The fishery food resource is still relatively strong up there, for starters. I've seen around 100 in a day around here (only once though), but the peak season is only a month or three. There's a resident pair within about a mile of my house though. We get close flyovers from time to time. But the only time I've seen a mob like in Will's picture was up north of Seattle on the Skagit River, which is a very famous birding location for that very reason.
We get Golden Eagles around here too, you'll see mixed flocks of juveniles occasionally. And then we have Ospreys all over the place, and they like to nest on navigation structures in the river. For many years there was a nest right at my favorite windsurfing launch spot, it was always a blast watching the babies grow up. Osprey will swoop down on your head if provoked, for real. My friends at USGS wore hockey helmets when they had to do work on nav beacon platforms or other places they have telemetry setups installed on out on the river.