As noted in the last post, the sediments have yielded up a fascinating collection of fossils over the years, including mosasaurs, plesiosaurs, ichthyosaurs, and even the fragmentary remains of a few dinosaurs. The valley was a seaway for the better part of 200 million years.
It is no longer a seaway. In the last few million years, California's tectonic framework underwent huge changes. The subduction zone was extinguished in the central and southern parts (subduction continues in the north state). A new plate boundary emerged: the San Andreas transform fault. Compressional forces along bends in the fault line raised the Coast Ranges, and the seaway of the Great Valley slowly gave way to river deltas, and then alluvial fans. In the last five million years, the Great Valley has been a terrestrial environment, with numerous river channels, gigantic lakes and vast grasslands. Large herds of grazing animals wandered the plains, pursued at times by terrifying predators (look for them in the next post of the series). Countless millions of birds made use of the wetlands during their long seasonal migrations.
The biggest and most far-ranging change in the Great Valley during the last five million years may have been that which took place only in the last 150: agricultural development. Millions of acres of grasslands, lakes and wetlands were drained and plowed. The rivers were dammed and diverted into artificial watercourses that took them hundreds of miles from their natural channels. Thousands of wells were drilled that brought prehistoric groundwater to the surface. Parts of the valley subsided thirty feet or more as the water was withdrawn. A mere 5% of the valley floor retains its natural character. Most of the animals are gone, shot to extinction (the California Grizzly Bear, for instance), or pushed to marginal environments in the Sierra Nevada or the Coast Ranges. The migratory birds are crowded into a few precious wildlife refuges up and down the valley.
Still, living here does have some benefits. Given our journey through the most dangerous plate boundary on the planet, the Great Valley doesn't exactly look geologically menacing. It is relatively far away from the most active earthquake faults of the state, and is even farther away from the most dangerous of the state's volcanoes. We don't get hurricanes, violent thunderstorms or blizzards, and tornadoes are exceedingly rare. The idea of dangers from landslides is laughable (except that I know of at least one fatality in Modesto caused by a slide). But we do have two threats: droughts and flooding.We are in the midst of the worst drought in recorded history, but with the development of an El Nino weather pattern in the Pacific, we could have catastrophic floods in only a few months. We can never really predict what can happen. In 1861-62 the floods were widespread that the state capitol had to be removed to San Francisco for a few months. The valley was a gigantic lake twenty miles wide in places.
|Flooding on the San Joaquin River in 2006.|
The map below shows what could happen if we get a repeat of the 1861-62 floods, an event the meteorologists call an atmospheric river storm (ARKStorm). They also refer to this as the "other big one", because the potential for damage is greater than that of a major earthquake, at least in the Great Valley.