My Thesis Area is Misbehaving: 6.0 Earthquake Near Walker, California

 

That was exciting. I live in the Central Valley of California. The valley is famously boring for a number of reasons, and one of the good ones is because we rarely feel earthquakes here. But that wasn't the case today. I was at my computer station and the desk started vibrating and I had to look and see if my wife was shaking it. She wasn't and she was not looking happy. 

It took a while for the USGS to untangle the many wave signatures in the seismic network (there was a brief report of a 4.8 quake just 20 miles from us), but as things have settled out, it looks like the quake took place smack dab in the middle of my masters thesis area in the eastern Sierra Nevada at the small town of Walker, California. The most recent estimate of the magnitude is 5.9 (upgraded to 6.0). The quake has the signature of a normal fault, the kind of faulting to be expected in a crustal region that is being stretched apart. 

Walker is the village in the picture above at the south end of the Antelope Valley, which exists because of faulting. The valley has sunk as the mountains on the right side of the photograph rose along the fault indicated by the solid black line. The epicenter of the quake would be just out of the picture on the lower right side (the picture is looking south).

Antelope Valley sits astride the boundary of the Sierra Nevada and Basin and Range provinces, where the solid block of the Sierra is being sliced up into a series of fault-bounded grabens. The picture below shows the upper (southern) end of the valley from Monitor Pass, with the Sweetwater Mountains in the distance. 

The next picture is from the hill behind Walker looking north. The break in slope on the left is where one would look for evidence of recent earthquakes, but slopewash has covered the fault terraces (scarps) in most places except for the streams and alluvial fans that cross the fault trace. That was what I was searching for when I was doing my masters thesis many years ago in this valley. I was very pleased when I found some.

The person who did the original mapping in the 1950's was working primarily on the rock exposures, and wasn't really looking for recently active faults. By the 1980's a number of people were looking a lot harder, trying to determine the seismic hazard for the region. Fresh alluvial fans provide a possibility of dating the occurrence and size of the last earthquake to cause ground rupture in an area.

The picture below is the Mill Creek fan, at the extreme south end of Antelope Valley. Under normal circumstances, an alluvial fan should be a smooth, gently sloping surface. Here at Mill Creek, the surface steps down to the left, forming a terrace. Immediately after the earthquake this terrace may have been essentially vertical (examples of scarps are shown on this post - Slinkard Valley lies immediately west of Antelope, and the post has a nice cut-away showing the arrangement of the fault blocks).

Scarps like these show that the last major earthquake took place in the recent geologic past, very likely less than 10,000 years ago, and maybe as recently as 3,000 years ago . The length of the fault and the size of the scarps are characteristic of quakes in the range of magnitude 6.5-7.0. A magnitude 5.8 event, the Double Springs Flat earthquake, shook the extreme north end of the Antelope Valley fault system in 1994. Today's quake one-upped that event, but do not be surprised if the magnitude is revised upward or downward (NOTE: the quake was revised upward to 6.0). It takes awhile to fully analyze the seismometer records. It is not inconceivable that some small cracks may appear along some of these older scarps.

I'm listening to reports of a rockfall off the cliffs above Meadowcliff Lodge. That would be very close to the epicenter.

I will revise this post as more information comes in. 

Postscript: I finally got to my office at Modesto Junior College to download the seismogram of the quake, and here it is. The shaking was off-scale for nearly two minutes.

The second shows a compressed version of the quake, along with some of the larger aftershocks.

Can't See the Forest for the Creek; Or is it the Other Way Around? California's Mega-droughts

The eastern Sierra Nevada is home to one of the strangest forests I know of. It's not the species of tree that is odd; they are mostly Ponderosa, a pretty but unremarkable tree which can also be found on the adjacent slopes. What's strange is that these trees are dead. Not the "recent forest fire" kind of dead. It's that they have been dead for the nearly a thousand years. Oh, and they are still standing, and are located in the middle of a good sized river.

The West Walker River drains a rather large region in the vicinity of Leavitt Meadows and Sonora Pass. Several forks come together near the junction of Highways 108 and 395, and the river plunges into a narrow gorge before spreading out into the Antelope Valley near Topaz Lake on the Nevada border. The trees, and there are several dozen, are found at the head of the gorge near the campground at Chris Flat. Studies have found that these trees sprouted, grew to maturity, and died within a relatively limited period of time between about 800 and 1350 C.E. (common era, equivalent to A.D.).

