The Things We Don't Think About: The Aftermath of a Major Earthquake from a Great Valley Perspective

The Great Shakeout is planned for Oct. 16, 2025, at 10:16 AM. 

Source: U.S. Geological Survey

We Californians live in earthquake country. The state is riddled with fault zones, and many of them are more than capable of producing damaging quakes. Some of the state's biggest cities are built on or adjacent to active faults, and much attention has been devoted to the probabilities and outcomes of major quakes in those cities, with predictions of possibly thousands of fatalities and hundreds of thousands of injuries. The dystopian models predict legions of newly homeless people, and badly disrupted services and ruptured transportation lines. It's grim reading. Government organizations are cognizant of the danger, and billions of dollars have been spent on preparing for such quakes, including reinforcement of vulnerable infrastructure, and educating the population about the dangers.

But what about the outlying regions of the state, somewhat removed from the most damaging aspects of the coming quakes? What is life going to be like after the fabled "BIG ONE" strikes? That's what I want to write about, since that's where I and many of my friends and acquaintances live. What's going to happen to us here in the Great Valley, in towns like Modesto and Turlock?

Source: Earthquake Country Alliance

First, the good news: the threat of severe shaking is considerably less in much of the Great Valley, with certain important exceptions (see the map above). Much of the valley floor is located tens of miles east of the most dangerous faults like the Hayward or San Andreas. The expected shaking will lead to some damage, but for the most part, buildings and infrastructure (roads, rails, pipelines) should fare reasonably well.

Liquefaction damage in San Francisco, 1906. Source: USGS

The sediments of the Great Valley can have a tendency towards liquefaction, the phenomenon of turning into "quicksand" during shaking, causing buildings to partially sink and severing buried pipelines and cables. This is especially true along river floodplains, including the San Joaquin River and within the Sacramento Delta (citizens of Tracy and Patterson need to take special note). This is the result of a shallow groundwater table. Luckily, much of middle and eastern parts of the valley have a deep water table, and liquefaction is less likely in the dry sediments.

Here's what happened last time in 1906. Source: Wikipedia open domain

So...let's imagine the worst has happened. A 7.8 magnitude earthquake has struck in the Bay Area, and devastation is widespread throughout the region. Hundreds, perhaps even several thousand people have died, and many tens of thousands have been injured. There are widespread power outages, and pipelines have been broken in many places, leaving many without water. Streets and freeways are blocked by debris and collapsed bridges, and emergency services are overwhelmed. Several hundred thousand homes have been rendered uninhabitable. Fires are burning in many areas. Dozens of aftershocks, some as great as magnitude 6, cause additional damage and panic. Life has taken a catastrophic turn, and millions are affected in profound ways. There's much uncertainty now as the population faces concerns of water, food, shelter, and danger from further seismic activity. It's the time that we find out if years of planning and preparation have been effective. Emergency services swing into action.

Agricultural fields in the western part of the Great Valley near Patterson (my photo)

Meanwhile in Modesto the situation is quite different. Although heavy shaking is felt by literally everyone, the sensation is more of a rolling motion, not the fierce movements that brought down bridges and unreinforced buildings in the Bay Area. Some chimneys are shaken loose, cracks occur in some older buildings, and items fall from shelves throughout the city. Some people are hurt by falling debris, but deaths are probably few in number. Electricity goes out due to the disruption statewide to the state power grid. The outages mean that water pressure can't be maintained, so faucets are dry throughout the city. Cell phone towers may have been damaged, and those that aren't are soon overwhelmed by phone traffic (it has been said that social media spreads earthquake news faster than the seismic waves themselves).

In the immediate aftermath, life has become inconvenient. The power is out, the water is off, sirens are blaring all over the city, but for the most part it's a time of waiting. Waiting for the power to come back on, the water to flow, and the phones to start ringing again. It could take days, maybe even a week or two. All things considered, for most of us it is an interruption.

But there are larger consequences with this incredible earthquake in places like Modesto. 

Emergency services will be largely unavailable in town. Ambulance and EMT crews will pressed into service in the Bay Area, and local hospitals will be filled to overflowing with the injured (many Bay Area hospitals will be overwhelmed and maybe even unusable). The same will be true of firefighters and perhaps law enforcement personnel as well.

