If you fish the river below Ashton Dam during daytime, why not just go for a swim. In Island Park terrestrial insects are just as abundant as on the lower river. But with cooler water fish are more active in taking them. They seem to like small (#18) rusty spinners, too. Tricos should be showing in significant numbers soon. Two rig nymphs (big pattern trailed by a small one) are working in Box Canyon. Water is dropping and warming in the Flat Ranch portion. This means larger fish there will be moving downstream.
If you intend to fish waters off the main park highways, expect big time delays because of hoards of tourists. Traffic lines at the west entrance extend for a half mile back west by mid morning. You can hear accents from all languages on the globe. The same is happening at other entrances. So get there early enough to be about the first in line. When you finally arrive at your fishing destination terrestrial insect patterns (including spruce moths around forested areas) are working as good as anything. Present these patterns with long, drag-free floats.
Creek mouths and springs are the places to fish these dog days of summer. Most of what’s being caught are cutts in the 15-18 inch range. Traditional Henry’s Lake flies work best. If you do not have one, pick up one of Bill Schiess’s book “Fishing Henry’s Lake.” It has tips for where to fish and how to approach this time of year as well as traditional Henry’s Lake fly patterns. It is a treasure trove of Henry’s Lake fly-fishing information.
Look at the boat ramp-parking areas. A glance at these tells how good the fishing is (for example Spring Creek overflows to near the highway during daytime). Mayflies, early morning mutant stones, PM caddis and terrestrial insects are active. Have patterns for all of these, and you will find action. At 9800 cfs at Irwin, 10700 cfs at Heise and 4850 cfs at Lorenzo, the river could not be in better shape.
Thanks to grasshoppers it does not matter where you go from the Teton River to smaller streams such as Little Warm River dry fly fishing will be good. To supplement the effectiveness of hopper patterns these days, be sure to have caddis life cycle, traditional attractor, ant, and beetle patterns. In a few streams small PMDs are still around, and tricos will be soon showing up. Kokanee are beginning to show in lower Big Elk Creek and western green drakes (flavs) should show up in significant numbers any day now.
Almost anywhere you go fishing is crowded. That applies not only streams and smaller still waters, but roads going there (travel as early in the day as possible). The best way to avoid crowds is to walk away from easily accessed waters once you reach them. That means try such as Fall River Basin, Lewis River between Shoshone and Lewis Lakes, second meadow above on Slough Creek, Cache Creek area on the Lamar River, Black Canyon of the Yellowstone and so forth. Even in those places you will have company, but to a lesser amount. For almost all streams presenting terrestrial insect patterns is the best way to fishing success.
Mayflies (PMDs & pink alberts) are out in the riffles up and down the river, and so are drift boats. If you fish early in the morning ( before around 9 AM), you will see mutant golden stoneflies emerging and fish keying on them.