Congrats. Beauty brown. Couple things on this fish that tell me it's a brown.
The big 2 are: the large number of large spots on the side that extend down past the lateral line of the fish. The spots are faint but can clearly be seen. Also, rainbows have many radiating spots on the tail that you would clearly see. This fish has very few small ones if any.
Other than those big 2, the tail looks pretty straight, no fork. If you fish for trout or salmon in the lake and in the tribs then you know there are some major changes that occur as they enter the tribs and they are usually bright silver while in the lake. This brown would have been back in the lake now for a bit and would have recooped from the fall spawn. Considering your location and the fact that this is the time to catch those browns in close to shore pretty much sums it up to me that you got yourself a nice female brown.
So, other than the reasons i stated above I got nothing else.
JFish