Many Bay Area freeways will be impassable, but emergency supplies must still reach the region. Interstate 5 may still be open, but Highway 99 will become a more critical lifeline than it is today. Traffic will increase. With refinery production in the Bay Area interrupted, gasoline will become scarce. Expect huge price increases and severe shortages at filling stations across the state.

As much as I don't like to think about it, most people will not have prepared for the eventuality of earthquakes. They won't have supplies of water and food and other materials on hand, so local store shelves will quickly empty out, and with a severely disrupted supply chain, those shelves will not quickly refill. There will be shortages. Restaurants and fast-food outlets will likely be closed for extended periods.

It's a given that our local community, our officials and authorities and our relatively undamaged infrastructure will be an important part of recovery efforts in Central California. My own institution, Modesto Junior College, has plans in place for sheltering and supporting thousands of people made homeless by the quake. They will likely be our guests for weeks or months. Many other schools and institutions have similar plans. The influx of hurting and anxious people will no doubt place a huge strain on our infrastructure, and I fervently hope we can rise to the occasion, just as I would hope that others will look upon us with compassion in case of catastrophic flooding in our valley (that's an entirely different subject for a future blog).

It's going to be a challenging time, and we can respond to it in two ways:

We can ignore the entire possibility of large earthquakes, and go on living our lives as if nothing will ever happen. That is frankly what most of us are doing now. It's seemingly worked well so far. We are of course living among fault zones that have been building up stress for a century or two, and they are all capable of producing damaging earthquakes right now. I don't think of this as a good choice. Not at all.

An aerial view of a devastated San Francisco shortly after the earthquake in 1906 (USGS)

Or...we could prepare. Take the possibility of a major quake with the intensity you would give it if you knew it was happening a few days. Gather supplies of food, water, and first aid supplies right now, before the shelves emptied by desperate quake refugees and victims. To the extent possible, keep your gas tank closer to full than empty. Have emergency go-bags ready in case you have to evacuate your home or community. Develop an emergency plan for your family, and maybe your neighborhood. Learn about the known seismic hazards in the place where you live. Check out my previous blog for more links and ideas. There is a great resource centered on Central California published by the California Earthquake Authority, the Earthquake Country Alliance, and the California Governor's Office of Emergency Services, that can be reached by clicking here.

It would be nice if we could know in advance that a quake is about to hit, but we don't. Since that's the case, we should be as ready as possible.

Source: USGS

Landslides and Slope Mitigation in California's Great Valley...Wait...What?

 

I used to talk to my students about the geological hazards that we face as inhabitants of California's Great Valley (or Central Valley, for those who don't appreciate its actual greatness). I would go down the list of things to worry about: earthquakes, droughts, wildfires, volcanic eruptions, flooding, and so on. But then I somewhat jokingly described things we didn't have to worry about such as hurricanes (Florida's problem), tsunamis (a problem for coastal cities), tornadoes (Oklahoma's problem), and mass wasting (also known by the generic term 'landsliding').

Unfortunately, over the years I've become aware that some of those unlikely hazards actually can be a factor in living in the valley. We've had a fair number of tornadoes in recent years, including two that came within a few miles of my house (they weren't anything like the monsters of Tornado Alley in the Midwest, but still a bit scary). A powerful tropical storm hit Southern California last summer that came up just short of being a hurricane, and the heavy downpours were statewide. And then there is mass wasting (slope failures and landslides). I know of at least two fatalities caused by mass wasting in the last few years. One was a homeless person who had dug a tunnel into a river embankment that later collapsed, and another was a person who was driving along a freeway in heavy rains when the freeway embankment collapsed as a mudflow and spread across the lanes causing a fatal accident.

The Great Valley is famously flat so mass wasting doesn't seem to be much of a danger to those who live here, since landslides and other slope failures require, well, a slope to happen. But the valley is not quite so flat as people may think. The valley is 400 miles long, and most of it is close to sea level. Much of it is low-lying river floodplains, but other sections sit at slightly higher elevations because of complex history of climate change and glacial ice ages over the last 1,000,000 years. These bluffs and terraces protect my city and others nearby because even the worst floods are contained within the floodplains and do not spill over onto the terrace surfaces where cities like Modesto and Turlock have been built near the Tuolumne River.

