The Monterey Bay outlet of the Sacramento and San Joaquin rivers was blocked off by uplift about 2 million years ago , and runoff from the Sierra began to transform the Central Valley into a gigantic lake , called Lake Clyde . This lake stretched north to south and was at least deep . About 650,000 years ago the lake catastrophically overflowed , draining into San Francisco Bay and creating the Carquinez Strait , the only major break for hundreds of miles in the Coast Ranges . The narrow outlet trapped some of the sediments of the rivers in the Central Valley , forming the inland Sacramento-San Joaquin River Delta . Since then , this inland sea has periodically reformed during times of intense flooding , the most recent being the Great Flood of 1862 . Dams , levees and floodways constructed during the 20th century have thus far prevented this phenomenon from re-occurring
Great Flood of 1862
Great Flood of 1862
Sacramento River