Re: Yellowstone Park - Northeast Portion - early Sept.
I've found the fish.. not to be too picky over there
It should be prime hopper season about then. Hoppers, Chernobyls work great.. and maybe a typical dropper - but hoppers were the go to fly.
It can get crowded, but if you move around.. easy to find spots, typical, most people fish the easiest accesible places.
Keep an eye open for wildlife.. all kinds there (ask Parks about Bear spray or not)
One of the times I was there.. we were not that far away, but a Bison herd was going to cut us off from gettting back to the car.. if th herd is big enough and slow enough, you'll have an extra long walk, or be prepared to wait them out