The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
This dun comes from the same location, and is about the same size, so is presumably the same species as this male spinner.
This mayfly was collected from Enfield Creek in Treman Park in New York on May 15th, 2007 and added to Troutnut.com by Troutnut on May 18th, 2007.