As with anywhere else you are considering to visit, expect crowds on Park waters. The best chance for getting away from crowds is to be willing to walk a bit. The second meadow above on Slough Creek would be a good choice. So would the Bechler Meadows streams Bechler River and Boundary Creek. In fact, the creek would be less crowded of the two, especially in the upper meadow. Walking downstream from Nez Perce Ford on Yellowstone River can get you into some large cutthroat trout, and the further you walk, the less the crowding. Lamar River above the cascades and in the meadows will be crowded, but walking upstream to such as Cache Creek will get you away from the bulk of crowds. Very few folks walk into the meadows along the middle of the Lewis River Channel between Lewis and Shoshone Lakes. The Gallatin River along the highway will be popular, but you can avoid crowds by walking up the Big Horn trail a few miles to a meadow reach or up Fan Creek a mile or so to another meadow reach. Spruce moths will supplement the other terrestrial insects on these.
I seem to be stressing meadow sections of these streams, and for good reason: terrestrial insects abound in these, and for now will provide your best chance of fishing success, even tho’ tricos are coming out on some waters. In all of these be “bear aware.” Bring the spray and a noise maker. The “noise maker” can be a loud talking buddy, but a boat horn, AKA “claxon horn,” is an even better choice. You can find one for under ten bucks at any marine supply shop or mega-store sporting goods department. It fits easily in a shirt or vest pocket and weighs next to nothing. Be assured that it will carry a lot further than any loud talking buddy you might have!