During the ice ages glaciers never reached the valley floor, but meltwater from the Sierra Nevada glaciers swelled the rivers to several times their average flow, and they carried tremendous amounts of muddy sediment that spread widely across the valley floor building up alluvial fans. When the glaciers receded, the rivers flowed less, but carried clear water that was more capable of eroding the soft sediments of the alluvial fans, forming channels and floodplains several tens of feet deep. Once these channels developed, floods never covered the terraces again. It's the bluffs that form the boundaries of these terraces that provide the conditions that can result in slope failure.

The heavy rains of 2022-23 led to widespread flooding across many parts of California including some real problems on the Tuolumne River Parkway Trail when I regularly go birdwatching. I wrote about these in January of 2023 in the aftermath of one of the biggest storms. The most serious problem was the access road to our town's water treatment plant. It's on the river floodplain about 60 feet below the river terrace. Slumping had caused major cracks to form in and near the pavement.

Eventually the rains subsided and the soil dried up. The slide seemed to stabilize, but the threat to the roadway remained and would eventually have to be dealt with. That is what was new this week: the cranes and were in place to start the slope mitigation process.

The main problem is that the access road traverses unstable debris and soil that slumped in the 2023 event. They would need to re-engineer the slope by rebuilding it from scratch. Their strategy was complicated by the fact that all the equipment and materials had to traverse the very road they were trying to repair. Truckloads of heavy boulders were going down the road every few minutes. Meanwhile a huge long-reach excavator was digging away at the slope below the road!

After digging away and smoothing off the slope they covered it with felt matting and then started piling many tons of boulders on the slope. The boulders are intended to buttress the slope and hopefully keep it stable during future weather events.

It's a lot of work being done to keep a single paved road open, but it's a pretty important road since it provides the only access to the water-treatment plant for the city of Waterford. And thus we are dealing with slope mitigation in what is supposedly the flattest place in the country!

A Rare Day for the Great Valley: The Sierra Crest, and a bit of Half Dome

California's Great Valley is many things: one of the most important agricultural regions in the world, America's Serengeti Plains where millions of migratory birds spend the winter, and one of the most polluted air basins in the country. The pollution is a shame, causing all manner of health problems for those who live here, and obscuring the incredible mountains that ring the valley. Except on a few select days out of the year. Today was one of those days.

Gray, Red and Merced Peaks in Yosemite National Park

We had a fairly intense storm last night, and it was followed up by windy cold conditions that cleared the air. I took my customary walk along the Tuolumne River Parkway Trail. A section of the trail follows the bluff and on days like this, I get a wonderful panorama of the Sierra Nevada covering Yosemite National Park. A zoomed-in look provides a view of Gray, Red and Merced Peaks in the southern part of Yosemite (above).

A shift to the north reveals the high peaks of the Sierra Crest, including Mt. Maclure, Rodgers Peak, and Mt. Lyell, three of the higher peaks in Yosemite National Park (Mt. Dana, the 2nd highest, isn't visible). Some of the foreground peaks are rather famous as well. Mt. Starr King and Sentinel Dome are visible, and at the very left edge of the photo is the top portion of Half Dome! It's hard to pick out, but the CalTopo provides some identification of the different peaks. 

Source: https://caltopo.com/view#ll=37.6344,-120.7662&e=15&t=n&z=9&c=0.31,85.35

There are better views of Half Dome from the valley floor (the intersection of Oakdale-Waterford Highway and Keyes Road is much clearer on days like this). I couldn't get any pictures today, but here's one from a different day (see some more at https://geotripper.blogspot.com/2019/02/it-was-that-kind-of-day-half-dome-from.html). 

A wonderful day.

A Great Valley View of the Eclipse

Today's eclipse at the greatest coverage of the Sun, about 80%

There was an eclipse today! This is perhaps not entirely news to my North American readers, as the eclipse was visible to some extent across the entire United States. The eclipse was not a total eclipse, as the Moon was farther from the Earth, and could not completely cover the disk of the Sun. This is called an annular eclipse. Those on the main path of the eclipse were treated with the "ring of fire" effect, as the edge of the Sun formed a perfect ring around the Moon. I didn't have the opportunity to be in that area, so I've settled for the "near miss" of about 80% coverage of the Sun.

