Looks like someone scooped me while I was writing this, but here's another similar view:
Bridges is definitely a notch more difficult than Filter Plant.
Bridges is a solid III (III+?) if you put in below Pine View and take out at the BTO (Bridges take out), which is about a third to a half mile above the nasty dam you'll see as you drive up. Including Pine View would kick it up to a IV.
Filter Plant is pretty much a class II, with the put in being either Gateway Park (which has a parking fee) or in the roadside parking area just downstream. The takeout is Picnic Rock. I've seen Mad Dog rapid, which is roughly halfway through the run, rated as a III, but I think most would agree that's a stretch. It's a fairly short, straightforward rapid compared to what you find on Bridges.