The weird part is that the trees pretty much filled the entire canyon bottom. They don't do well immersed in water, which seems to suggest that there was almost no room for a river channel when these trees were growing. This implies that the river was much smaller and therefore there were two crippling droughts that lasted on the order of 200 years  and 140 years respectively. Once the droughts ended, the river filled the channel with sediment again, perhaps supplemented by outwash gravels from the Matthes glaciation (the Little Ice Age).

These ghostly forests exist in other parts of the Sierra Nevada. Tenaya Lake, Lake Tahoe, and Fallen Leaf Lake all have submerged forests that grew to maturity during these periods when the lakes didn't have enough water to flow through their outlets. Oral histories of California's Native Americans also hint at terrible droughts.

In human time frames, droughts are a fact of life here in California. We had extended droughts from 1928-1934, 1960-61, 1976-77, 1988-92, and most notably, from 2011-2016.  Our population has grown so large that each drought becomes more problematic, and we muddle through on the strength of a few extraordinary precipitation years like 2017 that fill reservoirs for a time. What would we do if another century-long drought were to come? And what role will anthropogenic global warming play? I might not be here to find out, but my grandchildren might have a real struggle.

Feeling a Bit Sheepish: A Bit of Glacial Mutton in the Sierra Nevada

Pothole Dome in Tuolumne Meadows at Yosemite National Park

Sometimes glacial terms represent some kind of logical relationship to reality. "Horns" that are sharp pointed glacial peaks, "cirques" that are circular-shaped glacial bowls, "glacial polish" referring to rock worn smooth by glaciers, or "moraine lakes" that are lakes dammed by glacial moraines. Then there are the others: tarns, eskers, kettles, aretes, and cols, the kinds of terms that cause misery among my first-year geology students. But one term seems to depart from all reality...roche moutonnée

Unnamed roche moutonnée just north of Lembert Dome in Tuolumne Meadows

The term roche moutonnée describes an asymmetrical glacially scoured rock outcrop that has a smooth slope on the side facing the flowing ice, and a steep cliff on the side where the glacier pulled away from the outcrop ("stoss and lee structure" is a related term). The scale can range from a few meters to many hundreds. They are common features in regions of bedrock that have been scoured by massive continental ice sheets, but are a bit less common in mountainous areas eroded by alpine glaciers. These glaciers are the kind that filled valleys, but didn't overwhelm the surrounding peaks, such as those that flowed through most canyons of the Sierra Nevada. Yosemite Valley is the most famous, but glaciers filled dozens of other major river canyons from south of Sequoia National Park all the way north to the Lassen Volcanic National Park region. At their height, these glaciers covered about 30% of the mountain range.

Hiking the gentle slope of Lembert Dome in Tuolumne Meadows

The Tuolumne Meadows region of Yosemite National Park was an odd exception to the usual Sierra Nevada glaciers. The area surrounding the meadows was covered by a 2,000 foot thick mass of ice that was far more like a continental ice sheet than an alpine glacier. The ice spilled over ridges into several other drainages, including that of the Merced River, which carved Yosemite Valley. The addition of ice from the Tuolumne drainage gave the Yosemite glaciers an extra bit of "oomph", allowing them to erode deeper and farther than they would have otherwise.

Looking down the western edge of Lembert Dome. Note the glacial polish on the summit ridge.

There are numerous roche moutonnées in the Tuolumne Meadows area, including the easily seen Pothole Dome (first picture of this post), and Lembert Dome near the Tuolumne Meadows campground (below). The summits of each are easily climbed (from the right direction, anyway), and both provide stunning views of the region.

The problem with roche moutonnée as a geological term is that we geologists can only barely agree on its meaning. It's derived from French, and the "roche" part isn't a problem. It means "rock". But "moutonnée" is the tricky one. It can be translated loosely as "sheep" (think "mutton""), but not exactly (French: "mouton"). Moutonnée (with the extra e's) translates to "frizzy", and is taken as a reference to sheep's wool. The term originated in the 1700s with a naturalist named Horace-Bénédict de Saussure (it would be decades before the term "geologist" was coined) who noted that the rocks looked like a type of wig apparently well-known at the time whose locks were held in place with mutton grease. Except that there seem to be few or no references to wigs that were actually called that (the closest version was a tête de mouton).

