A New Debris Flow near Bridgeport

Debris Flow

Last weekend, while visiting the Bodie Hills for an outing with CNPS, I camped at Robinson Creek Campground, southwest of Bridgeport (Mono County, CA), near Twin Lakes. Walking out of the campground the evening of my arrival, I immediately noticed a very fresh-looking debris flow of mud, cobbles, and larger rocks issuing from a gully just across the valley. Being a fan of Sierran geomorphology I had to take a closer look.

Above: The flow covered at least 30 acres. Near the outer edges, it was all fine clay and silt, just a few inches thick.

Below: Looking far up the east side of Sawmill Ridge, you can see that something gave way on the left (south) side of this unnamed ravine, then gathered material and momentum as it raced down a very steep slope.

Debris Flow

Just below the bottom of the photo above, the material may have slowed down a bit and accumulated some approaching the top of a (Tahoe-stage?) moraine before hurtling again down through the existing steep cut in the moraine to the alluvial fan on the valley floor below.

This event obliterated the channel that previously followed a fairly straight path out from the cut in the moraine and several new channels have appeared heading to the northeast, east, and (pictured below) the southeast. Time will tell which becomes the primary channel for the next few centuries.

Debris Flow

A few very large boulders (like this one, below) that were already situated on the fan below the moraine, appear to have been unmoved as this latest debris flow moved around them. Some shrubs even benefited from the shelter provided by these larger boulders.

Debris Flow

In the mid to upper reaches of the flow across the alluvial fan, there are mud/gravel/cobble deposits of different textures and composition following different paths and sometimes overlapping each other (see below). These suggest there were a few different pulses of activity as materials of different viscosity made their way off the mountain.

Debris Flow

Approaching the narrow mouth of the cut in the moraine, the size of the rocks and the depths of both erosion and deposition increase markedly.

Debris Flow

Looking into the narrow, lower end of the moraine cut (below), this may have been where the volume and velocity of the debris flow were greatest. That tree trunk may have traveled some distance down the mountain.

Debris Flow

Much of the vegetation in this gully was thrashed, uprooted, and moved downstream, but the remaining plants will recover in a few years.

Debris Flow

At its deepest (below), the fresh incision through this gully appears to be about 20 feet deep.

Debris Flow

Debris Flow

Debris Flow

How fortunate there were no homes or campsites in the path of this debris flow!

So when, exactly, did this happen? I asked several people who were in the area at the time. None of their stories agreed (except that it was very loud and woke up a lot of people). It was before dawn (between 3:00 and 4:30 am, perhaps), on a Friday morning (perhaps) in late June (most likely), 2017. This was 2 or 3 weeks before these photos were taken on July 7–9.

P.S.: I wouldn’t hike up here during the next several summer rain storms. Although it’s hard when dry, much of this debris flow looks very unstable still, and will erode a lot more—perhaps suddenly at times—before it all settles into place.

Here’s a Google Map of the location:

Copyright © 2017 Tim Messick. All rights reserved.
See also Tim Messick Photography and the Bodie Hills Plants blog.

About Tim Messick

Photographer, cartographer, and botanist/naturalist. Home is in Davis, California. Home-away-from-home is the eastern Sierra Nevada. Compiling a flora of the Bodie Hills.
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