A Landing a Day

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Lake Tahoe, California

Posted by graywacke on January 14, 2019

First timer?  In this formerly once-a-day blog (and now pretty much a once-a-week blog), I use an app that provides 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 or towns 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) please see “About Landing” above.  To check out some relatively recent changes in how I do things, check out “About Landing (Revisited).”

Landing number 2427; A Landing A Day blog post number 862.

Dan:  Today’s lat/long (39o 10.361’ N, 120o 1.576’) puts me in E-Cen California:

For those familiar with CA / NV geography, it’s obvious that I landed near Lake Tahoe.  (OK, so this post’s title gives you a pretty good clue.)

But let’s see how close:

Couldn’t be closer!  Here’s my streams-only map, showing that the lake is drained by the Truckee River (2nd hit):

I zoomed back so that you can see that the Truckee ends up in the internally-drained Pyramid Lake.  This makes the Truckee the only lake-to-lake river I know of.

Using Google Earth (GE), I put the Orange Dude on a bridge over the Truckee just as it’s leaving the lake:

And here’s his downstream view:

Looking upstream, we can see a small water level control dam:

I found a nice unobstructed view of my landing spot on the south shore:

And here’s what the OD sees:

So.  It’s time to dive right into the crystalline waters of Lake Tahoe.  I’ll start (of course) with the geology. 

I’m going to keep it simple (and in my own words).  Probably beginning about 20 million years ago, tectonic forces were uplifting the Sierra Nevada Mountains to the west, and the Basin and Range landscape to the east across Nevada.  These two geologic provinces meet at Lake Tahoe.  Normal faults developed (where two blocks of earth move vertically relative to one another on either side of the fault).  One major fault was on the west side of the lake and another major fault was on the east side.

For reasons I don’t pretend to understand, about 3 million years ago the block of earth between the two faults ended up going down while the blocks on either side of the lake went up (relatively speaking).

Voila!  This downfaulted basin ended up filling with water. From TahoeCam.com:

A lake formed near the southern and lowest part of the basin, fed by snow, rain, and draining creeks and rivers. The lake level increased in depth until it found an outlet, then near the present town of Truckee. Several active volcanoes poured lava into the basin, eventually damming the outlet. The waters rose again, several hundred feet higher than the present level. Finally, a new outlet was cut (the present Truckee River outlet) and the lake level began to lower as the Truckee eroded its valley.

The lake is about 20 miles long and 10 miles wide, and very deep.  Here are some factoids:

  • After the five Great Lake, Lake Tahoe is the sixth largest lake by volume in the United States.
  • With a max depth of 1,645 feet, Tahoe is the second deepest lake (after Crater Lake, at 1,949).
  • Although Tahoe is only the 16th deepest lake in the world, it is the fifth deepest based on average depth.

Here’s a bathymetric map, showing the lake is, in fact, consistently deep (mostly deeper than 1,300 feet):

Here’ a cool 3-D view:

And another:

You can’t help but notice the big chunks (of rocks?) on the floor of the lake.  Here’s another view:

This view suggests that maybe there was a landslide, eh?  Well, there was!

I found an article by Andrew Alden on the website KQED Science entitled “The Tahoe Tsunami:  New Study Envisions Early Geologic Event.”  I’ve lifted some of his words:

Once upon a time, geologists tell us, a massive chunk of Lake Tahoe’s western shore collapsed into the water in a tremendous landslide. The water responded by sloshing high onto the surrounding shores in a series of landslide tsunamis. A major new study in the journal Geosphere adds much new detail to that story, tracing massive features around and beneath the lake. And it places the date of the fearsome event near the time that humans first visited it.

Lake Tahoe is a peaceful mountain resort area today, but its geologic past has been long and violent. Its very presence is due to tectonic stretching of the Earth’s crust across Nevada, which has opened large basins from California’s Sierra Nevada crest all the way to Salt Lake City, Utah. The Tahoe basin has been there for roughly 3 million years, during which time it’s seen outbreaks of volcanism and countless major earthquakes.

Forty years ago the first sonar survey of Lake Tahoe showed evidence that bite-shaped McKinney Bay, in the middle of the lake’s western shore, is a scar left by a very large landslide and that huge pieces of that slide, as much as a kilometer long, are strewn across the lake bottom.

The new paper in the August issue of the journal Geosphere, by veteran researchers James G. Moore, Richard Schweikert and Christopher Kitts, assembles the evidence old and new into a scenario of that convulsive day.

The landslide involved a body of rock made unstable by movement on a large-scale fault along the western shore. The slide, presumably triggered by an earthquake on that fault, sent some 12.5 cubic kilometers of rock and sediment into the lake, where it pushed a corresponding amount of water out of the way as huge tsunamis, perhaps 100 meters high. Much of this water burst over the lake’s outlet at Tahoe City and rushed down the Truckee River, where house-sized boulders litter the riverbed today as far downstream as Verdi at the Nevada border.

The rest of the water washed ashore all around the lake in what the authors call a “megasplash.” The lake would have sloshed back and forth for days afterward, and surely more landslides were being triggered at the time.

The authors say that the lake must have been muddy for years, and its shores a barren wasteland. All of the mud gradually blanketed the whole lake bed, making the landslide-related features look much older than they really are. Mapping on land also delineated a sheet of clean sand as thick as 2 meters spread across the flat lands at Lake Tahoe’s south end, a sign that the area was swept by large waves.

More detective work on land helped the authors narrow down the time of the megaslide and megasplash to some time between 21,000 and 12,000 thousand years ago. All of this evidence fits into a terrifying picture of geologic uproar.

As you might suspect, there are thousands of lovely pictures of the lake.  This one (from Utopian Luxury Vacation Homes) caught my eye:

And I figured I should close with a sunset shot over the lake.  I found this one (from the same spot and OK, it’s sunrise), by Aaron Keigher on the Minden Pictures website:

Dawn at Emerald Bay, Lake Tahoe, California

That’ll do it . . .

KS

Greg

 

© 2019 A Landing A Day

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