The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
I'm not sure if this cranefly larva is in the genus Hexatoma or Limnophila, but the habitat suggests Hexatoma. Its coloring is iridescent, and I've never heard of that before. However, there is an insect virus genus Iridovirus which can infect craneflies and causes iridescence. It has not been reported in this region as far as I can tell, but perhaps it is the culprit. It makes for beautiful pictures.