Saturday, October 9, 2021

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.


Anonymous said...

I don't know if this is a good question or not. The view of the valley east of Modesto in your picture has small hills. The model for the forearc basin is a shallow sea which explains why its so flat. Sediments settled. Why wouldn't the land to the east of the great valley be flat like an alluvial fan? Runoff channeling river paths is one explanation, but most paths of rivers seem to join in a larger channel at some point. Perhaps in comparison I could bring up the mima mounds you post about occasionally. Are the hills east of Modesto like those? Unexplained? I always appreciate your posts, photos, and explanations.

Garry Hayes said...

The original shape of the forearc basin has been much altered by subsequent events in the last 30 million years. Much of the sediments from the ocean floor have been pushed up by mountain-building processes to form the eastern margin of the Coast Ranges. On the east side, the forearc basin sediments were covered by sediments washing off the Sierra Nevada, and the the land was uplifted and gently tilted to the west, causing the rivers to flow faster and to erode through the older sediments, forming the hills that we see today.