[7] The factors contributing to the formation of Lake Corcoran are not fully understood[12] but it appears that Great Valley drainage for most of the Miocene epoch was to the south.

[2] Evaporation from this lake was a source of water for the Sierra Nevada and in lesser measure for the Basin and Range Province behind it.

[6] Six hundred thousand years ago a new outlet formed in the present day San Francisco Bay, where it remains today.

[8] Sediments found south of San Francisco indicate that by 400,000 years ago the drainage was fully established.

The overflow may have occurred at a time where glaciers were melting and when shifts in the jet stream during the marine oxygen isotope stage 6 caused increased precipitation in and runoff to the Central Valley.