Evening Primrose
(Oenothera caespitosa)

This is a low-growing plant, but has very large showy flowers. They bloom in the evening, then fade and wither soon after sunrise the next morning.
The flowers usually have no stem. What seems to be a stem is actually an elongated floral tube. Only insects with very long tongues can reach the nectar at the bottom.

Pale Evening Primrose
(Oenothera pallida)

This species grows about a foot tall. Its flowers are a little smaller than those pictured above (about 2 inches across), but still very showy.
Often many others will be blooming nearby at the same time, and the plants will seem to be covering the hillside.
Another very similar species (Oenothera albicaulis) also occurs in this area. I am not sure which one is portrayed in the photo at the right.