major earthquake in the Cascadia region. And, this is based on if the quake is actually centered far offshore at the subduction zone, rather than somewhere more inland.
Before this section comes to a close, let us consider a satellite photo of the Bonneville area, which is linked here, and what is shown in that photo. Note that the Bonneville Slide Complex, as shown in the photo, buried the area where the current Bridge of the Gods is located at Cascade Locks, Oregon. Note also that the landslide buried the area where the Bonneville Dam and powerhouse complex now resides. So, what could happen in the next massive Cascadia earthquake could cause a lot of problems for citizens of this region, especially if Bonneville Dam were seriously damaged or destroyed.
A photo linked here shows the debris from a mountain which appears to have crumbled and simply slid down. Note the steepness of the debris field. Another photo, linked here, shows a very steep rockslide which came down from some very steep mountains. Yet another photo, linked here, shows a very steeply angled debris field of rocky fragments. When a simple slide is the motivating force, it appears that it is common for the debris field to be steeply angled. But then, there is the Bonneville Landslide, which has a totally different story to tell.
In the page linked below there is a section titled Geologic story. At the bottom of that section is a photo with the following caption: "Aerial view and animation of the Bonneville landslide and its surroundings." Look at the extreme shallow angle of the Bonneville Landslide. This was not just a common landslide. The extreme shallow angle of the surface of the landslide indicates to the writer that there may have been a very powerful earthquake in operation here, while the debris field was being laid down. That is possibly why the debris field traveled so far and crossed and dammed the Columbia River.
A photo linked here was taken from the mountains on the Oregon side of the Columbia River. The photo is looking down on the Bonneville Dam and powerhouse complex, which is shown in the center of the photo. Just above the dam area is seen the Bonneville Landslide. Note the extreme shallow angle of the surface of the landslide, especially in the portion protruding across the river. Again, there is reason to suspect that a massive earthquake was shaking things, as this debris field was laid down across the river. A massive earthquake would allow for the extreme shallow angle of the landslide surface.
Based on the information, photos and animation provided through links in the section above, there is reason to suspect that a massive, relatively long-lasting earthquake was in operation when the Bonneville Landslide debris field was laid down. And then, an earlier paragraph in this presentation, linked here, indicates that the Bonneville Landslide may have potentially even happened during a Cascadia earthquake in the year 1700. Regarding this potential earthquake, the associated linked page calls it "the powerful offshore Cascadia Subduction Zone earthquake."
With the above information fully in mind, it is time for the very important question. If a powerful Cascadia Subduction Zone earthquake, centered roughly 70 miles offshore in the Pacific Ocean, was the driving force behind the splitting and collapse of a massive portion of that which is now called Table Mountain, what would the seismic energy have been doing in places closer to the Pacific Coast, such as where Portland, Oregon, and Vancouver, Washington, now reside? A shaking like that which leveled the surface of the Bonneville Landslide potentially could also level cities like Portland and Vancouver.
When considering a massive Cascadia earthquake in the days ahead, there is more to consider than just that which could potentially happen in the Bonneville Dam area on the Columbia River, plus what could potentially happen in Portland, Oregon, and Vancouver, Washington. Because the Cascadia fault or subduction zone (1)(2), or associated rift is a north-south trending geologic feature, there is reason to suspect that the situation which exists at Seattle, Washington, could be similar to the situation which exists in Portland, Oregon, plus other cities along the north-south Interstate-5 Freeway corridor.
Now, the page linked below may be from 2012, but it still provides some serious "food for thought". It notes that "a massive earthquake could strike the coast of the northwestern United States and southwestern Canada" at virtually any time. This massive earthquake could happen with little or no warning. The page speaks about "the potential earthquake threat to major population centers like Portland, Ore., Seattle, Wash., and Vancouver, British Columbia." The groundwork for understanding that there was actually a serious (1)(2)(3) earthquake threat in this region began to be laid in 1986 (1) by Brian Atwater.
At this point, there is something very important to note in the page linked below, when it comes to the next massive Cascadia earthquake. In earlier years, "scientists thought that the likely place for such a quake was off the coast." Well, research done prior to 2012 "extend[ed] the region likely to quake farther inland, thus indicating the earthquake threat to the people living in the Pacific Northwest is much higher than scientists realized." The page goes on to state: "It has never been thought that a large rupture would occur directly beneath Seattle, but these slow slip events show it will occur dangerously close."
LOOKING AT THE BIGGER PICTURE
For those desiring more information about these "slow slip events", also known as "silent earthquakes" or "deep tremors" or "episodic tremor and slip events", which happen near Seattle and elsewhere along the Interstate-5 Freeway corridor, check out the pages linked below. Now, there are those who believe that these slow slip events "could add to the pulling force on the locked plate above it, one day triggering a massive quake." But, when it comes to the nature of the next massive Cascadia earthquake, there are many theories out there. The day after the earthquake, we will know whose theory was correct.
The page linked below states: "Slow slip events are accompanied by a seismic phenomenon known as tremor, which registers on seismograms as imperceptible vibrations that are recorded up and down the Pacific Northwest." And, on the page is a map with an incredible amount of blue dots on it, which dots indicate the location of these small tremors which periodically occur beneath Vancouver Island, in Canada, plus in the Puget Sound area by Seattle and, inland, southward past the Portland area and all the way into northern California...basically beside the Interstate-5 freeway corridor.
