Saturday, January 21, 2023

A Short Primer on Mass Wasting, Courtesy of California's Atmospheric River Storms

I live in California's Great Valley, known to some as the plain old "Central Valley", and most know it as a very flat place. A VERY flat place. Over the four-hundred-mile length of the valley elevations barely rise above 300 feet above sea level, and much of the valley is floodplain. As we emerge from the unrelenting series of atmospheric river storms that dropped near-record (and some record) amounts of precipitation all over the state, one might assume the greatest problem here is flooding. Some areas have indeed been hit very hard, and lives were lost.

One might be surprised to hear that even though the rivers rose, some areas were less affected by the flooding. In the case of my home county, Stanislaus, there were (and continue to be) problems along the lower reaches of the Tuolumne and San Joaquin Rivers, but on the east side of the valley there were few ill effects. Along my usual walkway, the Tuolumne River Parkway Trail in Waterford, the damage was of a type not often associated with a flat valley floor: mass wasting, or mass movement.

The reason has to do with a quirk of the geological history of our region. During the Pleistocene ice ages over the last two million years, glaciers covered perhaps 30% of the Sierra Nevada on repeated occasions. The ice never reached the Great Valley, but the streams of ice ground up vast amounts of rock to sand and mud, and the rivers were swollen with muddy meltwater. Rivers like the Tuolumne and Merced built up vast alluvial fans that resulted in higher elevations near the mountain's edge, on the order of a few tens of feet. That doesn't sound like much, but when the glaciers ebbed, the muddy rivers turned clear, and the rivers began to erode into those old alluvial fans, forming terraces and bluffs.
On the one hand, these bluffs and terraces have protected towns like Modesto and Turlock from river flooding because even the worst of floods cannot overtop the bluffs where most of the region's cities are located. On the other hand, the bluffs are steep and are composed of loosely consolidated sediments. That's the ideal recipe for mass wasting, the downhill movement of loose debris and rock under the influence of gravity. I got an excellent introduction to a variety of mass wasting events after the final storm last week. It was a mess along the trail.

Mass wasting happens because of gravity, but an overaccumulation of water can substantially add to the intensity and degree of movement. The movement takes three forms: falls, flows, and slides. I saw examples of all three this week.

In the picture above, there was so much water built up in the soil that the slope failed rapidly and the fluid mix of silt and water flowed and covered part of the trail below. This is called a mudflow. In different circumstances, especially involving glaciers and erupting volcanoes or desert cloudbursts, mudflows are one of the most dangerous forms of mass wasting. A single volcanic mudflow in Colombia in 1985 killed some 25,000 people. 

A short distance away, the slope was more coherent, but water had added a great deal of weight to an already steepened slope (from the carving of the trail itself), and the slope failed as a single mass that slid downhill as a slump (above). Slumps are usually much slower-moving than a mudflow and thus rarely kill anyone. But they can do considerable damage to homes, roads and other developments. The slump shown above is inconsequential, but I saw a much more serious problem a short ways down the trail... 
The town's water treatment plant has been built on a lower terrace next to the Tuolumne River at the base of the steep bluff. A paved access road was necessary, and they carved it into the slope, oversteepening the upper slopes, and putting additional weight on the slope below the road. A slump has begun forming right next to the road, and is ominously slipping an inch or two a day so far. I don't know if it will stabilize now that we've had some dry weather, but they are going to have to do some mitigation work in coming weeks.
The over-steepened slope above the access road has always been a problem, as rockfalls have been a constant, if minor, problem even in dry weather. The rains made the problem far, far worse, and after the final storm, the road was a real mess. There had been some wild tobacco shrubs whose roots helped hold back the rock, but they could do little to stabilize things in the face of intense rain.

Mass wasting consists of flows, falls, and slides, but one of the most pervasive and efficient forms of mass-wasting is almost mundane in the face of all the drama seen above. Over time all exposed surface weather and develop into a loose ground cover called regolith. If the regolith can support plant life, it is referred to as soil. If any slope exists at all, the soil and regolith will move move downhill imperceptibly over many months or years. Soil creep is not dramatic, but in the big picture it probably moves more material than any other form of mass wasting. It never kills anyone, but it will deform and bulldoze structures built into the slope over time. It's why old barbwire fences on hilly country roads always seem to be tilting over. It can even tilt telephone poles.

