The Sierra Foothills aka “Gold Country” from Mariposa to Oroville is one of the top spring wildflower spots in California. It includes Hite Cove/south fork of the Merced River, Red Hills Roads, Hetch Hetchy in Yosemite, and Table Mountain near Oroville.
My favorite hike is Hite Cove, which is considered by many to be one of California’s top wildflower trails. It is along the South Fork of the Merced River and is twenty-three miles east of Mariposa by Savage’s Trading Post. The trail follows the river for 4 miles, but it isn’t necessary to go out far to see good wildflowers. A walk of 1/4 of a mile will be well rewarded.
The most informative source about the trail is Wildflowers of the Hite’s Cove Trail by Steve Botti and Ann Mendershausen. It has illustrations of the most common plants accompanied by descriptions including habitats. It also has a plant identification key and a more comprehensive list of the plants of Hite’s Cove.
Photographer Michael Frye often posts about Redbud and wildflower blooms in the Merced River Canyon in the spring on his blog: In the Moment: A Landscape Photography Blog
Toni Fauver describes Red Hill Road as “One of the all-time great wildflower spots in California.” Red Hills Rd is an excellent drive with good roadside flowers and some short walks. If your timing is right you might catch a field full of Five Spots: http://www.blm.gov/ca/st/en/fo/folsom/rhflowers.html
An article by Carol Leigh on Red Hill Roads: http://docs.google.com/viewer?a=v&q=cache%3A12yvTbzbYWoJ%3Awww.calphoto.com%2Fclcwl%2Fred.pdf+Red+Hills+road+wildflowers&hl=en&gl=us&sig=AHIEtbTpQq4F4m3HeCjndP303p7A4FLI4Q&pli=1
I was at Table Mt. and posted a report with photos and information on visiting on Natural History Wandering on April 3: https://sandysteinman.wordpress.com/2010/04/03/table-mountain-wildflower-report/
There are many other great spots in the Sierra Foothills and the best information source is: Wild Flower Walks and Roads of the Sierra Gold Country by Toni Fauver, which I find has the best information about where to drive and walk to see wildflowers in the Sierra Gold Country. It also has excellent drawings of many of the plants you are likely to see accompanied by descriptions, habitats and locations.
Sierra Foothills Chapter of the California Native Plants Society: http://www.sierrafoothillscnps.org/index.html is another resource on the Sierra Foothills and sometimes posts wildflower bloom reports.