So what are the factors then?
These are the reasons given by a Principal level fisheries biologist(my wife, lol
). I edited out the name of the consulting firm at the request of one of the employees.
Reason #1: It was caught in the lower Clackamas mainstem.
Rainbow are not found in the mainstem Clackamas. They are only found several miles upstream in headwater tributaries where the water is cooler and there is large wood and good cover. They live in those tributaries their entire lives. They are born, feed, breed, and die there. The mainstem Clackamas is a warm nasty mud hole in the summer months and Rainbow are very particular about their habitat. Cutthroat will live in a pile of crap if it has a small dribble of water in it. Rainbow will not.
Reason #2: It's not the right color.
Willamette Valley rainbow trout are distinctive. They have bright colors. This fish was muted and had silvered like a fish does when it goes out to see.
Reason #3: It's too big!
That was a fat, 17" long fish. A big rainbow in that system is probably more like 14".
Reason #4: It makes the most logical sense!
At this time of the year you'd expect to find a jack steelhead in that part of the Clackamas River. It's possible that a rainbow could have followed a misguided salmon out of a trib and up river to mow down some salmon eggs, but that's a long shot.
Bottom line, it's an Onchorynchus mykiss. The only way to know definitively whether it is a steelhead or not is to test a scale sample to see if it's hit salt water. That is the only way anyone can ever definitely tell a Steelhead from a Rainbow.