Check out how his winter weight-loss shows at his tail-end: his skin is throwing two big folds there.
When I posted the story on fb, someone remarked that this might be a Mojave Rattler, and I think that is possible. Going back through my earlier slides, I found that there were always a few among all the Diamondbacks that I photographed. It's not too difficult to tell them apart conclusively, but you have to take a closer look at them than I sometimes do. Here is a link to a blog written when I did.
On the same walk we saw two Northern Harriers, both gold-brown, a Red-tail, a Cooper's, the territorial Kestrel, a group of Lawrences's Goldfinches, a Belted Kingfisher, several Abert's Towhees, some Mallards, and flying overhead, two clouds of thousands of birds. The first cloud consisted of sparrow sized birds and I couldn't tell what they were, the second was made up of Blackbirds, mostly yellow-headed, and grackles.A lonely Monarch was fluttering among the willows along the river.