Millcreek on Apr 8, 2020April 8th, 2020, 5:28 am EDT
Definitely a member of Heptageniidae. Might be a Rhithrogena. If you still have the specimen, put it on it's back and see if the gills form a "suction cup", or if they meet in front and back, like in this picture.
Konchu on Apr 10, 2020April 10th, 2020, 6:50 am EDT
That looks like my search image of a living Rhithrogena. I'm only aware of R. undulata having been found in the Bitterroot R in MT, but like Millcreek said, we don't know as much about the nymphs as we'd like. The record I have was from a June emergence, FWIW.