Ancient blue oak trees are still widespread across the foothills of the Coast Ranges, Cascades, and Sierra Nevada in California. The most extensive tracts of intact old-growth blue oak woodland appear to survive on rugged and remote terrain in the south Coast Ranges and on the foothills west and southwest of Mt. Lassen. In our sampling of old-growth stands, most blue oak appear to have recruited to the canopy in the mid- to late-19th century. The oldest living blue oak tree sampled was over 459-years old and several dead blue oak logs had over 500 annual rings. Precipitation sensitive tree-ring chronologies up to 700-years long have been developed from old blue oak trees and logs. Annual ring-width chronologies of blue oak are strongly correlated with cool season precipitation totals, streamflow in the major rivers of California, and the estuarine water quality of San Francisco Bay. A new network of 36 blue oak chronologies records spatial anomalies in growth that arise from latitudinal changes in the mean storm track and location of landfalling atmospheric rivers. These long, climate-sensitive blue oak chronologies have been used to reconstruct hydroclimatic history in California and will help to better understand and manage water resources. The environmental history embedded in blue oak growth chronologies may help justify efforts to conserve these authentic old-growth native woodlands.