Soil creep was not much in evidence as a result of the storms, but it is clear that the trail builders knew it would be a problem over time. That's why many sections of the trail have walls built on the uphill side of the trail, to hold back the process for awhile (see below).
In one week, my modest hiking trail showed off nearly all the major forms of mass wasting, with the only exception (thankfully) of a debris avalanche that is capable of wreaking serious havoc, and solifluction, a form of creep known from artic environments. How did things play out where you live? I've heard a lot of stories of serious damage coming from around the state. I hope you've avoided the worst of it.

Sunday, January 1, 2023

Dry Creek: Anatomy of a Flood (One of Many Across California Today)

 

After days of gloomy and wet weather, New Year's Day dawned bright and sunny, and we couldn't resist driving out into the California prairie to have a look at the beautiful landscape. The streams across the prairies east of Modesto were full and flowing in a way we haven't seen for a number of years. And that's the problem of course. 

This creek, normally dry, was just one of many dozens of tributaries to Dry Creek, which is itself an unregulated, undammed tributary to the Tuolumne River. This entire area received upwards of five inches of precipitation in the last day or two, and all the water had to go somewhere.

I included a picture of Dry Creek in my post yesterday, when it was flowing at about 600 cubic feet per second (cfs). I am including another picture below, taken at the same time, but from an angle that shows the pasture to the left. I knew that more water would be coming downstream, maybe as much as 1,500 cfs, an amount that would actually be more, by a wide margin, than the main drainage in the area, the Tuolumne River.

That's not quite what happened...

When we crossed the Dry Creek Bridge north of Waterford today, the creek was running at 6,000 cubic feet per second, more than ten times the flow of the previous day. Take a look below at what happened to the pasture (not to mention all the shrubs and brambles at the base of the oak trees).

By the time we arrived in the prairies in the afternoon today, most of the floodwaters had subsided in the upper watershed, but we could see evidence everywhere that a significant flood event had taken place. Rocks were strewn across the roadways, and every watercourse showed evidence of having been feet deeper the previous day. One bridge we crossed would have been four feet underwater during the height of the storm. 

The flood hydrograph below tells the story. The data is taken from a stream gage downstream in Modesto. The bar graph at the top shows the pattern of the rainfall in the storm up in the watershed, and the subsequent rise of Dry Creek. Notice how the rise of the creek lagged behind the precipitation. This so-called lagtime makes sense because it takes time for the water to gather into the tributaries and then to flow the twenty miles or so downstream. Lagtime represents the critical hours that residents downstream can prepare for the oncoming flood.

Wouldn't it be nice if there were a government entity that could monitor all rivers and all flood events so that when such events unfold, there could be timely warnings? Perhaps even keeping records of storms over the course of a century or more, so that specific warnings could be made about the timing and the expected intensity of the oncoming flood? Unlike earthquakes, floods can be predicted, and there are in fact government institutions that are tasked with this job, mainly the United States Geological Survey (across the entire US), and the Department of Water Resources specifically in California.


Which brings us to the handy-dandy bottom portion of the hydrograph. The blue line on the graph is what happened already. The pink line is the prediction. We have another intense storm coming on Wednesday, and after dropping to around 300 cfs, Dry Creek is going to rise again to at least 6,000 cfs and maybe more. Isn't it nice that we have several days warning? That's just one huge example of the value of science in our society.

Of course, no one is perfect, and all models and predictions can be affected by unknown and unexpected factors. The storm this week offers one tragic example. Although most streams and rivers behaved more or less as predicted, the Cosumnes River defied the predictions and produced record flooding, well beyond the predicted levels. 

What went wrong? Those who do science fully understand that errors happen, and it their goal is to understand the reason for such errors. The factors in the Cosumnes River flooding are being analyzed and may include an unexpected slowing of the storm front causing increased precipitation, two or three broken levees, and the Caldor Fire of 2021 that ravaged much of the watershed upstream. If you want to follow the analysis, check out the Weather West blog by Daniel Swain (@Weather_West on Twitter).

So, there is my science homily for the day. But we were out to explore some nature, and in any case, we need to appreciate the gifts we have been given. The day, a respite from a long series of expected storms, was beautiful. 