To see the eclipse, one needed merely to go outside and hope it wasn't cloudy, but Mrs. Geotripper and I wanted to make the day a bit more special, so we headed out to the San Luis National Wildlife Refuge where we could see things in a setting resembling the primeval environment of California's Great Valley. We got to observe some of the early-arriving migrant birds like the Sandhill Cranes, Greater White-fronted Geese, and Snow Geese.

Sandhill Cranes at the San Luis National Wildlife Refuge

We were a bit curious if the bird behavior would be affected by the darkening skies, as sometimes happens with total eclipses, but we didn't notice anything unusual.

The entire affair unfolded over about two hours, so we drove the auto-tour route at the refuge, stopping to take pictures of the progress every fifteen minutes or so. This was far less stressful than my two experiences with total eclipses when totality lasted only a few short minutes, and much had to happen to get pictures. Our experience with the total eclipse in 2017 was stressful (and crowded), yet spectacular. Today was peaceful and we had the refuge to ourselves.

The sky must have darkened when 80% of the solar disk was covered by the Moon today, but it wasn't really noticeable. It happened so gradually. When we experienced a total eclipse in Baja in 1991 (see the pictures and story here), at totality the sky turned dark as night, stars became visible, and the temperature dropped around 20 degrees in mere moments. 

As the last hour progressed, more and more of the Sun became visible, and soon it seemed like a relatively normal day. The dragon had not consumed the life-giving Sun, and life could go on!

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.

The Prairies of California: the Most Extraordinary Landscape (and Ecosystem) in California

The Tuolumne River on the east side of the Great Valley

I admit this statement is a hard sell, especially among those who live here, but hear me out: the Great Valley prairie is the most extraordinary landscape and ecosystem in California. 

The Great Valley prairie remnant east of Modesto

"But", they will say, "the valley is boring. It is just one long flat plain full of farms, feedlots, oil derricks, and urban hellscapes. It's dusty and the air is bad. And there is nothing to do." And from their point of view, their statement is valid. But that's not the part that I am talking about. I'm talking about the valley that once was, and more importantly, the 5% of the valley that is not given over to agriculture and urban development. It is that small bit that teaches us what once was. 

I was thinking a lot about the original valley today.

Prairie and wetlands at the San Luis National Wildlife Refuge south of Turlock

The occasion was World Migratory Bird Day, Global Bird Weekend, and eBird's October Big Day. The purpose of the day is to gain a one-day snapshot of the world of birds, and birdwatchers all over the world contributed observations. For myself, I decided I wanted to capture as much of the primeval Great Valley prairie as I could, so I selected three habitats for the day: the valley wetlands and marshes, the riparian environment, and the dryland prairies that once filled the spaces between the rivers and wetlands. In the end Mrs. Geotripper and I explored the Merced National Wildlife Refuge and the San Luis National Wildlife Refuge, and in the afternoon I walked the Tuolumne River Parkway Trail (as I often do). As the sun began to set I took a drive through the prairies between the Tuolumne and Stanislaus Rivers east of Modesto. In all we saw nearly 60 species of birds.

One of the most extraordinary facts about the prairie lands of California is that they ever existed at all. The Great Valley is 400 miles long and 30-40 miles wide, and with the exception of a single volcanic center at Sutter Buttes, is as flat as any place on Earth. The entire American West is characterized by rugged mountainous topography, including the Coast Ranges and Sierra Nevada of California between which the Great Valley is sandwiched. Such a large mostly flat region is akin to finding a serene quiet lake surface in the midst of an intense hurricane.

The origin of the Great Valley lies in the violent tectonics of the subduction zone that existed off the coast of California for at least 200 million years. A region of oceanic crust was being driving beneath the western edge of the North American continent, forming a trench and a deformed mass of sediment called an accretionary wedge. Parts of the accretionary wedge were buoyed upwards to become the core of California's Coast Ranges.

As the slab of ocean crust sank into the Earth's mantle, portions of the plate melted and formed vast plutons of magma that melted their way through the overlying continental crust, forming an extensive volcanic chain called a magmatic arc. The deeper parts of the magmatic arc eventually cooled to become the granitic rocks of the Sierra Nevada.