So we teachers are left with trying to define the term as meaning "rock sheep" based on the nebulous idea that the rocks look like sheep grazing in meadows. Which they really don't. But it's still easier than trying to describe obscure French wigs from the 1700s and mutton grease.

A few weeks back we were on our field studies trip to the eastern Sierra Nevada, and we descended from Sonora Pass into the drainage of the West Walker River at Leavitt Meadows. I was describing various glacier features and even mentioned the presence of a few ill-defined roche moutonnées upstream of the meadow. You can see them in the picture below. But then I noticed something else in the midst of the meadow. I saw them, and now you are hoping that Geotripper won't go there, but he will...

Yes. It was actual....

...rock sheep! In a meadow.

But I still don't think they look like eroded glacial domes.

For an excellent description of the derivation of the geologic term, check out this great story from the Atlantic.

The Other California (uh, it's in Nevada...): Damned if you do dam, damned if you don't dam

Rivers are often damned by damming. Lakes are often made by dams. But sometimes lakes are damned by dams. And somehow these sentences relate to the title of today's post, which bears a similarity to yesterday's post about Topaz Lake. But the issues of today's post are strikingly serious and relate to the choices we make in our use of natural resources.

When does a lake die? I know that lakes aren't alive, but at what point does a lake no longer exist as an ecosystem that supports life? The answer to a question like this has multiple layers and complexities. There are many different answers. The Dead Sea in Israel/Jordan is the saltiest inland sea on the planet and nothing more complex than bacteria can survive in its waters. It is about as dead as a lake can get short of drying completely. Oil spills notwithstanding, the disappearance of the Aral Sea in four short decades (due to water diversions primarily for growing cotton) is considered one of the great environmental disasters of our times. What these lakes have in common is that they are terminus lakes. "Terminus" refers not to the fact that they are 'dead', but that these lakes have no outlets, and the only way for water to leave the system is by evaporation. Such lakes have unique environmental problems whether humans are involved or not.

There are several terminus lakes in the Basin and Range Province of the American West. Some of the lakes are famous, for instance, Great Salt Lake and Mono Lake. A few are less familiar, and are the subject of today's post. During the Ice Ages, vast amounts of glacial meltwater flowed into the Basin and Range Province, and except for a very few brief connections to ocean-bound rivers, never flowed out. The fault valleys filled with water, forming pluvial lakes. The biggest, Bonneville Lake, covered much of western Utah, some 10 times the area of present-day Great Salt Lake. Lake Manly filled Death Valley to a depth of 600 feet or more, and was 125 miles long. Possibly the least known lake was Lahontan, which covered a vast area of northwestern Nevada (see the map below). When Lahontan began drying up at the end of the last ice age about 13,000 years ago, just two river systems were big enough to support permanent lakes, the Truckee out of Lake Tahoe, and the Walker River out of the Sonora Pass region. The Truckee supplies Pyramid Lake, while the Walker supports Walker Lake, near Hawthorne, Nevada.

Map from Circular 1267 of the U.S. Geological Survey

Water is, and will always be a contentious issue in the arid western states. If water flows, somebody will have a use for it, and usually there are many somebodies, and the total use of the water will almost always exceed the average yearly flow of the river. Upstream users usually get first dibs to the water rights, and anyone sits at the end of the stream or river is lucky to get any leftovers. Such is the case with the Walker River and Walker Lake.

As outlined in several of the preceding posts, the West Walker River (and an equally interesting East Walker River) flows from the alpine mountains, through practically inaccessible gorges, and then across a series of wide and deep fault basins filled with sediment and fertile soils. The first Europeans to explore the region were after gold and silver. Hot on the heels of the miners came the pioneering ranchers and farmers, and lands were tilled in the Antelope, Smith and Mason valleys (see the USGS land-use map in the diagram below; full-scale map available here). Walker Lake, near Hawthorne, Nevada, was the end of the channel for the Walker River. So much water was diverted for agriculture that the lake level began dropping immediately, from an elevation of 4,083 feet in 1872 to 3,930 feet today. This represents a drop in volume from nearly 9 million acre-feet in 1872, to about 1.5 million acre-feet today. Total dissolved solids (TDS) in the lake increased from 2,500 parts per million in 1882 to about 16,775 ppm today, mostly in the form of salt (seawater has about 35,000 ppm). The lake that remains is 12 miles long, 5 miles wide, and only about 78 feet deep.