Mountain Bluebirds are not common on the valley floor, but we saw a small flock along the road.


An American Kestrel is a sharp-looking small member of the falcon family. This one remained perched near our car for a few moments.


Bald Eagles are not especially abundant in the region, but we found one. So had an 'unkindness' of Common Ravens, and they were making their displeasure known to the eagle.

And finally, an old horse seemed to appreciate the sunshine. The horses were brought to the continent by the Spaniards in the 1500s, but they actually have a long heritage here. They evolved in North America tens of millions of years ago! They migrated across the Bering Land Strait and spread throughout the world, but for some reason went extinct along with many other large mammal species in North America about 12,000 years ago.



Saturday, December 31, 2022

Year-end Look at the California Water Situation

 

It's the last day of 2022, and the third year of a stunning drought in California. It's been raining pretty much across the state during December, and there are hopes of alleviating the drought a bit. Let's hope so. The map above from the California Data Exchange Center gives a pretty clear idea of the situation. Reservoirs across the state are for the most part well below normal, and in the case of the biggest (Shasta, Oroville, etc.), still ominously low. I hope to revisit this diagram in a few weeks and see some changes, but we will see.

The long-term predictions earlier this year were for a continuation of dry conditions, so the current onslaught of atmospheric river storms is somewhat of a surprise, albeit a welcome one. But of course, one has to be wary of what one wishes for. There are flood watches up all around the state as one more storm will blow through to end the year.

There is also the cautionary tale of the previous rain year. We had some record storms in October and December last year, and things were looking great, but then January and February were about as dry as can be. I recorded a mere 0.08 inches in those two months.

Still, this year has some promise. From the weather station in the Geotripper backyard 13 miles east of Modesto, we've had 7.14 inches of precipitation in December, the highest total in the 32 years that I've been keeping statistics (I've recorded more than 5 inches in six different years, but never more than 6). Even with the earlier dry months, we should round out 2022 with about 8.6 inches as we move into the critical months of January and February when most of the precipitation should happen.

Despite the rain, I felt a need to check out our local barometer of runoff conditions. The Tuolumne River cannot serve in this capacity because of the numerous reservoirs upstream that very carefully control the daily flow levels. I checked instead at Dry Creek, of which there are many in California. This particular Dry Creek has its headwaters in the lower foothills of the Sierra Nevada Mother Lode, and flows for about 40 miles before joining the Tuolumne River in Modesto. Ironically, there is almost always some water in Dry Creek, obviously from rain runoff during the winter season, but also from irrigation overflow during the dry summer months. Without any substantial flood control structures, it is a good measure of runoff conditions during storms.


The creek was running about 600 cubic feet per second when I got this picture, which is about twice the current flow of the reservoir-controlled Tuolumne River. As can be seen from the discharge graph from the USGS Water Resources site, it has already been well over 1,500 cfs a couple of times in the last week. I haven't seen this much water in the creek in a couple of years.


Of course, the local picture is not the most important statistic. Everything in California's water infrastructure depends on the snow conditions, especially in the Sierra Nevada. Although these atmospheric river storms we are experiencing derive from tropical sources and are warmer than we might want, the current snow conditions are promising. Check out the report:

Let's hope this keeps up. A lot of forests, rivers, animals and people are depending on it.

What are conditions like in your region?


Wednesday, December 7, 2022

Believing in the Occult: The Occultation of Mars, anyway.

How often do you get to see something new, something you have never before seen? I got to have that experience this evening.
Some folks believe in the occult. I believe in a kind of occultation, but it involves our Moon and the planet Mars. This evening the full Moon, the Cold Moon, passed in front of the planet Mars. At that moment, the Earth, the Moon, and Mars were almost perfectly aligned.
The event unfolded over about twenty minutes from the time I got the camera out. At first Mars was easily seen, but as it got closer, the light of the full moon faded Mars out, but the camera had no problem.
I was photographing this with my handheld Panasonic Lumix DC-FZ80 with a 60x zoom. So the finer details weren't visible, but I bet some of the telescope rigs set up around the planet got some good features on Mars. I was happy to see the disk shape of the small planet!

I'd love to have captured the re-emergence of Mars on the other side, but I had to teach my night course. Such is life, but it sure is fun to see something for the very first time!

