Avalanches in southern SN

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gdurkee
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Re: Avalanches in southern SN

Post by gdurkee » Sun Mar 24, 2019 11:15 am

Just skimming through stuff and way, way late on this one. Interesting topic. I've skied pretty extensively from Yosemite south. Did snow surveys for a few years. There's zillions of avalanche paths though narrowing to slides to ground is interesting and more rare. An avalanche requires a "slab" -- some sort of cohesive layer of snow -- and something for that to slide on. Often that's a layer of what's called depth hoar, unconsolidated crystalline snow. Spring avalanches will do that, not because they're sliding from the ground layer (usually) but a layer will sometimes become lubricated by melt, or the density above becomes great enough to release on a lower, weaker layer. There's such a mass of heavy snow that it'll sometimes slide to ground from the sheer mass and force. Surfaces that slide in spring are usually bare granite with water running underneath. You'll see that a lot in the dome areas of Yosemite and, especially, above the Tioga road at Olmstead Pt. when it's undercut by snowplows.

But, if you haven't already thought of it, there's an even better study area available. If you look at, say, Harlen's photo of the north slope of Mt. Tyndall, notice the pronounced gullies towards the ridge and the smoother slopes below the gullies. The dividing line is the difference between the older larger glaciations and the more recent smaller late Pleistocene one.

The most recent glaciers of the late Pleistocene were smaller than earlier glaciations of the Sierra. Because they were more recent, there's been less erosion on the surfaces they left. The older -- larger -- glaciations mostly filled the entire canyons. Those older longer-exposed surfaces would have the greatest erosion after tens of thousands of years. The lower slopes would have been recently excavated by glaciers up to roughly their retreat about 10,000 years ago, plus or minus. So, although now also exposed, there hasn't been as much erosion, deepening the gullies, which are more pronounced on the upper slopes over a much, much longer period of time.

There would also be an interesting study area on the west facing slope of Mt. Whitney. The summit and slopes immediately below are completely unglaciated and without gullying. As you go down the slope, gullies begin to form in the areas of maximum glaciation, and are reduced when the come into the zone of recent glaciation.

Oh, also, I've often wondered about the maximum of the Little Ice Age (ca 1650 - 1850) and it's erosional effect. I think it was mostly in the gullies you see. There's a lot of terminal and recessional moraines on mostly north facing slopes. They would have contributed to erosion of those gullies as well, which might confuse the picture of just avalanches, wind, and water on exposed non-glaciated surfaces above the recent glacial maxim.

I would think someone's looked at this. It would be a good way to measure differential erosion over time (hundreds of thousands of years) on Sierra slopes. Original poster is probably long gone, but it seems a great line of research.








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oldranger
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Re: Avalanches in southern SN

Post by oldranger » Sun Mar 24, 2019 11:50 am

In 1986 most of the snowfall occurred during a 2 week period. There were significant avalanches thruout the Kings drainage. Most started high. Then because they were mostly huge, slow wet slabs they uprooted trees as they got down to tree level. Places I remember that were significantly impacted included Paradise Valley, the area between Bubbs Creek and East Lake, and the upper end of Big wet meadow. Big Wet avalanche was particularly significant for me as it looked like a D-9 cat had intentionally destroyed 2 of my favorite campsites. FYI the trail crew counted 500 rings on one (I believe Red Fir) tree taken down by the avalanche between Bubbs Creek and East Lake. My guess from the size of the trees I saw down at both Paradise Valley and Big Wet meadow that some of these trees were at least 300 years old. Also a couple of people in a Sierra Club National Trip lamented the damage caused by the avalanches. My response was how cool it was to see the impacts of the avalanches and view a little bit of the forces that created the landscape.

In 83 I did an Onion Valley to Cedar Grove ski trip. From Videttte Meadow to Sphinx Creek where we had to take off our skis there were several relatively fresh avalanche runout zones from the south facing slopes we had to cross.
Mike

Who can't do everything he used to and what he can do takes a hell of a lot longer!

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Harlen
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Re: Avalanches in southern SN

Post by Harlen » Sun Mar 24, 2019 4:19 pm

Mike wrote:
In 1986 most of the snowfall occurred during a 2 week period. There were significant avalanches thruout the Kings drainage. Most started high. Then because they were mostly huge, slow wet slabs they uprooted trees as they got down to tree level. Places I remember that were significantly impacted included Paradise Valley, the area between Bubbs Creek and East Lake, and the upper end of Big wet meadow.
I also did a lot of trips the summer of 1986. We ran into incredible avalanche scars with very large downed trees in two places in particular:

-The first we encountered was while descending from Laconte Divide into Goddard Canyon. There was a huge swath of flattened forest south of the Hell For Sure route. The avalanche came down the east-facing slope.

-The second area was along the route from Laconte Canyon up into Dusy Basin. This time the avalanche came down the west-facing slope.

I remember it was hell navigating through both of these avie paths due to the great size of the downed trees.

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