So, why worry about a 'dead' terminus lake in the middle of the desert? It's just that it wasn't, and isn't yet dead. There were four native species of fish living in the lake when settlement began: the Lahontan cutthroat trout (Oncorhynchus clarki henshawi), tui chub (Gila bicolor), speckled dace (Rhinichthys osculus), and Tahoe sucker (Catostomus tahoensis). Other species were introduced later on and thrived at first. Today, only one native species still survives in the lake waters, the tui chub, but it's continued success at reproduction is now in doubt. The original race of Lahontan cutthroat trout disappeared, but a different population was reintroduced, and is hanging on for now.

These fish, and the various forms of algae, diatoms, and insects in the lake are the basis of a complex food chain that includes thousands upon thousands of local and migratory birds. Mono Lake, with a much higher concentration of salt, hosts a food chain as well, but it involves only brine shrimp and brine flies to feed the birds, because nothing else can survive the waters. Walker Lake is a far more diverse system, but that diversity is ever more threatened.

Terminus lakes like Walker are highly sensitive to climate change. During the ice ages, the lake was a part of a huge freshwater lake that stretched across northwest Nevada. At other times the lake dried or nearly dried up with TDS much higher than seawater. The ecosystem of the lake responded to those changes, and when fresh water dominated, fish and other creatures would re-colonize the lake from their refugia in the Walker River or in freshwater springs within the lake. What has changed during the human history of the lake is the introduction of contaminants from mining or agricultural runoff, and the rapidity of the dessication of the lake.

Efforts are beginning to try and stabilize the situation at Walker Lake, mostly in the form of purchasing water rights from upstream users to increase the flow of fresh water in the downstream parts of the river. You can find out more here. A detailed analysis of the ecosystems of the entire Walker River system can be seen here. If I were a TV reporter, I would be closing this post with the words "Time will tell". But it doesn't seem that there is a whole lot of time.

One last, and totally unrelated note. If you look closely at the Google Earth image above at the terrain around the town of Hawthorne, you can see a network of roads and widely spaced buildings. It's an eerie place to visit, especially if you are a child of the Cold War era. The buildings are ammunition bunkers, widely spaced so that an atomic bomb couldn't destroy them all at once. The first time I visited, I felt like I was at the center of a giant bullseye target.

The Other California: Dammed if we do dam, dammed if we don't dam!

I thank my father for the title, which derives from a college term paper he wrote many years ago, although today's post is about a different place, Topaz Lake, on the border of California and Nevada just east of the Sierra Nevada province. It lies at the north end of Antelope Valley, which I explored in the previous post in my series on the "Other California". I also thought of calling this post "A River Runs Through It (but not exactly)".

Antelope Valley is a half graben, a valley formed as movements dropped the valley along a curving fault surface, forcing the West Walker River to flow along the mountain front. The fault system becomes complicated at the north end of the valley and some low peaks, the Gray Hills, interrupt the otherwise flat topography of the valley floor.

Prior to 1921, the basin between the Gray Hills and the Sierra was occupied by a dry lake bed called Alkali Lake (or Rickey Lake). The West Walker River was blocked from flowing into the lake by the alluvial fan that had built out from Slinkard Creek (at the bottom of the GoogleEarth photo). Early ranchers in the region, especially the owners of the vast Rickey Ranch, wanted better control of the water for irrigation purposes, and devised a scheme to divert the West Walker into Alkali Lake for storage. Their plans precipitated a water war with other ranchers, especially California cattle baron Henry Miller downstream and the litigation eventually reached the Supreme Court. The Rickey Ranch lost, but ironically the other ranches decided he had a great idea, and a levee was built to divert the river into the dry lake bed. A tunnel was built at a low point in the Gray Hills to control the water level. So, Topaz Lake is a reservoir without a dam! The reservoir has about 65,000 acre feet of "dead water" and about 59,000 acre feet of usable storage space. The outlet tunnel is at the low point in the hills on the far side of the lake in the picture below.

