Although it was easy to observe this bird with a spotting scope, my camera lens wasn't nearly strong enough for close-up photos.
Here are a couple for the record — a side profile (with a hint of the black breast patch), and another photo highlighting the long tail and feeding position:
Len posted some much better pictures here.
And if you're interested, the White Wagtail account (along with the figures and table) from the Rare Birds of California is informative.
P.S. The genus, Motacilla, means "moving tail" — and in this case, the name is quite appropriate. Check out this video posted by Stephen Berylant.