Where the Delta came from
GUEST COMMENTARY FROM THE NEWS-LEDGER — MAY 9, 2012 —
By Glen Holstein
Fifteen thousand years ago what became California had no Delta and was in a very different world in which much of North America and Europe were covered by vast continental ice sheets. By then people occupied most of the eastern hemisphere but few, if any, had yet reached the Americas.
Then what is now central California’s coastline was 26 miles west of its present location. The Farallons were then not islands but coastal headlands overlooking an open ocean dropping abruptly to great depths. What is now the continental shelf was a vast dry land plain bisected by an ancestral Sacramento River swollen to great volume by melting glaciers then widespread in the Sierra Nevada. It entered the Pacific south of the Farallons and flowed through the Coast Range 300 feet below present water level in deep canyons at what are now the Golden Gate and Carquinez Strait.
The climate then along the lower Sacramento was much like the present coast of southern Alaska and British Columbia, but the world was warming. The great continental ice sheets began retreating, and their meltwater caused seas to rise everywhere. By ten thousand years ago they neared the present shoreline and by eight thousand had entered the Golden Gate.
People were definitely in what would become California by then and had established villages in a broad valley just inside the outermost Coast Range ridge. Soon, however, rising seas following the ancestral Sacramento River’s channel inland completely flooded their valley and created what later arrivals would call San Francisco Bay. Inexorably seas pushed farther inland flooding more valleys and creating new bays like San Pablo and Suisun until they finally stopped near the present Montezuma Hills five thousand years ago.
There freshwater flowing downstream from the Sierras and Cascades through the Sacramento and San Joaquin rivers met seawater flowing inland through the Golden Gate. They mixed some, but the freshwater mostly flowed outward some distance in a shallow lens above the heavier salt water, which also acted as a hydraulic dam to stop most river flow at the Montezuma Hills and cause freshwater to backup and flood a vast area in the lowest part of the Central Valley.
This flooding starting just five thousand years ago created California’s Delta. Because sea level rise was gradual, the flooded area always remained very shallow beyond the deeper river channels and became covered by tall marsh plants called tules. Seas still slowly rose, though, and freshwater in the flooded Delta area also did just slowly enough for each new tule generation to grow on the last’s flooded remains.
What happened to it next is another tale.
Dr. Glen Holstein received his PhD in Botany from UC Davis and is a Senior Scientist with Zentner and Zentner, a local biological consulting company. Glen is Chapter Botanist for the Sacramento Valley Chapter of the California Native Plant Society , represents that Chapter at Habitat 2020 and was the California Prairie spokesman at its Wildflower Weekend in April , 2005 . He’s also on the Board of Tuleyome, a non-profit organization working to protect the wild and agricultural heritage of California’s Inner Coast Range and Western Sacramento Valley.