The flood of 1997 keeps coming up in my recent posts. It was an extraordinary event in which the river peaked at almost double the size of all previous floods, at 12,000 cubic feet per second. This flood, and an earlier smaller event in 1986 revealed a serious weakness of the reservoir system. Because of channel blockages below the diversion, there was no way to stop floodwaters, and especially debris from flowing into the lake, causing the lake to rise far above normal levees, and flooding some residential areas. Modifications to the levee system are currently being made (the inlet canal is marked by the line of Cottonwood trees in the picture below).

Topaz Lake today is fishing destination, and if you are one of those really desperate gambling junkies, it has one of those casinos that sits almost on the state boundary with Nevada. It also has some nice views of the mountains, and now that you know what you are looking for, it is a wonderful place to see the results of some heavy duty fault action.

One more post to go on the West Walker River series. The next post will involve the end of the river in Nevada, but the repercussions extend into our fair state of California.

The Other California: The West Walker River and the Antelope Valley

The West Walker River, the latest entry in our "Other California" series, changes character many times on its journey from the headwaters in the High Sierra to the end of the river in the desert at Walker Lake in Nevada. It begins as a glacially carved valley, plunges into a deep river-cut gorge, and then it flows out into a wide open alluvial plain, the Antelope Valley. Unlike the erosion that is taking place upstream, this is a place where the river deposits material. In other words, Antelope Valley was not carved by a river at all. As can be seen in the GoogleEarth image below, the use of the river changes as well: upstream is protected watershed and recreation/wilderness country, but the Antelope Valley is given over to agricultural development.

Notice in the image above how asymmetrical the valley is: on the left (west) it has a sharp boundary with the mountain flank; on the right (east), the valley slopes gradually up into the foothills of the Sweetwater Mountains. It is the expected shape of a fault-formed valley, a graben (really a half-graben, since the major faults are on one side, the left). Antelope Valley sits astride the boundary of the Sierra Nevada and Basin and Range provinces, where the solid block of the Sierra is being sliced up into a series of fault-bounded grabens. The picture below shows the upper (southern) end of the valley from Monitor Pass, with the Sweetwater Mountains in the distance.

Upon hearing that this is a fault valley, most people might ask if the faults are still active. In a word, yes. Besides the youthful appearance of the mountain front (look at the sharp slope break in the photo below where the valley is sliding down relative to the mountain slope), the river has been forced to flow mostly on the west side of the valley, where the valley has dropped the most. The break in slope is where one would look for evidence of recent earthquakes, but slopewash has covered the fault terraces (scarps) in most places. Except for the streams and alluvial fans that cross the fault trace. That was what I was searching for when I was doing my masters thesis many years ago in this valley. I was very pleased when I found some.

The person who did the original mapping in the 1950's was working primarily on the rock exposures, and wasn't really looking for recently active faults. By the 1980's a number of people were looking a lot harder, trying to determine the seismic hazard for the region. Fresh alluvial fans provide a possibility of dating the occurrence and size of the last earthquake to cause ground rupture in an area.

The picture below is the Mill Creek fan, at the extreme south end of Antelope Valley. Under normal circumstances, an alluvial fan should be a smooth, gently sloping surface. Here at Mill Creek, the surface steps down to the left, forming a terrace. Immediately after the earthquake this terrace may have been essentially vertical, much like the scarps formed in April in Baja California (examples of scarps are shown on this post from last month - Slinkard Valley lies immediately west of Antelope, and the post has a nice cut-away showing the arrangement of the fault blocks).

Scarps like these show that the last major earthquake took place in the recent geologic past, very likely less than 10,000 years ago, and maybe as recently as 3,000 years ago (the latest incarnation of the state fault hazard map is in agreement). The length of the fault and the size of the scarps are characteristic of quakes in the range of magnitude 6.5-7.0 (USGS analysis is here). A magnitude 5.8 event, the Double Springs Flat earthquake, shook the extreme north end of the Antelope Valley fault system in 1994.

