This is an Orange Sulphur (Colias eurytheme). The image above shows the upper surface of the wings, while the next photo shows the undersides.
I photographed a live Orange Sulphur on Bodega Head on 5 October 2013, so here's an individual in better condition for comparison (below). Although the butterfly on the beach is worn, you should be able to find similar markings — look closely for the pale rounded spots with dark outlines on both wings and several solid smudgy spots on the forewing.
Although it's called an Orange Sulphur, you probably noticed that the butterfly on the beach isn't very orange or yellow, except for those two small orange patches on the hindwings (see first picture of this post). Art Shapiro describes this coloration in his Field Guide to Butterflies of the San Francisco Bay and Sacramento Valley Regions:
"Up to half the females are white, due to an autosomal dominant gene called "alba," which is expressed only in that sex. "Alba" acts by interrupting the series of chemical reactions leading to the production of the usual orange pigment — but it does not affect the orange spot in the middle of the hindwing upperside."
Hmmphh. I wondered, "Why not?" There wasn't an explanation, but it was still interesting to learn about why some female Orange Sulphurs are only a little orange.