I've fished the Clackamas for many years, mostly trout...from the mouth up to and past the confluence with the Collowash River.
I use small mosquito and Adams a patterns (sz 12/14, I tie my own). I’ve always done well…Wooly buggers for lakes.
Look for slacker water fwd/aft of rapid areas but not too deep (3’ or less) of course behind larger rocks where the eddies form is good…just need to hit the spot right and be able to keep it there for a period.
I’m told wet flies are really good…I’m a little biased to dry flies…just an addiction from back in the 80’s.
Areas:
I fished for years just upstream from the I205 bridge, just above the Clackamas water intake structure, north side of the river. Park at High Rocks Pub/Safeway and walk the trail up. You’re casting upstream here with a slow drift down (very easy stretch to fish). I have come upon the occasional Steelie and Salmon in the area during certain times of the year.
Upper: 1 to 5 miles above the confluence with the Collowash River (above Ripplebrook RS). The river is smaller here than the lower section (duh). Great dry fly area with lots of logs/cover for trout. Most are just 6”-10” but I’m not a trophy guy so I find great fun in this fishery.
Northfork reservoir: Tolling a wooly bugger is one way to catch fish…you can cast and retrieve these also. I have caught fish on top with my dry flies (early and late day). Of course a boat (I kayak) helps.
Rivermill (Estacada Lake): Wooly buggers, again, early late dry fly patterns, small darker types.
Barton area: similar to upper river notes but the river is a little wider. Just gotta walk more to find fishable area.
Of course with all of these suggestions (except NF), waders are going to increase your area and fishability.
Hope this helps…now I want to go to the upper Clack this weekend…