Earthquakes are not the only hazards associated with the Antelope Valley. I had a personal experience with the massive flood of 1997 that I chronicled in a post last year. The flood was twice the size of the previous record flood, and resulted in a new channel being formed. The Army Corps of Engineers put the river back after a few weeks. I have video of the event, which I hope to find and post as soon as I can.

If you visit the Antelope Valley in the fall, you may have a unique and increasingly rare experience. The very first time I saw the valley, in 1976, I was on my second geology field trip. We were in a hurry to get somewhere, and we rolled into the valley at a high rate of speed, only to be brought to a complete stop by a ... stampede. Well, ok, to us flatlanders it was a stampede. It was actually a very large herd of cattle being directed down onto the valley floor to spend the winter after fattening up in the adjacent high country. Oh, and I must say that the 100-year-old Cottonwood trees along the highway at Coleville and Topaz have a wonderful blazing yellow glow when the weather is turning cold and the leaves are turning.

The Other California: An Enigmatic Gorge, the West Walker River Canyon

The West Walker River Gorge

Rivers used to be such simple things. I learned it in third grade. Water evaporated out of the ocean, fell on the mountains as rain or snow, and carved a canyon on the way to the sea. And then I found out it wasn't quite that straightforward. With a few exceptions (like rapidly growing volcanoes), mountains don't suddenly appear and then start eroding. Erosion is going on all the time the mountains are rising. And then one finds that few rivers are simple: they do strange things sometimes, like flowing right through mountain ranges. Or changing directions in illogical ways. Lots of rivers in my corner of the world just end, in the middle of the desert, or on the floor of my Central Valley. Some, especially in limestone country, disappear unexpectedly and then reappear somewhere else.

West Walker River Gorge from GoogleEarth, north at top

And so it is with one of my favorite rivers, the West Walker in the Eastern Sierra Nevada. I started my brief exploration last time at the headwaters of the river, where the glaciers had sculpted the river canyon. The valley was wide and open and the river meandered across open meadows for the most part. But near the junction of Highways 108 and 395, the character of the canyon abruptly changes. It plunges into a deep declivity, and in the next 12 miles it quite literally cuts through a mountain. It then flows into the wide fault graben of Antelope Valley and eventually crosses two more mountain ranges in Nevada before ending in the salty waters of Walker Lake.

How can a river cross a mountain range? There are several ways: the river may have been present before the mountain range, and as the mountain range rose, the river cut downwards, keeping up (down?) with the uplift (an antecedent stream). Or the mountains were originally buried under sediment, and a river flowing across the sediment quarried it away, exposing the mountains underneath (a superposed stream). One river can cut into a mountain range by way of headward erosion, eventually breaching the range and capturing a river on the other side (stream piracy). Or something exotic, like the river flowing under the mountain through caverns, where the caverns eventually grew and collapsed, forming a valley (sounds weird, but each of the last two have been seriously suggested as an origin for the Grand Canyon).

Large landslide in upper West Walker River gorge

So what happened to form the West Walker River gorge between Sonora Junction and Antelope Valley? It is a geologically young valley compared to the relatively gentle erosional surfaces that surround it. There are odd abandoned drainages here and there, and pirated stream channels. The orientation of the river seems most strongly influenced by both glacial activity and fault movements. Faults in the region have a strong north-south orientation and recent activity is clearly indicated by late Pleistocene or Holocene scarps at Sonora Junction and Antelope Valley at either end of the gorge. Any possible recent activity in the gorge itself has been obscured by erosion and slope wash, so the precise role of faulting is difficult to discern. The west canyon wall is generally higher than the east wall, so faulting very well may have guided the location of the river.

Severe bank erosion along West Walker River (large landslide in center distance)

The most recent glaciations (the Tahoe and Tioga) did not reach into the gorge, but the much older (~800,000 years BP) Sherwin glacier flowed the entire length of the canyon, leaving a train of erratic boulders as much as 1,200 feet above the present-day river. Glaciers certainly have a penchant for changing the orientations of rivers, and that probably happened here, but the present channel does not have any of the usual features of glaciers that could pinpoint the precise location of the glacial pathway, as vigorous downcutting in the last 800,000 years has removed any direct signs of the passage of ice in the inner canyon. Just the boulders on the canyon rim.

