Hercules Historical Society

San Pablo Bay

Stand at the mouth of Refugio Creek, look north, and take in the view of ten miles across the 90 square miles of San Pablo Bay. Eastward, the Carquinez Strait is the water gateway to the interior of California. Westward, forty per cent of all the waters of California flow through the Carquinez to the Pacific Ocean at the Golden Gate. 

San Pablo Bay is part of the 1,600 square mile San Francisco Bay estuary, the largest on the Pacific coast of the Americas. A tectonic shift 560,000 years ago spilled a large inland lake out of the central valley, cutting a gorge at Carquinez to take a course to today’s Golden Gate. Until the last ice age about 15,000 years ago, the San Francisco Bay was a dry valley with low hills and a river. As the world’s ice sheets melted about 11,000 years ago, sea level rose 300 feet, filling the river valley with Pacific Ocean water.