On the map, the slow slip or Episodic Tremor and Slip (ETS) events "fill a continuous zone along the entire Pacific West Coast from northern California to Vancouver Island." Note that high-density population centers are located by or in this zone. It should be noted, though, that at this time, the whole zone is not experiencing these tremors at the same time. Different patches experience tremors at different times. Now, looking further into the linked page, it appears there may be some type of a connection between Cascadia and the San Andreas fault system in California, plus interaction between them.
As the destructive potential for the next massive Cascadia earthquake is considered, keep in mind that continuous zone of slow slip tremor events which is located by the Interstate-5 Freeway corridor, as spoken about in the section above. Now, let us look into the pages linked below, which both present basically the same story. An earlier study indicated that the Cascadia fault could "rupture within 68 miles of downtown Seattle, pouring seismic energy into a densely populated urban area, threatening to knock down buildings both large and small, and endangering the lives of millions."
The linked pages state: "For decades, scientists, urban planners and emergency responders have taken small comfort in the fact that the tremor would stop near the coast of the Olympic Peninsula, perhaps as far as 118 miles west of Seattle." Yes, that is what they have commonly thought, plus what they have planned for in their emergency preparations. But, there is more to consider in this story. The earlier study showed "that the fault is locked down to a depth of 15 miles (25 kilometers), 6 miles deeper than previously thought."
This "6 miles deeper" of locked area on the Cascadia fault is happening on "a shallow-dipping fault." This places the extent of the locked area of the fault much closer to Seattle, than previously thought. This means that the shaking generated by the next Cascadia earthquake on this fault system could be located much closer to Seattle than previously thought. The pages also state: "Ground shaking could be up to five times stronger than anyone has planned for. Everything from small buildings to skyscrapers would be at risk of collapse." Well, in the aftermath of the quake, we will all know the real truth.
Now, let us again consider that zone for slow slip events which basically parallels the Interstate-5 Freeway corridor. There is reason to suspect that the situation which exists in the Seattle area --- with the locked section of the Cascadia fault extending further to the east and closer to high-density population centers than previously thought --- would apply all cities along the Interstate-5 Freeway corridor. This tends to indicate that the Portland, Oregon and Vancouver, Washington areas could also experience much greater shaking, plus much greater damage than previously thought, in a massive Cascadia quake.
At this point, there is a statement which the writer shall make. There is reason to suspect that the geologic computer models which the scientists base their theories about the Cascadia fault on, plus what it will do in the next massive earthquake event, may not be completely accurate. The writer has reason to suspect that in order to better understand what is the motivating force which is driving major earthquakes and volcanoes in the Cascadia region, people should consider that which appears to have happened with the ancient North American continent in earlier times.
As the original landmass on this earth (which scientists commonly call Pangea) was broken apart (1) or divided (1)(2), the various pieces moved (1) to different places on the face of this earth. The ancient base for the North American continent moved westward across the ocean. As it neared the end of its major westward travel, it appears that there were some massive collisions with other geologic features. In these massive collisions, it appears that former oceanic islands and mountains were embedded into the western edge of the moving continent.
In one of the collisions, oceanic islands apparently associated with the oceanic ridge and rift system were embedded deeply into the western edge of the moving continent. These islands became the Klamath Mountains (1)(2) in what is now southwest Oregon and northwest California. Further to the north, there appears to have been some undersea mountains. As the ancient North American continent collided with these mountains, plus snagged the opposite wall of the rift system, these things were embedded in the western edge of the landmass and became the Olympic Mountains (1)(2) in present Washington state.
Well, from the looks of things, it appears that the ancient North American continent, in its westward travel, overrode and deformed a north-south trending section of the oceanic ridge and rift system. So, where is that overridden section of ancient oceanic rift now residing? Well, there is reason to suspect that it may be located somewhere about 21 miles below the surface of the landmass, beneath the states of California, Oregon and Washington. Movement and volcanic activity associated with this ancient rift system may be a major driving force behind various earthquakes and volcanoes in the Cascadia region.
And now, there are other questions which should be asked. How swift was the westward movement of the ancient North American continent, after it was split loose from the original large continent? Did the collision of the ancient North American continent with the islands, undersea mountains and western wall of the north-south trending rift system end up changing the direction of the ancient Pacific Plate from that which it had originally been moving? And, if the direction of plate movement was changed, how fast did it happen?
Well, there is reason to suspect that the Hawaiian-Emperor seamount chain (1) can help us in finding answers to at least some of the questions presented above. There is reason to suspect that the sudden bend (1)(2)(3) in the seamount chain provides us with an answer about a change in travel direction for the Pacific Plate, plus how rapid the collision was between the westward moving North American continent and those various features (islands, undersea mountains, seafloor and western wall of the rift) which were originally associated with the north-south trending section of the oceanic rift system.
If the collision of the ancient North American continent with the features of the north-south trending section of the oceanic rift system had been slow moving, instead of a sharp bend in the Hawaiian-Emperor seamount chain, there would have been a rather large-radius, curving bend. The larger the radius of the bend, the slower moving would have been the collision. The smaller the radius of the bend would indicate a faster moving collision. That very abrupt bend in the Hawaiian-Emperor seamount chain tends to indicate that the collision was a sudden, extremely fast moving event.