If you’re not already aware of Calisphere you should have a look at it. A project of the California Digital Library and serving both the UC system and K-12 education, Calisphere is a gateway to a profusion of primary sources about… California!
The content comes from all UC campuses as well as non-UC institutions (including many specialized archives – see a list of contributors here), and represents 
California’s history and culture. The material is organized thematically, and special subject groups have been set up to conform with K-12 content standards (e.g. “Dust Bowl Migration”). You can also search, or browse the collection from a list of keywords, e.g. “Agricultural laborers” or “Reagan, Ronald”. In addition to images (scanned to archival standards) the collections include letters, newspapers, maps, and more.