I happened upon this site during a visit to Oroville Dam, on the Feather River on the northeast side of the Sacramento Valley. It's a little out of the way, and the signs pointing to it are not all that reliable. The plaque, and tree it celebrates, are adjacent to a California State Parks office (exact address: 400 Glen Drive, Oroville).
The plaque text:
Judge Joseph Lewis planted this Mediterranean sweet orange seedling near the toll bridge at Bidwell's Bar, Butte County, California, in 1856. It has survived hard times and is the oldest living orange tree in California. "From its example and largely from its offspring, a new industry was started in a new section hundreds of miles north of a known citrus region. It was a true pioneer." -- Dr. H.J. Webber, director, Citrus Experiment Station, Riverside, 1927. The mother orange was moved from Bidwell Bar during the construction of Oroville Dam in 1964 to protect it from inundation.
California Registered Historical Landmark No. 1043
Plaque placed by California State Parks in cooperation with the Butte County Historical Society and U.C. Cooperative Farm Extension Advisor for Citrus in Butte County, May 6, 2006.
Below: the mother orange tree (the green one, on the left) herself.