Severe erosion of alluvial fans along the river from 1997 floods

Whatever the origin of the gorge, the constriction of the river in this narrow and deep v-shaped canyon has geologically hazardous consequences for those who live and travel in the area. The only place for a highway is right next to the river, and there really isn't room for both of them. In 1997, a flood of catastrophic proportions removed most of the highway, and reshaped the canyon walls in many places. Rockfalls are common, and a few immense rockslides are present at several locations. A number of disastrous wildfires in recent years have led to mudflows. And the fact that much of the canyon is continuously in shadow means that black ice can linger on the road surface far longer in winter than on other parts of the highway. Even in dry conditions, the sharp curves lead to accidents. A bus rolled over here in 1987, killing 21 passengers.

On the other hand, the gorge is spectacular, and has many interesting mysteries. There are campgrounds, picnic areas and some really great fishing holes as well. Check it out!

The "Other California" is a continuing series about the less familiar places in our state with interesting geology. Our next post will be on the Antelope Valley, the downstream end of the West Walker in California.

The pioneering studies of the region :

Blackwelder, E., 1931, Pleistocene glaciations of the Sierra Nevada and Basin Ranges: Geological Society of America Bulletin, volume 42, pages 865-922.

Clark, M. M., 1967 Pleistocene glaciation of the drainage of the West Walker River, Sierra Nevada, California: Ph.D. Thesis, Stanford University, Stanford, CA, 170 pages.

Halsey, J. G., 1953, Geology of parts of the. Bridgeport, California and Wellington,. Nevada quadrangles (Ph.D. dissert.) Berkeley, University of California.

The Other California: The Other Side of the Sierra, Down The West Walker River

Ask most people their image of the Sierra, and you will hear about Yosemite Valley, Sequoia, granite, and the Mother Lode. Ask them about crossing the Sierra, and they may mention Tioga Pass or Donner Summit. There is a lot more to the Sierra, and this week I am going to highlight some of those places. The "Other California" series is about these kinds of places, the lesser-known spots of geological interest.

There are a total of four paved crossing of the Sierra Nevada between Yosemite's Tioga Pass and Donner Summit. Sonora Pass is crossed by Highway 108 out of Sonora and Pinecrest, and reaches an elevation of 9,624 feet, making it the second highest crossing of the range, just short of Tioga's 9,950 feet. The road drops into the drainage of one of California's little treasures, the West Walker River.

The West Walker is one of the longest rivers in the eastern Sierra Nevada. Most east-side rivers drop steeply and within a few miles disappear into the desert of the Owens Valley or Mono Basin. Because of a structural glitch in the range, the Walker runs mostly north through a relatively unbroken stretch of mountainous terrain before flowing into the Antelope and Smith Valleys in Nevada, and finally ending at Walker Lake, a bit more than 100 miles downstream.

The upper reaches of the West Walker are a bit different than other parts of the Sierra Nevada. The mountains are not composed of granite, but instead are somber red and gray volcanic rocks extruded in Miocene time, around 14-6 million years ago. They are assigned to the Relief Peak formation and the Stanislaus Group. They were originally mapped in the 1950's (by my thesis advisor for one), and are the object of ongoing research today (see below for an example).

Near the summit of Sonora Pass there is an interesting outcrop where the volcanic rocks lie in direct contact with the Sierra granites. There were canyons here that were filled when the volcanoes began erupting. Considering that granite forms several miles deep in the crust, that surface between volcanic and granitic rock represents the disappearance by way of erosion of perhaps four miles of rock between the late Mesozoic around 80 million years ago and the Miocene, about 14 million years.

Evidence of glaciation abounds in the high country in and around Sonora Pass. The West Walker River had the longest glaciers of any east-side canyon, with the longest reaching around 20 miles downstream. The Leavitt Valley, below, is a beautiful glacial trough. The main West Walker has its headwaters several miles above the point where the Highway 108 winds down a steep canyon wall.

