The Stanislaus River is believed to have originally formed sometime during the Miocene period, about 23 million years ago, flowing down from an ancient mountain range in the current location of the Sierra Nevada that has since eroded away.
As both uplift and erosion continued, the Stanislaus River gradually carved the rugged canyons it flows through today, and contributed to the vast alluvial deposits that make up the flat floor of the Central Valley [REF].