For thousands of years, the spring snow melt from several major mountain ranges, including the western side of the Sierra Nevada, the southern parts of the Cascade Range, and the eastern flanks of the Pacific Coast Range flooded huge swaths of the Sacramento Valley regularly. With this spring melt, a giant inland lake was formed in the valley. The runoff then flowed into the delta.

The delta is an estuary which acts as the funnel for a drainage basin covering thousands of square miles of California's land. Ultimately, all that water flowed into the Pacific Ocean through the narrow channel known as the Carquinez Strait. It flowed on to the San Francisco Bay and out into the Pacific Ocean through the Golden Gate.