Saturday, November 19, 2022

Why did the Road Cross the San Andreas Fault? 20 Years of Geologic Change (a new Update)

2002
I've been leading geology field studies trips to lots of places in the American West for 30 years and started to take digital pictures in 2001. I sometimes struggle to find new things to photograph when I visit a place for the 30th time, but in some cases it is not a problem. There are geologic changes that happen on a yearly basis, and with twenty years of photos (minus two due to Covid), the changes become obvious. This is a continuing update from a post in 2013, and I'll probably continue updating for the foreseeable future.
2004
Highway 25 in the California Coast Ranges connects the town of Hollister with the access road to Pinnacles National Park (formerly Pinnacles National Monument). Along the way the highway crosses the San Andreas fault in a section where the fault creeps an inch or so each year (36°35'54.27"N, 121°11'40.19"W). Most years we've stopped to have a look at the effect the movement has on the pavement. In 2002 and 2004, the damage was obvious.
2008
By 2008 someone had patched the road, and no fault motion was evident.
2009
Little damage was evident in 2009 either. But by 2010 cracks had begun to appear as the fault stressed the pavement.
2010
The fact that the fault creeps in this region is a good thing. It means that stress is not building along the fault surface, but instead is being released gradually. The sections of the fault to the north and south of the creeping section are locked by friction, and are building up the ominous stress that will eventually produce quakes with magnitudes in the range of 7.5 to 8.0. The quakes are coming and we need to be as prepared as possible.
2012
By 2012, the road had been completely repaved, and  yet the shearing was already evident.
2013
It became even more pronounced by 2013 and in 2014. Just by chance, the person working as a scale was the same individual as in 2004.
2014

In 2015 the fractures were moderately larger. They'll need to start thinking of road repairs before long.
2015
In 2016 Laura once again provided scale, as she did in 2014 and 2004.
2016
Here in 2017, long-time trip volunteer Mary provides scale. The cracks in the road are just a bit larger, but they didn't look appreciably different than the previous year except for a twist (pun intended).

2017
On Dec. 2, 2018, the break to my eye seems more continuous. It's now been six years since the road was completely repaved.
2018

Last year the paint was deformed (twisted), but not split (below).
2017
The offset paint strip reminds me of illustrations of elastic rebound theory, the idea that stress builds up on a fault line over time. In that model, the land on either side of the fault is distorted over time until the frictional resistance is overcome and the rock snaps back to its original shape. That won't be happening with the paint. Last year in 2017 I said "if they don't repair the road (as they often do; see above), it will probably show a clear break by next year." Here's what transpired:

First, a close-up on 2017's center stripe...
2017
And here's how it looked on Sunday, Dec. 2, 2018:
2018
As predicted, the break in the paint is complete...

In 2019 (those last few halcyon days before Covid) long-time volunteer Paul provided scale (he has been assisting MJC with field trips for 25 years!). The crack continues to grow, and I wouldn't have been surprised if it was patched by next year.
 The paint on the center strip is split even more.
November 2019
And then Covid happened and for a few years we were not able to conduct our field studies classes. Today we made a return visit with our students and here is the current condition of the highway. It doesn't appear that any repairs have been conducted yet. Our host is once again Laura, who was with us back in 2004 and subsequent years!
November 2022
Fault creep is not a constant. I didn't see a whole lot of change over the last three years, although I didn't get as many close-up shots. Here's a closer look with Paul, our other long-time volunteer. What do you see that is different?
November 2022

These little changes that happen at a rate visible in human lifetimes add up to huge changes when multiplied by thousands or millions of years. The nearby eroded volcano of Pinnacles National Park has been displaced 195 miles (315 kilometers) in the last 20 million years or so by movement along the San Andreas.

Sunday, October 30, 2022

How It Was Today: Fall in Yosemite Valley

How it ended...

