Oregon Hikers

You don't have to go very far east to risk seeing rattlers. I've encountered them on Dog Mountain (3x), Memaloose Hills (1x, 2 snakes), Lyle Cherry Orchard (1x), on a pullout along highway 199 near Kerby (2 snakes), Rogue River trail (2x).

The rockier the ground or higher the grass, and the hotter the day (say 80 degrees), the more likely you are to find them.

Supposedly Wind Mountain is the farthest west you'll find them on the Washington side of the Gorge. On the Oregon side, I haven't heard of them west of Memaloose, but it seems likely they'd be at Hood River Mountain too. (EDIT: a Google search turned up a few sightings on the HCRH bike path, so, yeah, I'd expect to find them all the way west to the Hood River Valley, but maybe not beyond the river.)

Oh, and add Tom McCall Point to my personal sightings list.

Smith Rocks

I've also seen them in Trinity Alps in northern California

I think rattlesnakes are found more often at places where humans visit.

(The rattlesnakes at places where humans don't visit aren't found because the humans don't visit there, that is, rattlesnakes are everywhere)