Between the geologically violent wedge and arc though was a shallow sea that simply collected sediments washed off the land. This sedimentary trough is known as a forearc basin. As the sediments accumulated, they pressed the crust downwards, providing room for additional sediments. Eventually these silt and sand layers reached a thickness of 25,000 feet or more. These sediments became the floor of the Great Valley.

Source: National Park Service

The valley was below sea level for most of its first 140 million years, but in the last four or five million years the floor of the valley was lifted slightly above sea level to become the plain it is today. It is dry in large part because the adjacent Coast Ranges act as a barrier to incoming storms from the Pacific Ocean. It is literally a desert in the southern portions from Fresno to Bakersfield, but the dry plain is crossed by numerous rivers flowing out of the Sierra Nevada.

In antiquity the valley was a vast prairie, a grassland stretching 400 miles from what is now Bakersfield to Redding. The drylands were broken up by a system of rivers, marshes and wetlands that supported groves of oak, cottonwood, and sycamore and vast thickets of willows and shrubs. These plants supported a rich fauna.

Until roughly 12,000 years ago, the prairies were populated by millions of grazing animals including horses, camels, deer, elk, bison, and even elephant species like mammoths and mastodons. This megafauna was preyed upon by a terrifying roster of predators including saber-tooth cats, 
lions, cheetahs, dire wolves, and bears, including the California Grizzly and the even larger short-faced bear. The cause of the extinction of most of these species is still debated, but the prime suspects are climate change and overhunting by newly-arrived humans. A minority view argues for an asteroid impact that primarily affected North America.

Sandhill Crane and Great Egret at the Merced National Wildlife Refuge

In the aftermath of the extinctions, the valley still supported a robust ecosystem. The mild winters provided shelter for millions of migratory birds, and the grasslands were grazed by elk, deer, and antelope. Predators included wolves, coyotes, foxes, mountain lions, and bears. The Tule Elk were thought to have been hunted to extinction in the years after the Gold Rush, but a small remnant of the species survived on a single ranch at the south end of the valley. The California grizzly and wolves were eliminated in the early 1900s. 

Tule Elk bull at the San Luis National Wildlife Refuge. We didn't see any today so I borrowed a shot I took in 2016

Today the remaining original prairie and wetlands environments are present due to sheer luck, and from concerted efforts by many environmental organizations and government agencies. A series of national wildlife refuges have been established like a string of pearls up and down the valley to provide habitat for the millions of migrating cranes, geese, swans and other species.

The White-faced Ibis is a tropical species that looks almost black until the sun shines on the iridescent feathers. The white face appears in the breeding season.

Some grass prairies are preserved in state parks and national monuments (most notably Carrizo Plains National Monument). But most persist because they are on cattle ranch property. These private lands are threatened by the rapid expansion of almond groves into the foothills. In my county alone, tens of thousands of acres have been planted, with no secure irrigation sources for the thirsty trees. I'm pretty sure they'll be abandoned within a decade or so as the groundwater they are using dries up.

Dry Creek, shown here, has earned its name this year, although it is capable of serious flooding in wet years. An invading and misguided almond orchard can be seen at upper right.

The riparian habitats have their own problems. The river woodlands are among the least-affected environments as attempts to develop them as farmlands tended to fail because of repeated flooding. Still, much damage was done in the attempt, and current efforts are directed at replanting native species in floodplains that have reverted to government or conservancy ownership.

The Tuolumne River near the Sierra Nevada foothills.

It is the rivers themselves that face the biggest challenges. There is no more contentious issue in the Great Valley than water use and distribution, especially in severe drought years. All of the major rivers are dammed and strictly controlled except for the occasional exceptional flood. The Tuolumne has been kept at a minimal flow, enough to maintain a string of river water into the delta. The slow-moving water is subject to warming to a dangerous level for river species, and the expansion of invasive species like river hyacinth. Through it all the river is a beautiful walk at all times of the year.

Sandhill Cranes at the San Luis National Wildlife Refuge

It was a serene day, but it was also the calm before the storm. The migratory birds are beginning to arrive in greater numbers, and soon these pools will be filled with Snow and Ross's Geese, Tundra Swans, and Aleutian Cackling Geese. There is nothing quite like seeing a flock of 10,000 geese take off all at once. It becomes so easy to imagine what once was in this most extraordinary place.