How it began...
I woke up late this morning. Mrs. Geotripper was finishing breakfast and asked if I'd like "to go up the river a little ways and find some fall colors". I had lots of grading to get through, so obviously I said yes, and around 11 or so we jumped in the car and headed up the Tuolumne River to see what we would find. There wasn't much, at least not in terms of fall color. So we went a little farther...
We followed Highway 132 up past Coulterville and on to Greeley Hill at a bit above 3,000 feet. Still not much in the way of fall color. So we went a bit farther up the hill and found ourselves at the Yosemite National Park entrance station at Big Oak Flat at 4,000 feet. We finally found a bit of color, although not at the intensity that a few more days of cold weather might bring. 
And we had made it this far, and Yosemite Valley was only 15 miles away. How could we pass it up? 
So on we went into the awesome gorge of the Merced River and into the valley itself. There was the first look at the distant cliffs of El Capitan (left), Half Dome (center), and Sentinel Rock and Dome (right).
Closer at hand were a lot of ripening acorns. The bears and woodpeckers will be happy.
We reached the valley floor and started our tour of Yosemite's greatest hits. We were surprised to find some wispy curtains of water flowing over Bridalveil Falls. The 620 foot-high waterfall is a classic example of a glacial hanging valley. The main trunk glacier flowing through Yosemite Valley was able to erode a much deeper trough than the small glacier in Bridalveil Creek, so the floor of the creek was left hanging high above the main valley floor.
It was not a cold day, mostly in the sixties, but the sun was intense. It brought out what colors there were in the oaks and dogwood trees.
We didn't see a great many varieties of birds, but there were some Acorn Woodpeckers busy collecting acorns and hiding them in tree "granaries". Such trees can have tens of thousands of drilled holes that can hold a single acorn each. The birds live in loose family groups who search for and guard their food supplies.
Yosemite Valley is not a 'typical' glacially-carved valley. Most such valleys have a U-shaped profile, and are relatively straight. Yosemite Valley is characterized instead by bold cliffs that extend out into the valley with dark recesses in-between. This is the result of having eight distinct intrusions of granitic rock, ranging in composition from 'true' granite to granodiorite, tonalite, and diorite. They differ from each other in the proportions of the minerals quartz, potassium feldspar, and plagioclase. They also vary in the amount of dark minerals they contain including biotite mica, hornblende, and a little augite. They also vary in their pattern of fracturing (jointing), and this is expressed in differing vulnerability to erosion by ice, water, and mass wasting (landsliding and rock falls).
Sentinel Dome (above) is a good example. It is composed of fairly resistant Sentinel granodiorite, but it is jointed and thus forms a somewhat narrow high cliff that looms over the valley.
Yosemite Point on the other hand is composed mostly of unjointed El Capitan granite and forms a wide bold cliff. Sometimes people sort of 'miss' this incredible cliff because much of the time there is a stunning waterfall pouring off the west flank of the precipice (the dark mark on the left side in the picture below). That waterfall is only fifth or seventh highest waterfall in the world, and is known by the moniker of Yosemite Falls, measuring in at 2,425 feet. It wasn't actually dry today, but one needed binoculars to see the small trickle at the top of the cliff.
The autumn season is one of the best times to view Half Dome from the middle of Sentinel Bridge. The Merced River is flowing at a low ebb and the still waters make for memorable reflections. Half Dome is another example of an unjointed monolith of granitic rock called the Half Dome granodiorite. It is the youngest of the igneous intrusions exposed in the valley, with an age of about 84 million years. The Sentinel granodiorite is about 88 million years, and the El Capitan granite around 103 million years. These dates fall within the Cretaceous period, which means that when these molten masses were intruding the crust, there were dinosaurs wandering the surface four or five miles above. The dinosaurs would have experienced occasional volcanic eruptions when some of the intruding magma escaped to the surface.

In the years since, erosion has removed the miles of overlying rock and dumped it into the Central Valley or the waters off the coast of ancient California. The region seems to have been eroded to a low elevation landscape that was later uplifted to form the modern Sierra Nevada.
We wandered around Cook's Meadow and stopped into the store at Curry Village to replenish my t-shirt collection. The sun was starting to get low, so we made our way west to our favorite evening viewpoint, Valley View.
Valley View is almost a secret to Yosemite visitors because it has a small parking lot (maybe room for ten cars) at a blind curve so that if you are in the right-hand lane you might miss it. Since the road is one-way at that point, you would have to repeat a five mile loop to get back. The small parking lot is a blessing because it limits the size of the crowd. It's a quiet spot to enjoy the fading light on the cliffs of El Capitan and the Cathedral Rocks. The river usually flows slowly here making for memorable reflections of the cliffs above. We enjoyed the few moments of peace, and then headed home.
And that's the way it was today...