A female Yellow-faced Bumble Bee, Bombus vosnesenskii, browsing on buckwheat flowers. She's really packed in the pollen but the color appears to be from California Poppies and not from that buckwheat. It appears that she's feeding on nectar. This buckwheat looks like a Saint Catherine's lace with a pink hint to the flowers and reddish pollen. This could be Eriogonum giganteum var. giganteum based on some images found on CalPhotos.
This was taken just outside the visitors center at Don Edwards in Alviso, CA. There are a lot of assorted buckwheats in bloom right now.