Around the Yellowstone region, the last two weeks of August and the first week of September are flying ant time. Various species of ants have developed wings for flight and are recolonizing before fall and winter arrive. Ant flights are impossible to predict; we only know the general time frame, and the ants adhere to it with remarkable precision.
I was on the Henry’s Fork two days ago, and witnessed a wonderful flight of ants. Big fish fed with abandon for several hours, as can be seen in the video. Bold, splashy rises are usually indicative of trout feeding on larger and/or mobile insects, but ants are an exception. The ants on this day were a size 16, and were in no danger of leaving the water. Trout love ants, and when they are present they will trump any other food item available.
If you’ve ever wondered why ants end up on rivers and lakes in the first place, it’s because they navigate by polarized light. Since water can reflect a lot of polarized light, when the conditions are right ants will be inescapably drawn to water.