A Landing a Day

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Posts Tagged ‘Gros Ventre landslide’

Jackson Hole, Wyoming (Part 2)

Posted by graywacke on January 22, 2014

First timer?  In this formerly once-a-day blog (and now more-or-less a twice a week blog), I have my computer select a random latitude and longitude that puts me somewhere in the continental United States (the lower 48).  I call this “landing.”  I keep track of the watersheds I land in, as well as the town I land near.  I do some internet research to hopefully find something of interest about my landing location.  To find out more about A Landing A Day (like who “Dan” is and what the various numbers and abbreviations mean in the first paragraph), please see “About Landing,” (and “Abbreviations” and “Cryptic Numbers”) above.

 Landing number 2077; A Landing A Day blog post number 505.

Dan –  Oh my.  I know, Dan, that you read my previous Jackson Hole WY post, and are looking forward to reading this Jackson Hole (Part 2) post.  Anyone else who hasn’t read my first Jackson Hole post, you must click HERE to read it.  When you’re done, come right on back!

 OK, so I’ll repeat my local landing map:


OK.  Here comes the new stuff.  Note the town of Kelly and the lake just east of Kelly.  Here’s a closer view:


You can see that I landed about 13.5 miles from “Lower Slide Lake.”  Hmmm, unusual name, eh?

 From Wiki, about Lower Slide Lake:

 Lower Slide Lake is located in Bridger-Teton National Forest, in the U. S. state of Wyoming.  The natural lake was created on June 23, 1925 when the Gros Ventre landslide dammed the Gros Ventre River. The lake was once much larger; however part of the rock dam failed less than two years later (on May 18, 1927) causing deadly flooding downstream.

Sounds like an ALAD story!  As you and my regular readers know, I’m interested in natural lakes, and the stranger the reason for their creation, the more I like it.  (Check out my Lake City CO post HERE, and my Clear Lake CA post HERE.)

 So, it looks like I need to check out the Gros Ventre landslide. 

 Here are some excerpts (with minor editing here and there) from a U.S. Forest Service article about the slide:

 On June 23, 1925, one of the largest fast-moving landslides in generations occurred near the town of Kelly, Wyoming. In just three minutes, huge amounts of rock and debris cascaded down the north slope of Sheep Mountain, changing the area forever.

Hurling down the slope at 50 mph, the mile-wide slide carried 50,000,000 cubic yards of debris. The mass rode 300 feet up the opposite slope, blocked the Gros Ventre River, and formed a five-mile long body of water known today as Lower Slide Lake.

 [ALAD Note:  The geologic formation that slid was the Tensleep Sandstone.  Underlying the Tensleep was the Amsden Shale.  The surface that represents the contact between the two formations was parallel to the slope.]

Three primary factors are thought to have contributed to the unusual event:

1. Heavy rains and rapidly melting snow saturated the Tensleep Sandstone, causing the Amsden Shale rock layer on Sheep Mountain to become exceptionally slippery;

2. The Gros Ventre river cut through the sandstone into the shale.  This created an unsupported downslope edge of the sandstone formation.

3. Swampy pools with no outlets on top of the mountain provided an extra source of water to produce the saturating conditions.

Earthquake tremors (which were occurring) added to these already unstable factors and could have precipitated a landslide.

[I found some cross sections from the Penn State University Geology Department, showing what is discussed above:]

psu geology cross section

[Back to the U.S. Forest Service piece:]

William Bierer, a long-time native to the area, had predicted a slide in the near future. Convinced of the validity of his theory, Bierer sold his ranch on Sheep Mountain to Guil Huff, an unsuspecting cattle rancher, in 1920. Bierer died in 1923 before his prophecy became reality.

Two years later, on the afternoon of June 23, 1925, Huff rode horseback down the river to the north side of Sheep Mountain where he had heard loud rumblings. He arrived at 4 p.m., in time to witness 50 million cubic yards of land mass descending rapidly toward him. He and his horse escaped the impact by a mere 20 feet.

By June 29, after heavy rains caused the dam to fill and overflow, the Huff house was floating in the lake.

A man-made dam has a built-in spillway so that the waters cannot top the dam, erode, and breech it. The slide dam, made by nature, was not equipped with a spillway.  Engineers, geologists, and scientists came to the area to study the slide; they determined that the dam formed as a result of the slide was permanent and safe.

 [Apparently, the flow of the river was able to infiltrate through the dam, rather than flowing over it (remember that the dam included blocks of sandstone bedrock).  Therefore, the dam was not being overtopped.]

Most of the local people accepted that decision and ceased worrying about a possible disaster, especially when the spring runoff in 1926 passed with no major problems.

The winter of 1927, however, was one of the most severe ever recorded in the state to that time. When spring arrived, the unusually deep snowpack melted quickly, aided by days of rain. On May 17, water began spilling over the low places of the dam.

Some local ranchers saw the water rising; they rode up towards the dam and saw that the top 60 feet of the dam had given way under the pressure of the excess water.  They turned around and headed for Kelly to warn the residents of the impending danger. By the time they arrived, the people had only 15 minutes in which to flee to safety.

Despite the warning, six lives were lost in the tragedy. The little town of Kelly was almost completely obliterated.

Here’s a back-in-the-day photo that accompanied the U.S. Forest Service piece.  It shows the raw slide scar (on the other side of the river).  I assume this photo was after the flood, since the river cuts a clear channel across the “dam”:

 old slide pic

This is a recent shot of the slide debris field with the slide scar in the background (Panoramio by NMNC):

 pano nmnc debris field and scar

Here’s a GE shot showing that Kelly is only about 4 miles downstream from the lake.  I wonder how they managed 15 minutes of warning to the poor folks of Kelly?  They must have seen the problem from quite a ways downstream of the dam.  But even so – let’s say the flood waters were moving at 20 mph (not unreasonable, could have been faster).  At 20 mph, it would only take 12 minutes for the water to travel four miles.  Oh, well . . .

 ge - kelly & slide area

Obviously, the dam didn’t completely fail; the lake’s still there, just much smaller than it was in 1927 prior to the big flood (it’s about 2 miles long today vs. 5 miles long before the flood).  Currently, the river overtops the dam (as it probably has since the flood).  Here’s a GE shot:

 ge - outfall over dam

I can only assume that some smart people have looked at the dam to be sure it’s stable now.  But hey, it has been there for 80 some years with no problems . . . .

 Here’s a GE Panoramio shot of the lower part of the lake (by Hobbes7714):

 pano hobbes 7714 lower part of lake

I wonder if the sunken trees are from 1925?  Nahhhhh . . . .    But really, why would there be trees in the lake if they weren’t from back before the slide???

 And a shot of the Gros Ventre river valley just below the lake, as it makes its way through the slide debris (Pano, Ralph Maughan):

 pano ralph, river just below lake

I’ll finish up with a bunch of lovely Panoramio photos from the vicinity of the lake.  Upstream from the lake in the Gros Ventre River valley is an area with very colorful rocks.  Here’s another shot by Ralph Maughan:

 pano red rock shot near the lake by ralph maughan

Ralph also took this artsy closeup of some of the rocks:

 pano great red rock shot near the lake by ralph maughan

Here’s yet another classic Teton Range shot, this one taken from just below the lake by Tim Jansa:

 pano tetons from the slide Tim Jansa

I’ll close with this great shot (also by Mr. Maughan) taken from above the lake looking west:

 pano ralph does it again red rocks, tetons 

That’ll do it.



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