California's Rarest Ecosystems: The Prairies and Vernal Pools (Part One of a Two-part Miniseries)

The lower foothills of the Sierra Nevada are one of the most unique and almost universally ignored parts of California. Caught between the famous and heavily-visited forests and alpine regions of Yosemite's high country and the utilitarian agricultural fields of the Great Valley, the lower foothills are made of soils too poor to be developed, and too "plain" to be noticed by people rushing up the highways to the mountain resorts. For much of the year they seem barren and lifeless. But these prairies are one of the most precious of California's diverse ecosystems.

The prairies of California once extended the length of the Great Valley, a 400-mile-long grassland supporting millions of grazing animals, birds, and predators, including Tule Elk, Pronghorn, Deer, Grizzly Bear, Mountain Lions, Coyotes, Foxes, and numerous rodents, reptiles, amphibians, fish, and insects. In earlier times (more than 10,000 years ago) there were camels, horses, bison, giant ground sloths, Columbian Mammoths, Saber-tooth Cats, and Dire Wolves.

There were also modern-day wolves, and it is ironic and coincidental that a wolf returned this week to the Great Valley for the first time in a century. It is a young male from Oregon who made an epic journey through Modoc County into the Sierra Nevada near Yosemite Valley, and then into the Sierra foothills of Fresno County, and across the Great Valley (including two freeway crossings). At last report it's now in Monterey County (it's wearing a radio transmitter). 

The prairies were covered with native grasses that were quickly replaced by invasive species when European colonizers arrived and started herding sheep and cattle. Farms eventually replaced the ranches, and the remaining prairies, no more than 5% of the original land cover, were relegated to the fringes where the soils were poor and no water was available for irrigation. Those remaining prairies form a belt through the foothills of Sierra Nevada.  A narrow remnant can be found along the Coast Ranges and the Carrizo Plains west of Bakersfield.

The prairie soils in the Sierra Nevada foothills are notably poor in nutrients because they have not been part of an active floodplain in tens of thousands of years, and the fertile components have been leached away, leaving behind reddish iron oxides and thick clays. Differential settling and wind deflation has left small hollows and depressions that collect rainfall during the relatively rare winter storms. The clay prevents the water from seeping underground, so pools will persist for weeks or months at a time. These pools are the basis for one of California's rarest ecosystems, the vernal pools.

The vernal pools are islands of diverse life in the otherwise barren grasslands. Numerous species of plants and wildflowers, amphibians, and invertebrates have evolved to survive the weeks of water and the months of dryness. As the waters evaporate away, one species of plant after another germinates, grows, and goes to seed. The pools are often rimmed in colorful zones of different flowers.

I'm writing about the vernal pools today because the limitations of travel in the times of a pandemic have kept me pretty close to home. I live on the edge of the prairie between the Tuolumne and Stanislaus rivers and I was out there today. A few pools are left, and flowers are blooming across the region. The views are expansive, extending to the snow-capped peaks of the High Sierra.

The pools and plants provide food and cover for multitudes of bugs and other wiggly organisms, so the pools are trafficked by a large variety of birds, and dozens of species can be identified by discerning and observant individuals, and occasionally by me as well. 

I saw a half-dozen American Avocets in one pond a few weeks ago.

The prairies are one of the best places to see the rare and threatened Burrowing Owls. The area around Crabtree, Willms, and Cooperstown Roads are among the most dependable places to see them in our region.

Savannah Sparrows are a common species, and they'll occasionally cooperate with photographers.

The Western Kingbirds seemed to arrive from their winter homes in Mexico all at once last week. I saw none a week ago, and dozens today, including this one that hung about long enough for a picture.

I have no picture, but I came upon a Gopher Snake in the road that was in serious danger of being run over. I stopped, looked at it carefully to make sure it wasn't a rattlesnake, and then I picked it up to put it in a safer spot. I was suddenly reminded of how Gopher Snakes sometime pretend to be rattlers! They can flatten their normally narrow heads into the triangle shape of a rattler, and will strike in a similar manner. I didn't hold it for long...how do you explain to the EMTs that you picked up a rattlesnake that you thought was something else?

If you are lucky enough to live near a California prairie, this is a good time of year to explore them. If you tend to rush through this unique part of the Sierra Nevada and the Great Valley on your way to Yosemite or some other mountain destination, slow down and give these precious and unique lands a quick look. You won't be disappointed.