The smaller glacier coming from Sonora Pass could not carve as deep a valley as the main glacier, so it forms a hanging valley, with a pretty cascade called Leavitt Falls visible from a pullout. It's a good spot to stop and let your brakes cool off. As long as I mention brakes, the Sonora Pass highway is not for the faint of heart in its highest reaches. Most cars do just fine, but a big RV would be a nightmare, not just for the driver, but for everyone else who has to make their way around it. In other words, this is one of my favorite paved roads in the state!

Highway 108 passes the Marine Base at Pickle Meadows and ends at Highway 395. At this point the river has a distinct change of personality, and that will be the subject of my next post.

Busby, C.J., et al., 2008, The ancestral Cascades arc: Cenozoic evolution of the central Sierra Nevada (California) and the birth of the new plate boundary, in Wright, J.E., and Shervais, J.W., eds., Ophiolites, Arcs, and Batholiths: A Tribute to Cliff Hopson: Geological Society of America Special Paper 438, p. 331-378. (Forrest, I assume this is your father?)

Friday's Fun Fotos: the Fluvial Forest Discussed

I posted some pictures of a "fluvial forest" this morning (fluvial=river) asking for an explanation about how these trees came to exposed in the river channel, and in short order got three excellent answers. I've provided a larger format picture above to give some perspective on the setting (click on the image to see the bigger picture). This is the West Walker River, a few miles downstream from the junction of Highway 395 and Highway 108 from Sonora Pass in California's Sierra Nevada.

It's rather striking and strange to see so many trunks of mature trees sitting in the active channel, as Jeffrey pine really can't tolerate soaking for more than a few weeks a year. They otherwise do quite well on the shady slopes to the right in the picture above (and not so well on the dry slopes to the left). I have to admit that this was my thesis area, but I didn't really give the trees much thought 25 years ago, yet they reveal an interesting story.

Anne had a great idea, saying that mining wastes could have caused the filling of the channel, smothering the trees. This would be a great explanation, and such a thing indeed happened in the Sierra Nevada during the hydraulic mining era of the Gold Rush (I posted a picture of a tree trunk formed this way some time back). The problem is that no mines of any consequence were present in the area.

Rob provides a glacial explanation, in which glacial outwash buries a forest, and then a surging glacier planes off the trunks. This is an appealing explanation because this part of the canyon was very close to the terminus of the Tioga and Tahoe glaciers. Glaciers would explain it, except the age of the trees are wrong. One set of the trees in this fluvial forest grew between AD 900 and 1100, while another set grew between AD 1210 and 1350. Smaller glacial episodes in the last two thousand years never approached the area.

Lockwood comes the closest to explaining what happened here, suggesting that a debris flow filled part of the canyon, killing and burying the trees, and then the river exhumed the forest. But there is an interesting twist: the valley is really narrow here (look at the top photo; there is barely enough room for the river and the highway, and when the river floods, there is no room for the highway at all. It disappears and has to be rebuilt, most recently in 1997). The trees, and there are dozens of them, were pretty much filling the entire canyon bottom. There was almost no room for a river channel when these trees were growing. This suggests that the river was much smaller in the two time periods the trees represent. In other words there were two crippling droughts that lasted 200 and 140 years respectively. Once the droughts ended, the river filled the channel with sediment again, and debris flows, as Lockwood suggests, undoubtedly had a role. Outwash from the Matthes glaciation may have been a factor as well.

There is confirming evidence for such mega-droughts in other areas of the Sierra Nevada. Tenaya Lake, Lake Tahoe, and Fallen Leaf Lake all have submerged forests that grew to maturity during these periods when the lakes didn't have enough water to breach their outlets. Oral histories of California's Native Americans also hint at terrible droughts.

In human time frames, droughts are a fact of life here in California. We had extended droughts from 1928-1934, 1960-61, 1976-77, and 1988-92. We may or may not be ending a three year drought this year as well. Our population has grown so large that each drought becomes more problematic, and we muddle through on the strength of a few extraordinary precipitation years that fill reservoirs for a time. What would we do if another century-long drought were to come? And what role will anthropogenic global warming play? I, for one, wouldn't particularly care, because I will be dead, but my grandchildren might have a real struggle.

Stine, S., 1994. Extreme and persistent drought in California and Patagonia during mediaeval time. Nature 369:546-549. I can't find a copy of Stine's paper on the web, but Stine's findings are discussed briefly here (page 15-16), and here (page 296)