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Rising Seas And The Delta Part 1: How The Delta Got Where It Is

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Now flooded with seawater, the Carquinez Strait was once a bottleneck that shaped the prehistoric Delta | Photo:Chris deRham/Flickr/Creative Commons License


An explanatory series focusing on one of the most complex issues facing California: water sharing. And at its core is the Sacramento-San Joaquin Bay Delta. Stay with kcet.org/baydelta for all the project's stories.

The Delta of 2015 looks very different from the Delta of 1815, as a century and a half of American society has harnessed the landscape for its own uses. The Delta of 2115 will look even more different. Though the science is complex enough that climatologists have been reluctant to offer precise predictions, it looks as though we can count on sea level rising by a meter by 2100, perhaps by significantly more.

That's a problem for any community at sea level, whether that community is human or ecological in nature. Along the California coast we can expect to see major economic damages, loss of shoreline habitat, and disruption of our lives. But in the Delta proper, the very features of the landscape that created the Delta in the first place make it supremely vulnerable to disruption from sea level rise.

Put it this way: Sea level rise may take the present-day Delta away from us, but it was sea level rise that gave us the Delta in the first place.

There had been wetlands in the Delta before, part of a vast inland sea that filled most of the Central Valley up until around half a million years ago. Runoff from the Sierra Nevada, which were far more snowy than they are these days, flowed down into the Valley the way it does now.

But unlike today, those westside Sierra rivers didn't flow to the sea through the Golden Gate. Geologists think these days that up until about two million years ago the combined Sacramento and San Joaquin rivers -- often called the California River in scientific papers -- flowed out to sea into what is now Monterey Bay, carving the gigantic Monterey Submarine Canyon in the process.

About two million years ago, the same tectonic forces active today along the coast caused the Santa Lucia and Gabilan ranges to rise, cutting off the California River's outlet to the sea.

With nowhere to go, that water started backing up in the valley, creating a freshwater lake up to 1,000 feet deep and 500 miles long stretching between the present-day sites of Lake Shasta and the Grapevine. Called "Lake Clyde" or "Lake Corcoran" by geologists, this freshwater inland sea would have waxed and waned with the climate cycles over its lifetime. At its peak Lake Clyde may have been larger than Lake Michigan, with an ecological richness it's hard to imagine.

About 650,000 years ago, likely during one of the lake's peak heights, it overtopped its banks at a relative low spot between what we now call the Diablo and Mayacamas ranges. Water poured westward out of Lake Clyde in what must have been an unimaginably catastrophic flood, submerging one coastal valley after another, eventually carving a path to the sea through present-day Carquinez Strait and the Golden Gate. The lake cut its new discharge channel deep enough into the coastal mountains that eventually most of the lake drained, and the Central Valley emerged again.

That's not to say the Valley was high and dry all the time. A chain of riparian wetlands remained along the valley floor, recharged by spring floods as the snow melted. The Present Day Delta would have been the largest of these wetlands. Though the California River could now reach the Pacific, the Carquinez Strait was a bit of a bottleneck. Large floods would have backed up behind it, causing a large accumulation of sediment as floodwaters waited to drain.

Where does sea level rise come into all this? Here's where: about 25 miles off the present-day coast of California. Lake Clyde had drained down to the Valley floor by somewhere around 500,000 years ago. Between that date and about 14,000 years ago, the Earth went through a number of glacial periods, in which much of the Earth's water was locked away in continental glaciers. That meant sea level would drop dramatically, and then rise again as the glaciers melted away.

The most recent glacial period started to wane about 14,500 years ago: by 12,000 years ago the melt was fully underway. At the last glacial maximum, sea levels had dropped by around 400 feet from their present levels, and the California River -- full of meltwater from a regional ice cap that covered the Sierra Nevada -- flowed out from the Delta area through the San Pablo and San Francisco Valleys, and then across a broad coastal plain to the sea, which would have been about where the Farallon Islands are today.

As the glaciers started to melt, sea level rose. By about 6,000 years ago, rising seas reached the level of the present-day Delta, flooding the Bay and causing even more of a bottleneck for those Sierra Nevada snowmelt floodwaters. Now those floods didn't just have to wait to file out through the narrow canyon at Carquinez Strait: there was a whole ocean pushing back at each high tide.

The freshwater wetlands had been biologically rich enough. But when sea level rose to the level of the Central Valley floor, it turned that biological productivity up a notch.

That meant that the Bay Delta's freshwater riparian wetlands, already biologically rich, had a new crisis and a new opportunity. Salt water from the Pacific started to intrude, drowning thriving riparian forests on the floor of what is now the Bay. Freshwater wetlands at modern-day sea level were forced to contend with shifting salinities. New organisms showed up: the ancestors of the marine and brackish-water fish, mollusks and crustaceans that now struggle to survive in the Delta's saltier parts, which had formerly kept to the Ice Age estuary off by the Farallones.

The freshwater wetlands had been biologically rich enough. But when sea level rose to the level of the Central Valley floor, it turned that biological productivity up a notch. The rich sediments that had accumulated mainly when the California River was in flood now accumulated constantly. Instead of water backing up only when there was too much flood for the Carquinez Strait to handle, it now backed up with every high tide.

And where minerals from the mountains now met minerals from the ocean, plants grew abundantly, then died. But where formerly they'd been in open air where fires could burn and winds could scour, they fell into stagnant water to turn slowly into peat.

Had sea level risen a hundred feet less, the Delta would still have been a rich riparian zone, but it would likely not have been the fertile magnet for settlement that 19th century farmers found at the confluence of the Sacramento and San Joaquin. And had sea level risen even a little more, the Delta would have become the easternmost extension of San Francisco Bay.

As we'll see in Part 2, sea level won't have to rise a whole lot more to make that second scenario come true.

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