Hi Michael - while this looks superficially like a female Common Flatwing, which it may well be, I can't rule out the possibility of a Powdered Flatwing, particularly in this mountain creek location where both species occur roughly equally. The side thoracic markings, which are the most reliable distinguishing feature, are semi-obscured by the wings. If anything, the lack of any 'caramel' colouration to the pale sub-terminal bands on the abdomen suggest to me that Common Flatwing is more likely.